How would you evaluate a patient with an isolated high RBC count but with a normal hemoglobin and hematocrit?

My first question would be, how long has the elevated red cell count been present? I ask this because, in a study of 10,000 individuals, erythrocytosis was initially found in 88 but after a year only 11 still had this finding (Ruggeri et al., PMID 13679323). If therefore, the observation is recent, ...

The high RBC with normal HCT and Hgb implies microcytosis (low MCV). The differential diagnosis for microcytosis is largely iron deficiency, hemoglobinopathy, and (less likely) some hereditary RBC skeleton protein mutations. The first step is to check the iron stores. If normal, then this is likely ...