RBC

Low RBC count usually suggests anemia, hemodilution, or reduced red-cell production. It is best interpreted with hemoglobin, hematocrit, MCV, reticulocytes, ferritin, B12, folate, kidney function, and inflammation.

Common causes: iron deficiency, B12 or folate deficiency, blood loss, chronic kidney disease with low erythropoietin, chronic inflammatory disease, marrow suppression, hemolysis, pregnancy, and overhydration.

Less common: leukemia, myeloma or other marrow disease, aplastic anemia, hypersplenism, inherited hemoglobin disorders, and medication or chemotherapy effects.

High RBC count suggests erythrocytosis or hemoconcentration. A transient increase is often from dehydration or recent intense exertion, so hematocrit and hemoglobin help separate concentration from true red-cell mass.

Common causes: smoking, sleep apnea, chronic lung or heart disease, high altitude exposure, testosterone or anabolic-androgen use, erythropoietin use, and other hypoxia-driven states.

Less common: polycythemia vera or other myeloproliferative disease, kidney tumors or cysts producing excess erythropoietin, renal artery stenosis, and high-affinity hemoglobin variants.