- 01Iron deficiency vs thalassemia trait
Both make small red cells, for opposite reasons. In iron deficiency the bone marrow runs short of iron, so ferritin is low, the red cell count drifts down and the cells vary in size (high RDW). In thalassemia trait an inherited gene makes less globin, the protein part of hemoglobin, so ferritin is normal, the red cell count is high and the cells are evenly small. Ferritin plus the red cell count usually tells you which one you are looking at 1, and only iron deficiency is helped by iron 2.
Your report has flagged MCV as low, and most likely MCH and MCHC beside it. Together they describe red cells that are small and pale. The two common causes are iron deficiency and an inherited thalassemia trait, and they need opposite handling. Iron fixes the first and does nothing for the second unless iron deficiency is also present, so a ferritin test comes before any supplement.
The three numbers that move together
Where an example MCV of 76 fL sitsfL
Low MCV, low MCH and low MCHC: why they usually come together
MCV is the average size of a red cell in femtolitres (fL), MCH the average hemoglobin per cell in picograms, and MCHC how densely that hemoglobin is packed in g/dL. Typical adult ranges are MCV 80 to 100 fL, MCH about 27 to 33 pg and MCHC about 32 to 36 g/dL 1. The analyser works out MCH and MCHC from measured numbers: MCH is hemoglobin divided by the red cell count, and MCHC is hemoglobin divided by hematocrit. A hemoglobin of 11 g/dL with a hematocrit of 36% gives an MCHC of about 30.6 g/dL. Full definitions live on the MCV, MCH and MCHC pages.
The three fall together because a red cell is built around hemoglobin. When the bone marrow (where red cells are made) cannot make enough hemoglobin, whether from too little iron or too little globin protein, each new cell gets less of it and comes out smaller. Smaller cells with less hemoglobin show up as a low MCV and a low MCH, and once the shortage is marked, the hemoglobin inside each cell is also thinner, so MCHC drops. Reports call this microcytic hypochromic, meaning small and pale.
| What is low on your report | What it usually points to |
|---|---|
| MCV, MCH and MCHC all low | Iron deficiency that has gone on for months, or thalassemia trait |
| MCV and MCH low, MCHC normal | The common early picture, or thalassemia trait (MCHC often stays normal in a trait) |
| MCH low, MCV normal | Early iron deficiency, or iron shortage mixed with low B12 or folate that pulls the average size back up |
| MCHC low alone | Can be early iron deficiency or a temporary rise in young red cells; worth a repeat and a ferritin rather than ignoring it |
| MCV low, RBC count high | Strong hint of thalassemia trait; see the comparison below |
How low is low? Reading your number
The size of the drop carries information. A borderline MCV is common in early iron deficiency and easily missed, while a very low MCV with a near-normal hemoglobin is the classic thalassemia trait picture 1. Read the bands below alongside ferritin and the red cell count; on their own they decide nothing.
| Marker | Result | What it tends to mean |
|---|---|---|
| MCV | 78 to 80 fL | Borderline small. Often early iron deficiency or a mild trait; worth a ferritin and a repeat. See MCV 78, 79 and 80. |
| MCV | 70 to 77 fL | Clearly microcytic. Iron deficiency or thalassemia trait are both likely. See MCV 76. |
| MCV | Below 70 fL | Marked microcytosis: longstanding iron deficiency or thalassemia trait, and worth prompt follow-up |
| MCH | 25 to 27 pg (for example 26 or 26.8) | Mildly low, often the first index to slip |
| MCH | Below 25 pg | Clearly low; read with ferritin and RBC count |
| MCHC | Above 30 up to 32 g/dL (for example 31 or 31.9) | Mildly low; common in iron deficiency that has been present for a while |
| MCHC | 30 g/dL or below | Marked hypochromia, usually significant iron deficiency |
Children run smaller. A rough rule for the lower limit of MCV in a child is 70 fL plus the child's age in years, up to about age 10 1, so an MCV of 74 is normal at age 4 and low at age 12. Pregnancy increases iron needs sharply, so a falling MCV in pregnancy deserves a ferritin check rather than waiting.
Is it iron deficiency or thalassemia trait?
Iron deficiency is the most common cause of a low MCV, and thalassemia trait is the next most common 1. Thalassemia trait (also called thalassemia minor) is an inherited change in one of the globin genes that build hemoglobin. It is common in people with Mediterranean, Middle Eastern, South Asian, Southeast Asian and African ancestry, it is lifelong, and it usually causes no illness. The same report can often separate the two before any extra test.
| Clue | Points to iron deficiency | Points to thalassemia trait |
|---|---|---|
| Ferritin | Low | Normal or high |
| Red cell count (RBC) | Low or normal | Normal or high |
| RDW (variation in cell size) | Often high | Often normal |
| Hemoglobin vs MCV | Hemoglobin falls about as much as MCV | MCV very low for a near-normal hemoglobin |
| Past results | MCV used to be normal | MCV has always been low |
| Family | No pattern | Relatives with a low MCV or known trait |
A quick screening clue is the Mentzer index: MCV divided by the RBC count in millions per microlitre. Below 13 favours thalassemia trait and above 13 favours iron deficiency 1. An MCV of 68 with an RBC count of 5.8 gives 68 ÷ 5.8 = 11.7, which leans toward a trait. An MCV of 72 with an RBC count of 4.1 gives 72 ÷ 4.1 = 17.6, which leans toward iron deficiency. The index is not reliable enough to diagnose on its own, so it guides the next test and settles nothing alone.
Beta thalassemia trait is confirmed by a hemoglobin analysis blood test (electrophoresis or HPLC, which sort your hemoglobin into its types), where HbA2, a minor type of hemoglobin, above about 3.5% is the usual marker 1. Alpha thalassemia trait often shows a normal HbA2 and is confirmed by DNA testing or diagnosed once iron deficiency and beta trait are excluded. Labs prefer to check iron status first, because iron deficiency can lower HbA2 and hide a beta trait 1. The two can also coexist in the same person, and then iron is still the right treatment for the iron deficiency. If your RBC count is the high number on your report, the high RBC page covers that side.
Why MCV is a late sign of iron deficiency
How iron runs out before the cells shrink
A normal MCV with a low ferritin is still iron deficiency, caught early 2, and a low MCV means the shortage has usually been going on for months.
Other causes of small red cells
Anemia of inflammation, sometimes called anemia of chronic disease, is the third common cause. Long-running inflammation from chronic kidney disease, autoimmune disease, infection or cancer raises a hormone called hepcidin, which blocks iron absorption from the gut and locks iron inside storage cells 6. The cells go short even though stores are full, so ferritin is normal or high while transferrin saturation is low. This anemia is usually normocytic (normal-sized cells) and only mildly microcytic in some people 6. A raised CRP alongside it is the tell; the high CRP page covers that marker.
Rare causes include lead exposure, sideroblastic anemia (an inherited or acquired fault in heme production), copper deficiency and some medicines 1. Mixed deficiencies can also hide a low MCV: iron shortage makes cells smaller while B12 or folate shortage makes them larger, and the average can land in the normal range with a high RDW as the only clue. The high RDW page explains that pattern.
The follow-up tests usually ordered
Ferritin comes first because it is the most specific single test for iron stores. The cutoff a clinician uses depends on the setting, which is why two labs can read the same ferritin differently.
In apparently healthy individuals, ferritin concentrations below 12 μg/L in children under 5 years of age, and below 15 μg/L for individuals 5 years of age and older, define iron deficiency.World Health Organization, guideline on use of ferritin concentrations to assess iron status, 2020 3
| Ferritin cutoff (ng/mL, the same number as µg/L) | Who uses it | What it means |
|---|---|---|
| Below 15 | WHO, healthy people aged 5 and over | Iron stores are depleted 3 |
| Below 30 | Many labs and clinicians | Iron deficiency likely, when there is no inflammation 2 |
| Below 45 | British Society of Gastroenterology | Cutoff used when assessing iron deficiency anemia 4 |
| Below 70 | WHO, people aged 5 and over with infection or inflammation (below 30 under age 5) | Can still mean iron deficiency, because inflammation pushes ferritin up 3 |
Where do your iron stores sit?
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Alongside ferritin, a clinician will usually add transferrin saturation (below about 16% suggests too little iron reaching the marrow 2), CRP to judge whether ferritin is inflated, and sometimes reticulocyte hemoglobin. If iron studies are normal and the MCV is still low, hemoglobin analysis for thalassemia trait is the usual next step. The iron studies guide walks through the full panel, and ferritin vs iron explains why serum iron alone misleads.
What is usually checked next
If it is iron: finding where it went
Iron deficiency always has a cause, and finding it matters as much as replacing the iron. The common ones are heavy periods, pregnancy, frequent blood donation, a diet low in iron (more common on vegetarian and vegan diets), endurance sport, and poor absorption from celiac disease, stomach surgery, long-term acid-suppressing medicines or H. pylori infection. The iron deficiency page covers each in detail.
The British Society of Gastroenterology recommends celiac blood testing for every adult with iron deficiency anemia, and investigation of the stomach and bowel (endoscopy and colonoscopy) for men and postmenopausal women with confirmed iron deficiency anemia, because slow blood loss from the gut, including from bowel cancer, is a recognised cause 4. See celiac disease for that test.
Treating hypochromic anemia from low iron, and how long MCV takes to recover
When iron deficiency is confirmed, oral iron is the usual first treatment, with the form and schedule set by a clinician. Timing matters for absorption. In iron-deficient women, iron given on alternate days was absorbed better in total than the same doses on consecutive days, because each dose raises hepcidin for about 24 hours and blocks the next one 5. Tea, coffee and calcium reduce absorption, and vitamin C taken with iron may help. Iron does nothing for thalassemia trait with a normal ferritin, and unnecessary iron can add to body iron over time.
Supplements studied in confirmed iron deficiency
Recovery is slow because red cells live about 120 days, and MCV only rises as new, normal-sized cells replace the small ones 2. Reticulocytes, the newest red cells, rise within about 1 to 2 weeks 2. Hemoglobin should climb by about 1 to 2 g/dL over 2 to 4 weeks, and treatment usually continues for about 3 months after hemoglobin normalises to refill stores 4. A repeat CBC and ferritin at 8 to 12 weeks is the usual check.
Symptoms and when it is urgent
Microcytic hypochromic anemia symptoms come from the anemia or from iron shortage in the tissues, since a low MCV causes none by itself: tiredness, breathlessness on stairs, headaches, hair shedding, restless legs and pica, a craving to chew ice or non-food items 2. Thalassemia trait usually causes none of these. If your hemoglobin is also low, the anemia page covers the wider picture.
If you have thalassemia trait and are planning a pregnancy, ask about testing your partner. Two carriers can have a child with a serious form of thalassemia, and the check is a simple blood test.
To see how a low MCV fits the rest of a blood count, read CBC explained, the blood count topic or the iron topic. If your MCV is high instead, see high MCV.
Free, no accountRead your blood testRead MCV, MCH, MCHC, RDW and ferritin from your own report together.Read my results
Citations
- Van Vranken M. Evaluation of microcytosis. Am Fam Physician 2010;82(9):1117-22
- Camaschella C. Iron-deficiency anemia. N Engl J Med 2015;372:1832-43
- World Health Organization. WHO guideline on use of ferritin concentrations to assess iron status in individuals and populations, 2020
- Snook J et al. British Society of Gastroenterology guidelines for the management of iron deficiency anaemia in adults. Gut 2021;70:2030-51
- Stoffel NU et al. Iron absorption from oral iron supplements given on consecutive versus alternate days. Lancet Haematol 2017;4(11):e524-33
- Weiss G, Ganz T, Goodnough LT. Anemia of inflammation. Blood 2019;133(1):40-50
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.
