This article is from the Interpretation Of Lab Test Profiles, by Ed Uthman firstname.lastname@example.org with numerous contributions by others.
The RBC count is most useful as raw data for calculation of the
erythrocyte indices MCV and MCH [see below]. Decreased RBC is
usually seen in anemia of any cause with the possible exception
of thalassemia minor, where a mild or borderline anemia is seen
with a high or borderline-high RBC. Increased RBC is seen in
erythrocytotic states, whether absolute (polycythemia vera,
erythrocytosis of chronic hypoxia) or relative (dehydration,
stress polycthemia), and in thalassemia minor [see "Hemoglobin,"
below, for discussion of anemias and erythrocytoses].