Hematocrit, or HCT, is a measure of the percentage of red blood cells contained in a volume of whole blood. Another term for hematocrit is packed cell volume, or PCV. Values obtained in this test are dependent on the number and size of red blood cells. Normal hematocrit levels are 38.8% to 50% for adult men and 34.9% to 44.5% for adult women. Ranges for children 15 years old and younger will differ according to age and sex.
It is helpful to know what to expect when this blood count procedure is performed. The lab technician will draw a blood sample from a vein, normally from the inside of the elbow or back side of the hand. Technicians will first tie an elastic band around the arm to cause the vein to swell with blood. Following this, a needle will be inserted into the vein, causing blood to collect in a vial. Once the entire blood sample has been obtained, the needle will be removed and a bandage will be placed over the puncture to stop bleeding.
After the blood sample has been drawn, the process of determining whether it contains normal hematocrit levels begins by placing it in a spinning device known as a centrifuge. This machine will apply a very fast twirling motion to a test tube, which will result in the blood separating into three components. The three parts of blood consist of the fluid, otherwise known as the plasma, the red blood cells, and the remaining blood cells. Once the blood has divided into its component parts, the lab technician can determine the percentage of red blood cells.
Lower-than-normal and higher-than-normal hematocrit levels may result from a vareity of different medical conditions or deficiencies. Lower HCT readings can be caused by anemia or leukemia, as well as destruction of red blood cells, overhydration, and malnutrition. Nutritional deficiencies of iron, vitamin B12, and vitamin B6 will also create lower levels of HCT. Different medical disorders can also produce higher-than-normal hematocrit levels, such as the blood conditions of erythrocytosis, polycythemia vera, and low blood oxygen levels. Other illnesses include congenital heart disease, cor pulmonale, and pulmonary fibrosis, or a simpler reason, dehydration.
Certain conditions can influence the outcome of a hematocrit test, causing the results to be inaccurate. Such situations include living at a high altitude, pregnancy, or severe dehydration. A significant recent blood loss or recent transfusion may also produce misleading results. Doctors will take these factors into consideration when they interpret the readings.