Microcytosis is a reduction in the mean volume of red blood cells, i.e. an MCV below normal (usually < 80 fL in adults). It is not a diagnosis but a sign that a doctor sees in a complete blood count.
Small red blood cells most often accompany iron-deficiency anemia, but they also occur in other conditions, so it is important to work out the cause.
What microcytosis means
The size of a red blood cell depends on how much hemoglobin it contains. When there is not enough 'building material' (iron above all), the cells come out small and often pale (hypochromic). That is why microcytosis usually goes hand in hand with a reduced MCH.
Main causes
The most common cause is iron deficiency (blood loss, poor absorption, low dietary intake). Less often, microcytosis is caused by thalassemias and other inherited hemoglobin disorders, anemia of chronic disease (via hepcidin) and lead poisoning. Ferritin helps tell iron deficiency and thalassemia apart: in thalassemia, iron stores are normal or elevated.
What to do
Microcytosis on a test is a reason to check iron metabolism (ferritin, transferrin saturation). If iron is normal but microcytosis persists, a hemoglobin study (electrophoresis) is discussed to rule out thalassemia. Treatment targets not microcytosis itself but its cause.
Reference ranges
Reference points for mean red blood cell volume (MCV) in adults:
| Normal MCV | 80–100 fL |
| Microcytosis | < 80 fL |
| Hypochromia (MCH) | < 27 pg |
Calculate using this marker
Calculators where Microcytosis is used directly:
Frequently asked questions
Does microcytosis always mean a lack of iron?
No. Most often yes, but microcytosis also occurs in thalassemia, anemia of chronic disease and lead poisoning. Ferritin and other iron-metabolism tests help tell them apart.
Can microcytosis occur without anemia?
Yes. Small red blood cells can appear with normal hemoglobin — for example, at an early stage of iron deficiency or in a minor form of thalassemia (carrier state).