Blood consists of a fluid portion (plasma) and a solid portion that includes RBCs, WBCs, and platelets. More than 99 percent of the total blood cell mass is composed of RBCs. The Hct or packed RBC volume measures the proportion of RBCs in a volume of whole blood and is expressed as a percentage.

Several methods can be used to perform the test. In the classic method, anticoagulated venous blood is pipetted into a tube 100 mm long and then centrifuged for 30 minutes so that the plasma and blood cells separate. The volumes of packed RBCs and plasma are read directly from the millimeter marks along the side of the tube. In the micro method, venous or capillary blood is used to fill a small capillary tube, which is then centrifuged for 4 to 5 minutes. The proportions of plasma and RBCs are determined by means of a calibrated reading device. Both techniques allow visual estimation of the volume of WBCs and platelets.²⁴

With the newer, automated methods of cell counting, the Hct is calculated indirectly as the product of the RBC count and mean cell volume. Although this method is generally quite accurate, certain clinical situations may cause errors in interpreting the Hct. Abnormalities in RBC size and extremely elevated WBC counts may produce false Hct values. Elevated blood glucose and sodium may produce elevated Hct values because of the resultant swelling of the erythrocyte.²⁵

Normally, the Hct parallels the RBC count. Thus, factors influencing the RBC count also affect the results of the Hct.

Note: Values vary across the life cycle and between genders.

• Abnormalities in RBC size and extremely elevated WBC counts may alter Hct values.
• Elevated blood glucose and sodium may produce elevated Hct values because of swelling of the erythrocyte.
• Factors that alter the RBC count such as hemodilution and dehydration also influence the Hct.

• Routine screening as part of a CBC
• Along with an Hgb (i.e., an "H and H"), to monitor blood loss and response to blood replacement
• Along with an Hgb, to evaluate known or suspected anemia and related treatment
• Along with an Hgb, to monitor hematologic status during pregnancy
• Monitoring responses to fluid imbalances or to therapy for fluid imbalances:
  • A decreased Hct may indicate hemodilution.
  • An increased Hct may indicate dehydration.

Nursing Care Before the Procedure

Client preparation is the same as that for any study involving the collection of a peripheral blood sample (see Appendix I).

The volume of the sample needed depends on the method used to determine the Hct. With the exception of the classic method of Hct determination, a capillary sample is usually sufficient to perform the test. If a venipuncture is performed, the sample is collected in a lavender-topped tube.

Nursing Care After the Procedure

Care and assessment after the procedure are the same as for any study involving the collection of a peripheral blood sample (see Appendix I).

• Critical values: Notify the physician at once if the Hct is greater than 60 percent or less than 14 percent. Prepare the client for possible transfusion of blood products or infusion of intravenous fluids and for further procedures to evaluate the cause or source of the blood loss or hemoconcentration.