Microcytic Anemia: What a Low MCV Means

TL;DR30-second take

A low MCV means the red blood cells are smaller than average, under about 80 femtoliters in an adult. Iron deficiency is the most common cause, and ferritin is the first test to order. The second common cause is thalassemia trait, an inherited difference in how hemoglobin is built that makes small red cells for life. Fishtown Medicine reads the red cell indices alongside iron studies and, when the picture fits, adds a hemoglobin electrophoresis before an invasive procedure. When iron deficiency is confirmed in an adult man or a postmenopausal woman, the gastrointestinal evaluation still stands, because carrying thalassemia trait does not protect anyone from a bleeding ulcer or colon cancer.

TL;DR: MCV is a number on your blood count. It tells you how big your red blood cells are. A low MCV means they are small. Low iron is the most common reason, so the first test is a ferritin level along with the rest of an iron panel. Some people are born with small red cells instead. That is called thalassemia trait, and it is passed down in families. It is more common in people whose families come from the Mediterranean, the Middle East, South Asia, Southeast Asia, and Africa. A blood test called a hemoglobin electrophoresis checks for it. Here is the order that helps: check ferritin and iron first, then check for thalassemia trait if the numbers do not add up. If your iron is low and you are a man, or a woman past menopause, you also need a camera test of your stomach and colon. Low iron in those groups can mean slow bleeding inside, and finding the cause early saves lives. Carrying thalassemia trait does not change that part.

A low MCV on a routine blood count often arrives alongside a message about mild anemia and a request to schedule a colonoscopy. That sequence is reasonable and it saves lives. It can also skip past a question that a single extra blood test can answer, which is whether the small red cells are something you developed or something you were born with.

What does a low MCV mean on a complete blood count?

MCV stands for mean corpuscular volume, the average size of your red blood cells, and it comes standard on every complete blood count (CBC). In an adult, a value below about 80 femtoliters is called microcytosis, meaning the cells are smaller than average.¹ The number by itself is a description rather than a diagnosis: it tells you the cells are small and says nothing about why they are small.

Two causes account for most of what turns up. Iron deficiency is the first, because hemoglobin cannot be built without iron, and a cell short on hemoglobin ends up short on size. Thalassemia trait is the second, an inherited difference in how the protein chains of hemoglobin are made that produces small cells from birth and keeps producing them for life.¹ Anemia of chronic disease, lead exposure, and sideroblastic anemia fill out the rest of the list and come up far less often.¹

Why is iron deficiency the first thing to check?

Iron deficiency is the first thing to check on a low MCV because it is the most common cause and because it usually points at a treatable problem somewhere else in the body. The test that answers it is ferritin, the protein that stores iron, and it belongs on the first blood draw along with serum iron, total iron-binding capacity, and transferrin saturation.¹

The threshold matters more than most people are told. The American Gastroenterological Association recommends using a ferritin cutoff of 45 ng/mL rather than 15 ng/mL to diagnose iron deficiency in someone who is anemic, because the lower number misses too much. At 15 ng/mL the test identifies about 59% of iron-deficient patients; at 45 ng/mL it identifies about 85%, while specificity falls only from 99% to 92%.² A ferritin of 30 alongside a low MCV is a low ferritin in the way that counts, even when the lab report does not flag it.

Ferritin has a second job as an inflammation marker, so it climbs during infection, liver stress, and metabolic strain, which means it can read falsely normal in someone who is both iron deficient and inflamed. That is why the transferrin saturation belongs on the same panel, and it is the mirror image of the problem covered in our guide to a high ferritin.

When is a low MCV inherited rather than acquired?

Hemoglobin is built from 2 pairs of protein chains, alpha and beta, and thalassemia is the name for an inherited shortfall in making one of them. A person with beta thalassemia trait carries a single altered beta-globin gene, and a person with alpha thalassemia trait is missing 2 of the 4 alpha-globin genes.⁴ Either way the red cells come out small and pale, the blood count reads abnormal for a lifetime, and the person is generally well.

That last part is what causes the confusion. Thalassemia trait is a carrier state rather than a disease, so there are no symptoms to prompt anyone to look for it, and the CBC is the only place it announces itself. Someone can reach 50 with a documented low MCV on every panel they have ever had and still be told this year that it is new iron deficiency, because nobody pulled the older results forward.

The practical clue is time. Iron deficiency develops, so there is a before and an after. Thalassemia trait has no before. If you can find a blood count from 10 or 15 years ago with the same low MCV, that history is doing serious diagnostic work, and it is one of the most useful things a patient can bring to a visit.

How do you tell thalassemia trait from iron deficiency on a CBC?

There are 4 features on the blood count itself that lean one way or the other, and the honest version is that they point rather than prove.

How low the MCV is compared with how low the hemoglobin is. In thalassemia trait the MCV is often strikingly low, into the 60s and low 70s, while the hemoglobin is only slightly below normal or even normal. Iron deficiency tends to be more proportional, so as the MCV falls the hemoglobin falls with it. Very small cells carrying a near-normal hemoglobin is the classic trait pattern.
The red blood cell count. In thalassemia trait the marrow makes plenty of red cells and they are simply small, so the RBC count is often normal or high. In iron deficiency the count usually drops along with everything else.
The RDW. Red cell distribution width measures how much the cells vary in size. Iron deficiency creates a mixed population, older normal-sized cells alongside newer small ones, so the RDW rises. Thalassemia trait creates cells that are uniformly small, so the RDW is often normal.
The Mentzer index. Described in 1973, it is the MCV divided by the red blood cell count. A value below 13 leans toward thalassemia trait and a value above 13 leans toward iron deficiency.⁶

None of that settles the question. When RDW was tested as the main sorting variable across a series of microcytic cases, nearly half the thalassemia patients also fell into the raised-RDW group, which is enough overlap to make it unusable as a rule.⁸ Reported accuracy for the Mentzer index swings widely from study to study, and head-to-head comparisons of 12 of these discrimination formulas have yet to produce one that stands alone as a diagnosis.⁷

There is also the case that breaks every formula, which is a carrier who is also iron deficient. Both conditions in the same person produce a blood count that reads like neither, and that combination is common in the very populations where thalassemia trait is most frequent.

Who carries thalassemia trait, and why does ancestry belong in the picture?

Thalassemia became common where malaria was endemic, because carrying a single altered globin gene offered some protection against severe malaria, and the trait remained in those populations long after families moved elsewhere. That history is why carrier rates today are higher in people whose ancestry traces to the Mediterranean basin, the Middle East, the Indian subcontinent, Southeast Asia, sub-Saharan Africa, Melanesia, and the Pacific islands, with reported carrier frequencies running from roughly 1% to 20% depending on the region.⁴ ⁵

Ancestry belongs in the workup the way family history does, as one input that raises or lowers the chance of a diagnosis before any test comes back. It never rules a condition in and it never rules one out. A person of Northern European ancestry can carry beta thalassemia trait, and a person whose grandparents came from Cyprus or Kerala can have ordinary iron deficiency from a bleeding ulcer. What ancestry does is tell a physician when an inexpensive confirmatory test is likely to pay for itself before an invasive one. Obstetric practice has already moved in that direction: the American College of Obstetricians and Gynecologists now recommends offering carrier screening for thalassemias and hemoglobinopathies to every patient who is pregnant or planning a pregnancy, regardless of ethnicity.¹²

Philadelphia makes the point on its own. The city's ancestry map has been redrawn several times over, and a practice here sees patients whose families came from Italy, Greece, Vietnam, Cambodia, India, Pakistan, Nigeria, and Puerto Rico, sometimes in the same week. A workup built on the assumption that microcytosis equals iron deficiency will be wrong often enough in that population to matter.

What does a hemoglobin electrophoresis show, and what does it miss?

A hemoglobin electrophoresis separates the different types of hemoglobin in a blood sample and reports each as a percentage. Most labs now run it by high-performance liquid chromatography or capillary electrophoresis and still list it as an electrophoresis on the order screen. It is an ordinary blood draw, and the result usually comes back within a few days.

For beta thalassemia trait it is the confirmatory test. Carriers make less beta chain, so hemoglobin A2, which is built from alpha and delta chains, takes up a larger share of the total, and an HbA2 above about 3.5% establishes the diagnosis.⁹ Hemoglobin F is sometimes mildly raised alongside it.

Two limitations decide how the result should be read.

The first is alpha thalassemia. Alpha thalassemia trait does not raise HbA2 and usually produces a normal hemoglobin electrophoresis, so a normal result does not exclude it. Confirming alpha thalassemia trait requires alpha-globin gene testing, which looks directly for the common deletions and, less often, for point variants.¹⁰ In practice, someone with lifelong microcytosis, normal iron studies, and a normal electrophoresis is a candidate for alpha-globin analysis rather than a person with no explanation.

The second is iron. Iron deficiency lowers HbA2 slightly, on the order of 0.2% on average in one large screening population, and alpha thalassemia trait lowers it as well, so a carrier who is iron deficient can produce a borderline HbA2 that reads as normal.¹¹ The effect is small, and other groups have found that overt iron deficiency rarely pushes a true carrier below the diagnostic threshold, so the practical rule stays narrow: when HbA2 comes back borderline, in the low 3s, in someone who is iron deficient, replace the iron and repeat the test rather than closing the question.

Does thalassemia trait mean you can skip the colonoscopy?

No. This is where the reasoning has to hold both halves at once, because getting it wrong in either direction causes harm.

The indication for a gastrointestinal evaluation is confirmed iron deficiency anemia rather than a low MCV on its own. For asymptomatic postmenopausal women and men who have iron deficiency anemia, the American Gastroenterological Association recommends bidirectional endoscopy, meaning an upper endoscopy and a colonoscopy in the same setting, and it suggests the same approach for asymptomatic premenopausal women.² The reason is the yield: in study populations made up mostly of men and postmenopausal women with iron deficiency anemia, bidirectional endoscopy found a lower gastrointestinal cancer in about 8.9% and an upper gastrointestinal cancer in about 2.0%.² Those figures come from a diagnostic setting rather than a screening one, and they run many times higher than the cancer rate found at routine screening colonoscopy. The British Society of Gastroenterology reaches the same conclusion about who needs investigating.³

So what changes is the sequence, and often the destination does not change at all. Confirm iron deficiency with a ferritin and full iron studies. If iron deficiency is confirmed in an adult man or a postmenopausal woman, the endoscopy is indicated whether or not thalassemia trait is also present, because carrying the trait does not protect anyone from a bleeding polyp, an ulcer, or a colon cancer. Where the electrophoresis earns its place is the other branch: when the iron studies are normal, or when the microcytosis is far out of proportion to a barely-low ferritin, the small cells may be inherited, and the case for an invasive procedure was never as strong as the CBC made it look.

Fishtown Medicine

A 90-minute conversation with Dr. Ash. A written plan you can actually follow.

Start your intake

The reverse error deserves naming too. Someone who knows about a family trait can carry that label for decades and use it to explain away a falling hemoglobin, a dropping ferritin, or a change in bowel habits. Thalassemia trait is a fixed, lifelong finding, so anything that is new or getting worse is a separate problem until proven otherwise. If you have the trait and your numbers are moving, that movement needs its own answer. Our guides to colorectal cancer screening and colon cancer in young adults cover what that evaluation involves.

What else causes a low MCV?

Beyond iron deficiency and thalassemia trait, the remaining list of microcytosis causes is short.¹

Anemia of chronic disease usually produces normal-sized cells, but with enough time and enough inflammation it can turn microcytic. The mechanism is hepcidin, a hormone that rises with inflammation and locks iron inside storage cells where the bone marrow cannot use it, so the body runs functionally short of iron while its stores stay full. The giveaway is a ferritin that is normal or high while the serum iron and transferrin saturation are low.

Lead exposure is uncommon and belongs on the list here because Philadelphia's housing stock is old. Adults who sand, strip, or demolish painted surfaces in a rowhome built before 1978, or who work in battery manufacturing, radiator repair, or metal recycling, can absorb enough lead to interfere with heme production. A blood lead level answers the question and is worth ordering when the exposure history fits.

Sideroblastic anemia, in which the marrow takes in iron but cannot build it into heme, is rare. It can be inherited or acquired, including from heavy alcohol use, certain medications, copper deficiency, and myelodysplastic syndromes. Confirming it involves a bone marrow examination, so it is a hematology question rather than a primary care one.

Why does knowing you carry the trait matter for family planning?

Knowing you carry thalassemia trait matters for family planning because thalassemia is inherited in an autosomal recessive pattern. A carrier is generally healthy and stays that way. When both partners carry a beta thalassemia variant, though, each pregnancy carries a 25% chance the child inherits 2 copies and has beta thalassemia major, a transfusion-dependent condition, a 50% chance of being a carrier, and a 25% chance of inheriting neither variant.⁹

This is why the American College of Obstetricians and Gynecologists recommends offering carrier screening for thalassemias and hemoglobinopathies, along with a complete blood count, to every patient who is pregnant or considering pregnancy. When the first partner tested turns out to be a carrier, the other partner is tested, and couples who both carry a variant are offered genetic counseling and options that include prenatal diagnosis and preimplantation genetic testing.¹² Doing this before pregnancy leaves the most room to think and decide calmly.

That is the quiet upside of working a low MCV up properly. A hemoglobin electrophoresis ordered to explain a blood count can end up answering a question the person had not thought to ask yet, and it puts information in the hands of their siblings and their children as well.

How Fishtown Medicine works up a low MCV in Philadelphia

We treat a low MCV as a starting point rather than an answer, and the first move is usually to pull the history forward. If we can find an old CBC, we look at what the MCV was doing 5 or 10 years ago, because a low value that has been there all along is telling a different story from one that appeared last year.

From there the panel is built to answer both questions at once. Ferritin, serum iron, total iron-binding capacity, and transferrin saturation establish whether iron is the problem, and we read ferritin against the 45 ng/mL threshold rather than the older cutoff, with an inflammation marker alongside it when the picture calls for one. When the red cell indices point toward an inherited cause, or when ancestry and family history make a carrier state likely, we add a hemoglobin electrophoresis on the same draw so the answer arrives in days instead of after a procedure.

When iron deficiency is confirmed, we do the part that gets skipped, which is finding out where the iron went. That means looking hard at menstrual bleeding, gut symptoms, celiac disease, medications that promote bleeding, and diet, and it means arranging the gastrointestinal evaluation when guidelines call for it and coordinating with gastroenterologists we trust across Philadelphia. Our guide to iron, heavy periods, and hair loss covers the most common cause in premenopausal women.

Some findings should move faster than a routine follow-up. Get medical attention sooner if you notice:

Black, tarry, or bloody stools, or blood when you vomit
A hemoglobin that is falling from one panel to the next
Chest pain, breathlessness at rest, or lightheadedness on standing
Unintended weight loss, a new persistent change in bowel habits, or abdominal pain
Iron pills that have been taken correctly for 3 months with no rise in hemoglobin or ferritin

Guidance from the Clinic

"The mistake I see is treating a low MCV as if it only carries one meaning. Someone arrives with a colonoscopy already scheduled and nobody has asked whether their red cells have been small on every panel since college. If the ferritin is low, that colonoscopy is the right call and I want them to keep it, because iron deficiency in an adult man or a woman past menopause is a bleeding question until proven otherwise. What I want people to know is that a hemoglobin electrophoresis is a simple blood draw, it costs a small fraction of what a procedure costs, and it sometimes explains a lifetime of abnormal blood counts in a single line. Ordering it first costs you a few days. Skipping it can cost somebody a procedure they never needed, or send them home with an iron pill that was never going to work."

Actionable steps for a low MCV in Philly

What to do, and in what order.

Find your old blood counts. Log into every patient portal you have and write down the MCV from each CBC going back as far as you can. A low MCV that has been stable for years changes the whole conversation.
Ask for the full iron panel by name. Ferritin, serum iron, total iron-binding capacity, and transferrin saturation, and write down the ferritin number itself instead of the word normal. Read it against 45 ng/mL, not 15.
Write down what you know about your family. Where your parents and grandparents came from, and whether any relative has been told they have thalassemia trait, thalassemia minor, or lifelong mild anemia.
Ask about a hemoglobin electrophoresis before an invasive procedure, if your iron studies are normal or your microcytosis looks out of proportion to your ferritin. It is a blood draw, and the answer usually arrives within a few days.
Keep the endoscopy if your iron is truly low. Confirmed iron deficiency in an adult man or a postmenopausal woman needs a look at the stomach and colon, and thalassemia trait does not cancel that.
Do not start iron on your own. Iron given to someone whose small cells come from thalassemia trait will not help, and iron can accumulate where it is not needed. Test first, then treat.

Key Takeaways

A low MCV describes small red blood cells and names no cause. Microcytosis in an adult means an MCV under about 80 femtoliters, and iron deficiency and thalassemia trait account for most of it.
Ferritin and a full iron panel come first, read against the American Gastroenterological Association's 45 ng/mL cutoff rather than the older 15 ng/mL threshold.
A very low MCV with a near-normal hemoglobin, a normal or high red cell count, and a normal RDW points toward thalassemia trait, though RDW and the Mentzer index overlap too much between the 2 conditions to be diagnostic on their own.
A hemoglobin electrophoresis confirms beta thalassemia trait through an HbA2 above about 3.5%, and it does not detect alpha thalassemia trait, which needs alpha-globin gene testing.
Iron deficiency can lower HbA2, so a borderline result in an iron-deficient person should be repeated after iron is replaced.
Confirmed iron deficiency anemia in an adult man or a postmenopausal woman still requires gastrointestinal evaluation, since bidirectional endoscopy finds a lower gastrointestinal cancer in roughly 8.9% and an upper one in roughly 2.0% of these patients.
Knowing you carry thalassemia trait matters beyond your own blood count, because 2 carriers face a 25% chance per pregnancy of a child with a transfusion-dependent form.

High Ferritin: What It Means, from Iron Overload to Inflammation - the other end of the iron spectrum, and why ferritin can mislead
Iron, Heavy Periods, and Hair Loss - the most common source of iron loss in premenopausal women
Colorectal Cancer Screening: Which Test, and When? - what the evaluation involves once iron deficiency is confirmed
Colon Cancer in Young Adults: The Rise and the Red Flags - why age alone no longer rules out a GI source
What Your Blood Count Reveals - the other signals hiding in a routine CBC
Iron Bisglycinate: High Absorption, No Gut Pain - choosing an iron you can tolerate once deficiency is confirmed
Restless Legs: A Brain Running Low on Iron - when low iron shows up as legs that will not settle
Making Labs and Imaging Affordable - getting a full iron panel and an electrophoresis without a big bill

Scientific References

Van Vranken M. "Evaluation of microcytosis." American Family Physician. 2010;82(9):1117-1122. PMID 21121557. https://www.aafp.org/pubs/afp/issues/2010/1101/p1117.html
Ko CW, Siddique SM, Patel A, et al. "AGA Clinical Practice Guidelines on the Gastrointestinal Evaluation of Iron Deficiency Anemia." Gastroenterology. 2020;159(3):1085-1094. PMID 32810434. https://pubmed.ncbi.nlm.nih.gov/32810434/
Snook J, Bhala N, Beales ILP, et al. "British Society of Gastroenterology guidelines for the management of iron deficiency anaemia in adults." Gut. 2021;70(11):2030-2051. PMID 34497146. https://pubmed.ncbi.nlm.nih.gov/34497146/
Kattamis A, Kwiatkowski JL, Aydinok Y. "Thalassaemia." Lancet. 2022;399(10343):2310-2324. PMID 35691301. https://pubmed.ncbi.nlm.nih.gov/35691301/
Kattamis A, Forni GL, Aydinok Y, Viprakasit V. "Changing patterns in the epidemiology of β-thalassemia." European Journal of Haematology. 2020;105(6):692-703. https://onlinelibrary.wiley.com/doi/10.1111/ejh.13512
Mentzer WC Jr. "Differentiation of iron deficiency from thalassaemia trait." Lancet. 1973;1(7808):882. PMID 4123424.
Vehapoglu A, Ozgurhan G, Demir AD, et al. "Hematological indices for differential diagnosis of beta thalassemia trait and iron deficiency anemia." Anemia. 2014;2014:576738. PMID 24818016. https://onlinelibrary.wiley.com/doi/10.1155/2014/576738
Flynn MM, Reppun TS, Bhagavan NV. "Limitations of red blood cell distribution width (RDW) in evaluation of microcytosis." American Journal of Clinical Pathology. 1986;85(4):445-449. PMID 3953499. https://academic.oup.com/ajcp/article-abstract/85/4/445/1785832
Origa R. "Beta-Thalassemia." In: GeneReviews. Seattle (WA): University of Washington, Seattle. NCBI Bookshelf NBK1426. https://www.ncbi.nlm.nih.gov/books/NBK1426/
Tamary H, Greenberg-Kushnir N, Dgany O. "Alpha-Thalassemia." In: GeneReviews. Seattle (WA): University of Washington, Seattle. NCBI Bookshelf NBK1435. https://www.ncbi.nlm.nih.gov/books/NBK1435/
Denic S, et al. "Hemoglobin A2 lowered by iron deficiency and α-thalassemia: should screening recommendation for β-thalassemia change?" ISRN Hematology. 2013;2013:858294. PMID 23577263. https://onlinelibrary.wiley.com/doi/10.1155/2013/858294
American College of Obstetricians and Gynecologists. "Hemoglobinopathies in Pregnancy." Practice Advisory, August 2022. https://www.acog.org/clinical/clinical-guidance/practice-advisory/articles/2022/08/hemoglobinopathies-in-pregnancy

Fishtown Medicine | Diagnostics

2418 E York St, Philadelphia, PA 19125(267) 360-7927 hello@fishtownmedicine.comHSA/FSA Eligible

Start your intake

Frequently Asked Questions

Common Questions

No, a low MCV does not always mean iron deficiency. Microcytosis, defined as a mean corpuscular volume below about 80 femtoliters in an adult, has 2 common causes: iron deficiency and thalassemia trait, an inherited condition that produces small red blood cells from birth. Anemia of chronic disease, lead exposure, and sideroblastic anemia account for most of the rest. Ferritin and a full iron panel are the first tests that sort iron deficiency from everything else.

Thalassemia trait is the carrier state for thalassemia, meaning a person carries a genetic change affecting one of the protein chains in hemoglobin but does not have the disease. It causes small red blood cells and sometimes mild anemia for life, and most carriers feel well and need no treatment for it. Its importance is in 2 places: it explains a low MCV that would otherwise be blamed on iron, and it matters for family planning, because 2 carriers can have a child with a serious form of thalassemia.

Yes, if iron deficiency anemia has been confirmed. The American Gastroenterological Association recommends bidirectional endoscopy, an upper endoscopy plus a colonoscopy, for men and postmenopausal women with iron deficiency anemia, because that evaluation finds a lower gastrointestinal cancer in about 8.9% of them and an upper gastrointestinal cancer in about 2.0%. Thalassemia trait explains small red cells, but it does not explain a low ferritin and it does not protect anyone from a bleeding ulcer or a colon cancer. The trait changes the order of testing rather than the need for the evaluation once iron deficiency is confirmed.

Only after iron studies confirm you are iron deficient. In thalassemia trait the red blood cells are small because of how hemoglobin is built, so iron supplements will not raise the MCV and will not correct the anemia. Taking iron without a deficiency also adds iron the body has no use for. Fishtown Medicine checks ferritin, serum iron, total iron-binding capacity, and transferrin saturation before starting iron, then finds the source of any confirmed deficiency rather than refilling the tank indefinitely.

The Mentzer index is the MCV divided by the red blood cell count, described in 1973, and it is used to lean toward one cause of microcytosis over another. A value below 13 suggests thalassemia trait and a value above 13 suggests iron deficiency. Its reported accuracy varies widely across studies, and head-to-head comparisons of 12 similar formulas have not produced one accurate enough to stand alone. The Mentzer index is a pointer that helps decide what to order next, and confirmation still comes from iron studies and a hemoglobin electrophoresis.

No, a hemoglobin electrophoresis cannot rule out all thalassemia. It is the confirmatory test for beta thalassemia trait, which raises hemoglobin A2 above about 3.5%, but alpha thalassemia trait usually produces a normal electrophoresis. Confirming alpha thalassemia trait requires alpha-globin gene testing. A person with lifelong small red cells, normal iron studies, and a normal electrophoresis has a meaningful result rather than an empty one, because that combination points toward alpha thalassemia and genetic testing.

Deep-Dive Questions

Thalassemia trait produces small cells with a nearly normal hemoglobin because the defect limits how much hemoglobin fits inside each red blood cell rather than how many cells the marrow can make. Hemoglobin is built from balanced pairs of alpha and beta protein chains, and a carrier makes a reduced amount of one chain, so each developing cell divides an extra time and finishes smaller and paler than usual. The bone marrow compensates by producing more of these small cells, which is why the red blood cell count in thalassemia trait is often normal or even high while the MCV is strikingly low. Iron deficiency behaves differently, because the raw material for hemoglobin runs out and both the size and the number of cells fall together. That difference is the reason a very low MCV alongside a near-normal hemoglobin and a high red cell count is the pattern that should prompt a hemoglobin electrophoresis.

Iron deficiency can hide beta thalassemia trait on a hemoglobin electrophoresis because it lowers hemoglobin A2, the marker the test relies on. When intracellular iron is short, alpha-chain production falls relative to the non-alpha chains, and the remaining beta chains compete more successfully than delta chains for the available alpha chains, so less hemoglobin A2 is formed. In one large screening population, mean HbA2 was about 0.2% lower in iron-deficient subjects than in iron-replete ones, and alpha thalassemia trait lowered it further, which put a small share of people with borderline values at risk of a false negative result. The effect is modest, and several groups have reported that overt iron deficiency rarely drags a true beta thalassemia carrier below the 3.5% threshold. The practical response is narrow rather than sweeping: interpret HbA2 alongside iron status, and when the value comes back borderline in someone who is iron deficient, correct the iron and repeat the test before calling the question closed.

Alpha thalassemia trait is harder to confirm than beta thalassemia trait because it leaves no fingerprint on a hemoglobin electrophoresis. Beta thalassemia trait raises hemoglobin A2 above about 3.5%, a change the test measures directly. Alpha thalassemia trait, which usually results from losing 2 of the 4 alpha-globin genes, reduces alpha chains across all hemoglobin types at once, so the proportions stay roughly normal and the electrophoresis reads as unremarkable. Confirming it requires alpha-globin gene analysis, which looks for the common deletions and, in a minority of cases, for point variants. For someone with lifelong microcytosis, normal iron studies, and a normal electrophoresis, alpha-globin testing is the logical next step, and the answer often ends years of repeated iron panels.

Someone with both iron deficiency and thalassemia trait is managed by treating each part on its own terms, because the 2 conditions overlap without cancelling each other. The iron deficiency is confirmed with ferritin and iron studies, the cause of the iron loss is found and addressed, and iron is replaced until stores are refilled, which is the part that will improve the anemia. The thalassemia trait is documented so that the residual low MCV afterward is recognized as the baseline rather than treated as ongoing deficiency, which prevents years of unnecessary iron. Guideline-directed evaluation for the source of the iron loss still applies in full, including bidirectional endoscopy in men and postmenopausal women. Once iron is replete, it is also reasonable to repeat the hemoglobin electrophoresis if the earlier HbA2 was borderline, since iron deficiency can suppress that value.

Ready when you are

Start your intake

The chat is our AI assistant, answering from our published guides. To talk it through with Dr. Ash himself, schedule the free 20-minute call.

TL;DR30-second take

TL;DR: MCV is a number on your blood count. It tells you how big your red blood cells are. A low MCV means they are small. Low iron is the most common reason, so the first test is a ferritin level along with the rest of an iron panel. Some people are born with small red cells instead. That is called thalassemia trait, and it is passed down in families. It is more common in people whose families come from the Mediterranean, the Middle East, South Asia, Southeast Asia, and Africa. A blood test called a hemoglobin electrophoresis checks for it. Here is the order that helps: check ferritin and iron first, then check for thalassemia trait if the numbers do not add up. If your iron is low and you are a man, or a woman past menopause, you also need a camera test of your stomach and colon. Low iron in those groups can mean slow bleeding inside, and finding the cause early saves lives. Carrying thalassemia trait does not change that part.

What does a low MCV mean on a complete blood count?

Why is iron deficiency the first thing to check?

When is a low MCV inherited rather than acquired?

How do you tell thalassemia trait from iron deficiency on a CBC?

There are 4 features on the blood count itself that lean one way or the other, and the honest version is that they point rather than prove.

How low the MCV is compared with how low the hemoglobin is. In thalassemia trait the MCV is often strikingly low, into the 60s and low 70s, while the hemoglobin is only slightly below normal or even normal. Iron deficiency tends to be more proportional, so as the MCV falls the hemoglobin falls with it. Very small cells carrying a near-normal hemoglobin is the classic trait pattern.
The red blood cell count. In thalassemia trait the marrow makes plenty of red cells and they are simply small, so the RBC count is often normal or high. In iron deficiency the count usually drops along with everything else.
The RDW. Red cell distribution width measures how much the cells vary in size. Iron deficiency creates a mixed population, older normal-sized cells alongside newer small ones, so the RDW rises. Thalassemia trait creates cells that are uniformly small, so the RDW is often normal.
The Mentzer index. Described in 1973, it is the MCV divided by the red blood cell count. A value below 13 leans toward thalassemia trait and a value above 13 leans toward iron deficiency.⁶

Who carries thalassemia trait, and why does ancestry belong in the picture?

What does a hemoglobin electrophoresis show, and what does it miss?

Two limitations decide how the result should be read.

Does thalassemia trait mean you can skip the colonoscopy?

No. This is where the reasoning has to hold both halves at once, because getting it wrong in either direction causes harm.

Fishtown Medicine

A 90-minute conversation with Dr. Ash. A written plan you can actually follow.

Start your intake

What else causes a low MCV?

Beyond iron deficiency and thalassemia trait, the remaining list of microcytosis causes is short.¹

Why does knowing you carry the trait matter for family planning?

How Fishtown Medicine works up a low MCV in Philadelphia

Some findings should move faster than a routine follow-up. Get medical attention sooner if you notice:

Black, tarry, or bloody stools, or blood when you vomit
A hemoglobin that is falling from one panel to the next
Chest pain, breathlessness at rest, or lightheadedness on standing
Unintended weight loss, a new persistent change in bowel habits, or abdominal pain
Iron pills that have been taken correctly for 3 months with no rise in hemoglobin or ferritin

Guidance from the Clinic

Actionable steps for a low MCV in Philly

What to do, and in what order.

Find your old blood counts. Log into every patient portal you have and write down the MCV from each CBC going back as far as you can. A low MCV that has been stable for years changes the whole conversation.
Ask for the full iron panel by name. Ferritin, serum iron, total iron-binding capacity, and transferrin saturation, and write down the ferritin number itself instead of the word normal. Read it against 45 ng/mL, not 15.
Write down what you know about your family. Where your parents and grandparents came from, and whether any relative has been told they have thalassemia trait, thalassemia minor, or lifelong mild anemia.
Ask about a hemoglobin electrophoresis before an invasive procedure, if your iron studies are normal or your microcytosis looks out of proportion to your ferritin. It is a blood draw, and the answer usually arrives within a few days.
Keep the endoscopy if your iron is truly low. Confirmed iron deficiency in an adult man or a postmenopausal woman needs a look at the stomach and colon, and thalassemia trait does not cancel that.
Do not start iron on your own. Iron given to someone whose small cells come from thalassemia trait will not help, and iron can accumulate where it is not needed. Test first, then treat.

Key Takeaways

A low MCV describes small red blood cells and names no cause. Microcytosis in an adult means an MCV under about 80 femtoliters, and iron deficiency and thalassemia trait account for most of it.
Ferritin and a full iron panel come first, read against the American Gastroenterological Association's 45 ng/mL cutoff rather than the older 15 ng/mL threshold.
A very low MCV with a near-normal hemoglobin, a normal or high red cell count, and a normal RDW points toward thalassemia trait, though RDW and the Mentzer index overlap too much between the 2 conditions to be diagnostic on their own.
A hemoglobin electrophoresis confirms beta thalassemia trait through an HbA2 above about 3.5%, and it does not detect alpha thalassemia trait, which needs alpha-globin gene testing.
Iron deficiency can lower HbA2, so a borderline result in an iron-deficient person should be repeated after iron is replaced.
Confirmed iron deficiency anemia in an adult man or a postmenopausal woman still requires gastrointestinal evaluation, since bidirectional endoscopy finds a lower gastrointestinal cancer in roughly 8.9% and an upper one in roughly 2.0% of these patients.
Knowing you carry thalassemia trait matters beyond your own blood count, because 2 carriers face a 25% chance per pregnancy of a child with a transfusion-dependent form.

High Ferritin: What It Means, from Iron Overload to Inflammation - the other end of the iron spectrum, and why ferritin can mislead
Iron, Heavy Periods, and Hair Loss - the most common source of iron loss in premenopausal women
Colorectal Cancer Screening: Which Test, and When? - what the evaluation involves once iron deficiency is confirmed
Colon Cancer in Young Adults: The Rise and the Red Flags - why age alone no longer rules out a GI source
What Your Blood Count Reveals - the other signals hiding in a routine CBC
Iron Bisglycinate: High Absorption, No Gut Pain - choosing an iron you can tolerate once deficiency is confirmed
Restless Legs: A Brain Running Low on Iron - when low iron shows up as legs that will not settle
Making Labs and Imaging Affordable - getting a full iron panel and an electrophoresis without a big bill