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Macrocytosis

There have been relatively few studies of the causes of macrocytosis in outpatients. In one study (Lab. Hematology, 3:104-109, 1997) the incidence and etiology of macrocytosis (MCV>100fL) was evaluated in over 39,000 consecutive outpatient hematology profiles submitted to an outpatient laboratory.

The observed incidence of macrocytosis in this group was 0.7%. Chronic alcohol abuse was the commonest cause, accounting for 27% of cases (see Table). Chemotherapy or other drugs known to produce macrocytosis was the second commonest cause, accounting for 23% of cases. This was followed by myelodysplasia or other primary bone marrow disorders (18%), liver disease (11%, of whom more than half also abused alcohol), and vitamin B12 or folate deficiency, which accounted for only 11% of cases. Other less common causes of macrocytosis included reticulocytosis, HIV infection (most of whom were on zidovudine), and hypothyroidism.

Causes of Macrocytosis in Two Studies Fifteen Years Apart

Disorder

1981

1995

Alcohol

0%

27%

Chemotherapy

1%

23%

Bone marrow disorders

13%

18%

Liver disease

3%

11%

B12 or folate deficiency

39%

11%

Unexplained

8%

7%

Reticulocytosis/rouleaux

6%

5%

HIV infection

0%

4%

Hypothyroidism

3%

3%

These findings constitute a significant change from earlier investigations, such as a study in 1981, which found that vitamin B12 or folate deficiency was the commonest cause of macrocytosis. A more recent study of inpatients in a large metropolitan urban hospital showed the commonest condition associated with macrocytosis to be zidovudine therapy in AIDS patients. Clearly the commonest causes of macrocytosis have changed from what has been reported in textbooks or older literature.

The authors also make the point that 49% of the patients with macrocytosis in this study were not anemic, and that an underlying reversible condition was present in 42%. This supports the recommendation that patients with unsuspected macrocytosis should be followed up, even if anemia is not present.

Also see folate and Vitamin B12.

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