MCV
Mean corpuscular volume (MCV) is the average size of a person's red blood cells, expressed in femtolitres (fL, 10⁻¹⁵ L). It is the most useful red cell index for classifying anaemia. A low MCV (microcytosis) usually points to a problem with haemoglobin synthesis, most often iron deficiency or thalassaemia. A high MCV (macrocytosis) points to vitamin B12 or folate deficiency, alcohol, liver disease, medicines or bone-marrow disorders.
How MCV is measured
Automated analysers measure the volume of each red cell directly as it passes through an aperture (impedance) or a laser beam, and report the average. MCV can also be calculated: MCV (fL) = haematocrit (%) × 10 / RBC (×10¹²/L). Because it is an average, a mixture of small and large cells, for example combined iron and B12 deficiency, can give a "normal" MCV. The red cell distribution width (RDW), which describes the spread of cell sizes, then helps reveal the mixture.
Reference values
Most adult laboratory ranges are about 80–100 fL; some laboratories use 80–96 fL. The Clinical Methods reference gives 87 ± 7 fL. MCV is higher in newborns and falls during the first months of life. Always compare your result with the range printed by your own laboratory.
Low MCV — microcytosis
- Iron deficiency — the most common cause worldwide; ferritin is low, RDW is often high
- Thalassaemia trait — very low MCV with a near-normal or high red cell count; the Mentzer index (MCV/RBC) below 13 suggests thalassaemia
- Anaemia of chronic inflammation — usually mildly low or normal MCV, normal or high ferritin
- Sideroblastic anaemia, lead poisoning (rare)
In adults, unexplained iron-deficiency microcytosis can be the first sign of chronic blood loss from the gut, including colorectal cancer. It requires investigation, not only iron tablets.
High MCV — macrocytosis
| Group | Causes |
|---|---|
| Megaloblastic | Vitamin B12 deficiency (pernicious anaemia, vegan diet, metformin, after gastric surgery), folate deficiency |
| Non-megaloblastic | Alcohol use, liver disease, hypothyroidism, reticulocytosis after bleeding or haemolysis |
| Medicines | Hydroxyurea, methotrexate, azathioprine, zidovudine and other antivirals, some anticonvulsants |
| Bone-marrow disorders | Myelodysplastic syndromes, aplastic anaemia — especially in older people with other cytopenias |
Macrocytosis can precede anaemia by months. Vitamin B12 deficiency can cause nerve damage, such as numbness, tingling, unsteady gait and memory problems, even before the blood count changes markedly, so it should be found and treated early.
Anaemia with normal MCV
Normocytic anaemia is typical of acute blood loss, chronic kidney disease, chronic inflammation, haemolysis and bone-marrow failure. The next step is the reticulocyte count: high values suggest blood loss or haemolysis, while low values suggest reduced production.
When to see a doctor
Tiredness, pale skin, heavy periods, black or bloody stools, weight loss, family history of thalassaemia.
Numbness or tingling, balance problems, sore tongue, memory changes, heavy alcohol use, other abnormal blood counts.
References
- Sarma PR. Red cell indices. In: Walker HK, Hall WD, Hurst JW, eds. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd ed. Boston: Butterworths; 1990. Chapter 152. ncbi.nlm.nih.gov/books/NBK260
- MedlinePlus. RBC indices. U.S. National Library of Medicine. medlineplus.gov/ency/article/003648.htm
- Kaferle J, Strzoda CE. Evaluation of macrocytosis. Am Fam Physician. 2009;79(3):203–208. PMID: 19202968
- Camaschella C. Iron-deficiency anemia. N Engl J Med. 2015;372(19):1832–1843. doi:10.1056/NEJMra1401038
- Mentzer WC Jr. Differentiation of iron deficiency from thalassaemia trait. Lancet. 1973;1(7808):882. doi:10.1016/S0140-6736(73)91446-3
This article is for general information and does not replace a consultation with a doctor. Laboratory results must be interpreted by a physician together with symptoms and other tests.
Published on 11 February 2020