Hematocrit and hemoglobin measurements are blood tests. They are part of a complete blood count, or CBC. Hematocrit measures the percentage of total blood volume that is red blood cells. For example, 40 percent means that there are 40 milliliters of packed red blood cells in a 100 milliliter sample. Technicians pack the red blood cells by spinning the blood. There are 1000 milliliters in a liter. A liter is a little more than a quart.

Hemoglobin is the part of a red blood cell that carries oxygen from the lungs to all cells. The body requires iron to build hemoglobin. A total hemoglobin test measures the weight of hemoglobin in 100 milliliters of whole blood. It is usually reported as grams of hemoglobin per deciliter whole blood. A gram is a metric unit of weight. A deciliter is the same as 100 milliliters.

Together, the hematocrit and hemoglobin tests help diagnose anemia and polycythemia. Anemia is a shortage of red blood cells. Polycythemia is production of too many red blood cells. Other explanations for low values include internal bleeding and fluid retention.

These tests require taking a blood sample from the arm. This is a relatively painless procedure that can occur in a clinic, office, hospital, or lab. A nurse or technician wraps a rubber strap tightly around the upper arm. This and the needle puncture may cause slight discomfort. For infants and young children, they generally take blood from a finger or ear lobe. Occasionally, a small amount of blood collects under the skin at the puncture site. A warm washcloth held against it will help reduce swelling.

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