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Hematocrit (HCT) - Test
Also known as: Hct, Crit, Packed cell volume (PCV)
A hematocrit (HCT) test may be done separately or as part of a complete blood count. It measures percentage by volume of packed red blood cells (RBCs) in a whole blood sample; for example, an HCT of 40% indicates that a 100-ml sample of blood contains 40 ml of packed RBCs. Packing is achieved by centrifuging anti coagulated whole blood in a capillary tube so that red cells are tightly packed without hemolysis.
- To aid diagnosis of polycythemia, anemia, or abnormal states of hydration.
- To aid calculation of two red cell indices: mean corpuscular volume and mean corpuscular hemoglobin concentration.
- Explain to the patient that HCT is tested to detect anemia and other abnormal blood conditions.
- Tell him that a capillary blood sample will be taken. Explain who will perform the test and when.
- If the patient is a child, explain to him (if he's old enough) and his parents that a small amount of blood will be taken from the finger or earlobe.
- Inform the patient that food or fluids need not be restricted before the test.
Procedure and posttest care
- Perform a finger stick, using a heparinized capillary tube with a red band on the anticoagulant end. Blood drawn from a capillary puncture has slightly lower HCT levels than blood drawn from a venipuncture.
- Fill the capillary tube from the redbanded end to about two-thirds capacity, and seal this end with clay.
- Alternatively, perform a venipuncture and fill a 7-ml lavender-top tube.
- If a hematoma develops at the venipuncture site, apply warm soaks.
- Send the sample to the laboratory immediately.
- If you perform the test, place the tube in the centrifuge with the red end pointing outward.
- Fill the collection tube completely.
- Invert the tube gently several times to mix the sample.
HCT is usually measured electronically. The results are 3% lower than manual measurements, which trap plasma in the column of packed RBCs.
Reference values vary, depending on the type of sample, the laboratory performing the test, and the patient's age and sex, as follows:
- neonates ages 0 to 2 weeks: 44% to 64%
- infants ages 2 weeks to 2 months: 39% to 59%
- infants ages 2 to 6 months: 35% to 49%
- infants ages 6 months to I year: 29% to 43%
- children ages 1 to 6: 30% to 40%
- children ages 6 to 16: 32% to 42%
- adult males: 42% to 52%
- adult females: 36% to 48%.
Low HCT suggests anemia, hemodilution, or massive blood loss. High HCT Indicates polycythemia or hemoconcentration due to blood loss and dehydration.
- Failure to fill the tube properly, to use the proper anticoagulant, or to adequately mix the sample and the anticoagulant.
- Hemolysis due to rough handling of the sample or drawing the blood through a small-gauge needle for venipuncture.
- Hemoconcentration due to tourniquet constriction for longer than 1 minute (increase, typically 2.5% to 5%).
- Drawing the blood from the same arm used for I. V. infusion.
- Elevated blood glucose or sodium (possible increase due to erythrocyte swelling)
- Hemodilution or dehydration.