Microcytic Anemia: Causes, Key Blood Tests and Treatment Explained

Key Takeaways

• Microcytic anemia means anemia with smaller-than-normal red blood cells; a low MCV is a finding that needs an explanation, not a diagnosis.
• Iron deficiency is a well-recognised cause, but thalassemia trait and anemia of inflammation are important alternatives that call for a different work-up.
• Serum ferritin is the single most useful marker of iron deficiency anemia, but inflammation can make ferritin look falsely normal.
• In men and postmenopausal women with confirmed iron deficiency anemia, the source should be looked for, including the GI tract.
• Microcytosis can signal an underlying cancer even when hemoglobin is normal, although a benign explanation is more likely.
• In my clinical experience, microcytic anemia is the most common anemia I see, and iron deficiency usually explains it.
• In my experience, ferritin often falls before hemoglobin or MCV change, and dark stools can be the clue to bleeding from the gut.
• In my practice, many patients tolerate iron tablets poorly, and alternate-day or lower-dose iron helps some of them.
• An iron infusion is not a trivial procedure; in my practice it usually requires documented low iron together with intolerance to oral iron or a problem absorbing it.

Introduction: Microcytic Anemia

Small red blood cells are a common finding on a routine blood count and can easily be overlooked. In 2021, 24.3% of the global population across all ages had anemia, equal to 1.92 billion cases, and dietary iron deficiency was the leading cause of years lived with disability due to anemia.[1] Microcytic anemia means anemia in which the red blood cells are smaller than normal.

From a sports medicine perspective, the problem tends to appear in two forms: the patient whose tiredness has been put down to a busy life, and the athlete whose training has quietly stopped producing results. In both, the size of the red cells on the blood count can be the first objective clue.

Microcytic anemia is probably the most common type of anemia I see in my practice, and in most cases it is caused by iron deficiency in one way or another. In my clinical experience, iron deficiency anemia is the most common anemia among my patients. The global figures above set the scene, but at the level of the individual patient the more useful question is why the iron is missing.

This article covers what microcytic anemia means, its main causes, how iron deficiency is separated from thalassemia trait and anemia of inflammation, why an unexplained low MCV deserves an explanation, and what the evidence says about treatment.

What Is Microcytic Anemia?

The key number is the mean cell volume (MCV), the average size of the red blood cells. The threshold differs between countries. A large UK primary care study defined microcytosis as an MCV below 85 femtolitres (fL), the cut-off commonly used in UK practice, and noted that 80 fL is the threshold commonly used in North America.[2]

Modern blood count analysers report two red cell changes typical of iron deficiency: a low mean cell hemoglobin (MCH), called hypochromia, and a low MCV, called microcytosis. The British Society of Gastroenterology (BSG) guideline notes that MCH may be more sensitive for iron deficiency than MCV.[3]

Microcytic anemia is therefore a description, not a diagnosis. The blood count shows that the red cells are small; it does not show why.

The Main Causes of Microcytic Anemia

Iron deficiency

Iron deficiency is a well-recognised cause of microcytic anemia. According to the BSG guideline, it can arise from poor dietary intake, malabsorption of dietary iron and several significant gastrointestinal (GI) conditions. Because blood is rich in iron, chronic blood loss, for example from menstruation or the GI tract, is a common route to iron deficiency anemia (IDA).[3]

The guideline also lists blood donation, non-steroidal anti-inflammatory drugs (NSAIDs) and long-term proton pump inhibitor (PPI) use among the contributing factors, and coeliac disease is found in 3%–5% of cases of IDA.[3]

In practice, the reasons behind iron deficiency in my patients are fairly predictable. Most often iron is poorly absorbed from the diet, or the patient is vegetarian. Another large group is menstruating women, particularly those with heavy periods, which in my experience is a very common cause of iron deficiency. Bleeding from the gut is less common, but it can be very serious. A patient may become anemic quite quickly without noticing it before symptoms develop, and I often find that dark stools are the clue in these patients.

Thalassemia trait and other hemoglobinopathies

Small, pale red cells are not specific to iron deficiency. They also occur in many hemoglobinopathies, such as thalassemia, in which the MCV is typically reduced out of proportion to the degree of anemia.[3] Globally, hemoglobinopathies and hemolytic anemias were the second most common cause of years lived with disability due to anemia in 2021, after dietary iron deficiency.[1]

It is also worth keeping inherited hemoglobin disorders in mind, especially in patients whose families come from regions where they are common. In my experience, patients with sickle cell disease usually know about their condition, but occasionally it can come as a surprise when microcytic anemia is investigated.

Anemia of inflammation

Anemia of inflammation (AI), also called anemia of chronic disease, is considered the second most prevalent anemia worldwide after iron deficiency anemia.[4] Inflammatory cytokines and hepcidin, the master regulator of iron balance, block iron absorption from the gut and trap iron in reticuloendothelial cells, which restricts the iron available for red cell production.[4]

Typically, AI is a mild to moderate anemia with normal-sized, normally colored red cells, which distinguishes it from the small, pale cells of IDA.[4] Small, pale cells can nevertheless occur in some cases of anemia of chronic disease and in sideroblastic anemia.[3] Between 20% and 85% of patients with AI also have true iron deficiency, so the two conditions can overlap.[4]

Why a Low MCV Needs an Explanation

On examination, a person with IDA may show general signs of anemia, such as breathlessness, fatigue or heart failure, and occasionally features more specific to iron deficiency, including angular stomatitis, glossitis, koilonychia (spoon-shaped nails), restless legs syndrome, pagophagia (a craving for ice) and blue sclerae.[3]

The more important question is what lies behind microcytic anemia. About a third of men and postmenopausal women presenting with IDA have an underlying pathological abnormality, most often in the GI tract.[3] Age, sex, hemoglobin concentration and MCV each independently predict the risk of GI cancer in IDA.[3]

Microcytosis carries a signal even when hemoglobin is normal. In a UK primary care cohort of patients aged 40 years or older, 497 of 12 289 patients with microcytosis (4.0%) received a new cancer diagnosis within 1 year, compared with 1465 of 73 150 patients without microcytosis (2.0%). Among males with microcytosis, 298 of 4800 (6.2%) were diagnosed with cancer. Among patients with microcytosis but normal hemoglobin, cancer was diagnosed in 86 of 2637 males (3.3%) and 101 of 5055 females (2.0%).[2] The authors concluded that microcytosis predicts underlying cancer even when hemoglobin is normal, although a benign explanation is more likely.[2]

Microcytic Anemia in Athletes

Iron deficiency anemia is common in endurance athletes; the BSG notes that the mechanism is uncertain, but that high hepcidin levels may contribute.[3] A review on hepcidin and sports anemia describes a high rate of iron deficiency in athletes, particularly those with heavy training loads, resulting from iron losses through hemolysis, hematuria, sweating and gastrointestinal bleeding during training.[5] In the authors’ interpretation, exercise-induced up-regulation of hepcidin might be the main reason for iron deficiency in athletes.[5]

From a sports medicine perspective, this creates a trap: it is tempting to assume that microcytic anemia or a low MCV in an athlete is simply “training-related”. In my view, the same diagnostic logic applies to athletes as to anyone else, and the BSG recommends that confirmed IDA in young men warrants the same investigational approach as in older people.[3]

Diagnosing Microcytic Anemia: Which Tests Separate the Causes?

Ferritin first. Serum ferritin is the single most useful marker of IDA, although other tests, such as transferrin saturation, can help when a falsely normal ferritin is suspected.[3] A ferritin below 15 µg/L indicates absent iron stores, and a value below 30 µg/L generally indicates low body iron stores.[3]

Account for inflammation. Ferritin is an acute-phase protein, so it can look normal despite iron deficiency when an inflammatory disease process is present. A cut-off of 45 µg/L has been suggested as the best practical trade-off between sensitivity and specificity, and a ferritin above 150 µg/L is unlikely in absolute iron deficiency, even with inflammation.[3] In inflammatory diseases, ferritin largely loses its value as a marker of body iron stores.[4]

In my clinical practice, when I find iron deficiency anemia, I usually order a broader set of tests. These typically include ferritin, serum iron and often transferrin saturation. Vitamin B12 and folate also belong to a basic anemia work-up, but in my experience their deficiency usually causes macrocytic or megaloblastic anemia, the opposite of microcytosis.

Think of thalassemia when iron studies are normal. To avoid unnecessary GI investigations, the BSG recommends hemoglobin electrophoresis for people with microcytosis and normal iron studies, particularly if there is an appropriate ethnic background.[3]

Use the Mentzer index with caution. The Mentzer index divides the MCV by the red blood cell count; a value above 13 suggests IDA and a value below 13 suggests thalassemia trait.[6] In a study of 434 children, the index had 74% sensitivity and 63% specificity for beta-thalassemia trait, and the authors judged it not highly reliable for distinguishing IDA from thalassemia trait in that population.[6] In my view, it is a screening clue in microcytic anemia, not a replacement for iron studies and hemoglobin analysis.

Look for the source. The BSG recommends that the initial investigation of confirmed IDA include urine testing, screening for coeliac disease and, in appropriate cases, endoscopic examination of the upper and lower GI tract. In men and postmenopausal women with newly diagnosed IDA, gastroscopy and colonoscopy are generally the first-line GI investigations.[3]

Treating the Cause of Microcytic Anemia

Treatment follows the cause. For confirmed IDA, the BSG recommends starting with one tablet a day of ferrous sulphate, fumarate or gluconate, and considering one tablet every other day, another oral preparation or parenteral iron if this is not tolerated.[3]

Why less frequent dosing can work better. In a randomized study of iron-depleted women (serum ferritin ≤25 µg/L) taking 60 mg of iron, cumulative fractional iron absorption was 16.3% with consecutive-day dosing and 21.8% with alternate-day dosing, and cumulative total iron absorption was 131.0 mg versus 175.3 mg. Splitting the dose into two daily doses led to higher serum hepcidin than a single daily dose.[7] In women with IDA taking 100 or 200 mg of iron, fractional absorption from the alternate-day dose was 40–50% higher than from the consecutive-day dose, and each oral dose raised hepcidin acutely for 24 hours.[8]

How to take it. Taking iron with meals can reduce its bioavailability by up to 75%, and the BSG does not recommend adding vitamin C to oral iron.[3]

How to know it is working. In an anemic patient, a hemoglobin rise of at least 10 g/L within 2 weeks of starting iron is highly suggestive of absolute iron deficiency. The BSG recommends continuing treatment for around 3 months after hemoglobin normalizes, to replenish the marrow iron stores.[3]

When oral iron is not enough. The BSG advises considering parenteral iron when oral iron is contraindicated, ineffective or not tolerated.[3]

If a patient continues to have microcytic anemia despite oral iron, the indications may at some point favour an iron infusion. In my practice, an infusion usually requires documented low iron levels together with intolerance to oral iron or a problem absorbing it. Iron infusions are also seldom given in the Finnish public sector, in my experience, when there is no anemia or microcytosis. Restless legs are a different matter: there, I consider correcting iron deficiency a recognised treatment option. An iron infusion is not a trivial procedure either: hypersensitivity-type and infusion reactions occur in roughly 0.5% of cases and are more common than with oral iron or placebo, although rates of serious adverse reactions are low.[3]

Athletes. A meta-analysis of 13 studies (449 participants) found that oral iron supplementation increased serum ferritin in athletes, whereas blood hemoglobin did not change significantly. The increase in ferritin was evident in those starting with a serum ferritin ≤12 µg/L, effective doses ranged from 16 to 100 mg of elemental iron daily over 6 to 8 weeks, and shorter protocols were ineffective.[9]

In my experience, ferritin often falls before any microcytosis, or any anemia at all, develops. The BSG describes this early phase, in which body iron stores are depleted while hemoglobin is still within the normal range, as non-anaemic iron deficiency.[3] In practice, a patient can therefore have a completely normal hemoglobin and MCV but a low ferritin. Whether this stage needs treatment is a recurring debate. During my medical studies at the University of Helsinki, the hematologists were firmly of the view that it does not need treatment. In athletes, however, I tend to recommend treating a low ferritin. And if a patient is exhausted and has low iron values without anemia or microcytosis, they are free to take an iron supplement, even if I would not necessarily recommend it in every case.

Restless legs syndrome and periodic limb movement disorder are the exception where I aim for a clearly higher ferritin. For restless legs syndrome, a task force of the International Restless Legs Syndrome Study Group concluded that oral iron (65 mg of elemental iron) is possibly effective when serum ferritin is ≤75 µg/L.[10]

In my clinical experience, a surprisingly large number of patients do not tolerate iron supplements well, and their stomach gets upset. Diarrhea and bloating can occur, and stools may turn dark. In these situations, the BSG suggests one tablet every other day when daily iron is not tolerated,[3] and this helps some of my patients tolerate the tablets. Some of my patients instead use sucrosomial iron, which many find easier on the stomach. I do not know whether this is due to its structure or simply the lower dose: the products my patients use typically contain considerably less iron per tablet than prescription iron, and that difference may matter. Either way, many of my patients seem to tolerate it better.

Practical Recommendations

• Treat a low MCV as a finding that needs an explanation, not as a diagnosis.
• Interpret ferritin alongside inflammatory markers such as CRP, and treat a “normal” ferritin with caution when inflammation is present.
• If iron studies are normal, consider thalassemia trait and hemoglobin analysis before further investigations.
• In men and postmenopausal women with confirmed iron deficiency anemia, look for the source, including the GI tract.
• If oral iron is used, taking it away from meals may improve absorption, and alternate-day single doses may also help.
• Recheck hemoglobin early to confirm response, and continue treatment after normalization to rebuild iron stores.
• Athletes are not an exception: confirm the cause before attributing a low MCV to training.
• In my clinical experience, poor absorption of dietary iron, a vegetarian diet and heavy periods are the most common reasons behind iron deficiency, and many patients tolerate alternate-day or lower-dose iron better.

Conclusion

Anemia affected 1.92 billion people worldwide in 2021, and dietary iron deficiency was its leading cause of years lived with disability.[1] Microcytic anemia is commonly explained by iron deficiency, but thalassemia trait and anemia of inflammation are important alternatives that call for a different work-up. In people aged 40 or older, microcytosis was associated with a 1-year cancer incidence of 4.0%, compared with 2.0% with a normal MCV.[2]

This matches what I see in practice: microcytic anemia is probably the most common anemia in my clinical work, and iron deficiency usually explains it. Ferritin often falls before hemoglobin or MCV change, and in my experience tolerability is a common practical challenge once iron treatment starts.

The practical message is simple: measure ferritin, interpret it in the light of inflammation, identify the cause, and treat iron deficiency with a regimen the patient can actually follow. Small red cells are a clue. In my view, following that clue properly is what protects both health and performance.

References

1. https://doi.org/10.1016/S2352-3026(23)00160-6
2. https://doi.org/10.3399/bjgp20X709577
3. https://doi.org/10.1136/gutjnl-2021-325210
4. https://doi.org/10.1182/blood-2018-06-856500
5. https://doi.org/10.1186/2045-3701-4-19
6. https://doi.org/10.3389/fmed.2024.1361805
7. https://doi.org/10.1016/S2352-3026(17)30182-5
8. https://doi.org/10.3324/haematol.2019.220830
9. https://doi.org/10.1007/s40279-024-01992-8
10. https://doi.org/10.1016/j.sleep.2017.11.1126