The concentration of which of the following is usually expressed as grams per deciliter (g/dL)?

• A. Red blood cells
• B. White blood cells
• C. Hemoglobin
• D. Platelets

Answer: (C)

Hemoglobin concentration is expressed in grams per deciliter because it measures the mass of the protein present in a given volume of blood. This mass-per-volume unit directly reflects how much hemoglobin is available to bind and transport oxygen. Red blood cells, white blood cells, and platelets are counted as numbers of cells per unit volume (for example, cells per microliter), not by mass, so their typical reporting units aren’t g/dL. Hematocrit, meanwhile, is the percentage of blood volume that is occupied by red cells, not a mass per volume value. So the concentration most commonly given in g/dL is the amount of hemoglobin.