A complete blood count, or CBC, is often used as a diagnostic tool. The CBC measures the number of red blood cells, the number of white blood cells, and the total amount of hemoglobin in red blood cells. The hematocrit is a measure of the proportion of the whole blood that is made up of red blood cells. The value is then given as a percentage, such as 45%, which means that red blood cells take up 45% of the blood volume.

How the Test Is Performed

Blood is usually drawn from the inside of the elbow or the back of the hand. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In the laboratory, some of the blood is centrifuged (spun in a machine). This forces the cells to the bottom of the container. The cellular portion is compared with the total amount of blood and expressed as a percent. The cellular portion is composed almost entirely of red blood cells. The percentage of white blood cells is very small.

What Abnormal Results May Mean

Low hematocrit may indicate

• anemia
• blood loss (hemorrhage)
• bone marrow failure (e.g., due to radiation, toxin, fibrosis, tumour) which can affect stem cell production
• destruction of red blood cells
• leukemia
• malnutrition or specific nutritional deficiency
• multiple myeloma (cancer of plasma cells in the bone marrow)
• rheumatoid arthritis

High hematocrit may indicate

• dehydration
• burns
• diarrhea
• erythrocytosis (excessive red blood cell production)