Is an MCV of 100 consistent with microcytic anemia?

No, an MCV of 100 is NOT microcytic—it is macrocytic

An MCV of 100 fL falls well above the normal range and indicates macrocytosis, not microcytosis. 1, 2

MCV Classification

Microcytic anemia is defined by:

• MCV <80 fL in adults 2
• MCV <83 fL by some definitions 3

Macrocytic anemia is defined by:

• MCV >100 fL 4

Your MCV of 100 fL places you at the threshold of macrocytosis, indicating larger-than-normal red blood cells. 4

Clinical Significance of MCV 100

With an MCV of 100 fL, you should be evaluated for causes of macrocytosis, not microcytosis:

Common causes to investigate include:

• Vitamin B12 or folate deficiency (check serum B12, folate, methylmalonic acid, and homocysteine) 4
• Medication effects, particularly thiopurines (azathioprine, 6-mercaptopurine), methotrexate, or other myelosuppressive agents 4
• Reticulocytosis from hemolysis or recent hemorrhage (check reticulocyte count) 4
• Alcohol use 4
• Liver disease 4
• Hypothyroidism 4

Critical Diagnostic Pitfall

Do not confuse MCV values with their opposites:

• Microcytic = small red cells (MCV <80 fL) 1, 2
• Macrocytic = large red cells (MCV >100 fL) 4

Mixed Picture Consideration

One important caveat: If you have both iron deficiency (which causes microcytosis) and B12/folate deficiency (which causes macrocytosis) simultaneously, these can cancel each other out, resulting in a falsely normal or borderline elevated MCV. 4 In this scenario, an elevated RDW (>14.0%) would be the key clue that mixed deficiencies are present. 4, 5

Check your RDW: If your RDW is elevated with MCV of 100, consider checking both iron studies (ferritin, transferrin saturation) and vitamin levels (B12, folate) to rule out combined deficiencies. 4, 5