The red blood cell count is the number of erythrocytes in a given volume of blood. Together with hemoglobin and hematocrit it describes the size of the red cell population, and it is the count from which the red cell indices are calculated. On its own it says less about oxygen delivery than hemoglobin does, because cells vary in how much hemoglobin each one carries.
No readings for this marker yet.
A lowered count most often accompanies anemia from blood loss, from iron, B12 or folate deficiency, from chronic disease or kidney disease, or from shortened red cell survival. A raised count most often reflects dehydration, or a sustained response to low oxygen such as smoking, altitude or chronic lung disease; rarely it points to a primary bone marrow overproduction. In some conditions the count and hemoglobin move apart, with many small underfilled cells, which is itself informative. Direction indicates where to look next, and interpretation belongs with a doctor.
Plasma volume shifts the result without changing the actual red cell mass, so dehydration, heavy sweating, pregnancy and intravenous fluids all move it. Posture at the draw and prolonged tourniquet time cause small changes. It is interpreted alongside hemoglobin and hematocrit, and with MCV, MCH, MCHC and RDW, which turn the raw count into a description of cell size and hemoglobin content.
Reference ranges are general adult values and depend on lab, assay, age and sex. For orientation, not diagnosis — discuss results with your doctor.