The “expected” values were derived using the formulas from Pearson et al.139 These 4 examples illustrate that it is not possible to predict the red cell mass from the hematocrit alone and that in polycythemia vera a normal hematocrit value does not ensure that the red cell mass is normal. In hypoxic erythrocytosis, the red cell mass expands at the expense of the plasma volume; with androgen therapy, there is often a reduction in the plasma volume which gives the false impression that erythrocytosis is present. In polycythemia vera, expansion of the plasma volume can mask expansion of the red cell mass; in the case of the 49-year-old woman, splenomegaly masked the elevated red cell mass and for at least 8 years she was thought to have idiopathic myelofibrosis.

Adapted from Handin et al369 with permission of the publisher, Lippincott, Williams, and Wilkins.