Background

Hematocrit (HCT), also called packed cell volume (PCV), means "to separate blood" and is the packed spun volume of blood that consists of intact red blood cells (RBCs), expressed as a percentage. HCT can be measured directly following centrifugation of a blood sample or calculated using the formula:

HCT = (RBC x MCV)/10. Mean corpuscular volume (MCV) is the average volume (size) of the patient's RBCs.

HCT is part of the complete blood cell count and indirectly measures the RBC mass. The results are expressed as the percentage by volume of packed RBCs in whole blood (PCV). It is an important measurement in the determination of anemia or polycythemia.

Normal reference values in adults

* Women: 36% to 48%

* Men: 42% to 52%.

Clinical alert

An HCT under 20% can lead to heart failure and death; an HCT over 60% is associated with spontaneous thrombosis.

Clinical implications

* Decreased HCT is an indicator of anemia, a disorder in which there is a reduction in the PCV. An HCT under 30% means that the patient is moderately to severely anemic. Decreased values also occur in the following conditions:

- leukemias, lymphomas, Hodgkin disease, myeloproliferative disorders

- adrenal insufficiency

- chronic disease

- acute and chronic blood loss

- hemolytic transfusion reactions.

* The HCT may not be reliable immediately after even a moderate loss of blood or immediately after transfusion.

* The HCT may be normal immediately after acute hemorrhage. During the recovery phase, both the HCT and the RBC count drop markedly.

* Usually, the HCT parallels the RBC count when the cells are of normal size. As the number of normal-sized erythrocytes increases, so does the HCT.

- However, for the patient with microcytic or macrocytic anemia, this relationship does not hold true.

- If a patient has iron-deficiency anemia with small RBCs, the HCT decreases because the microcytic cells pack to a smaller volume. The RBC count, however, may be normal or higher than normal.

* Increased HCT occurs in:

- erythrocytosis

- polycythemia vera

- hemoconcentration from hypovolemia.

Interfering factors

* People living at high altitudes have a high HCT.

* Normally, the HCT slightly decreases in the physiologic hemodilution of pregnancy.

* The normal values for HCT vary with age and gender. The normal value for infants is higher because the newborn has many macrocytic red cells. The HCT in women is usually slightly lower than in men.

* There is also a tendency toward a lower HCT in men and women over age 60, corresponding to lower RBC count values in this age group.

* Severe dehydration from any cause falsely raises the HCT.

Source: Fischbach FT, Fischbach MA. A Manual of Laboratory and Diagnostic Tests. 10th ed. Philadelphia, PA: Wolters Kluwer; 2018.