Red blood cell count (RBC) measures the number of red blood cells in circulation. Typical reference ranges are about 4.5–5.9 × 10⁶/μL for men and 4.1–5.1 × 10⁶/μL for women, though this varies slightly by lab. Low RBC count can reflect anemia from blood loss, nutritional deficiency, low marrow production, or hemolysis. High RBC count may occur with dehydration, chronic low-oxygen states, high altitude, or less commonly polycythemia. It is best interpreted alongside hemoglobin and hematocrit rather than on its own.