Hematocrit (Hct)

What it measures

The fraction of whole blood volume occupied by red cells. It follows hemoglobin closely and helps assess anemia, polycythemia and fluid status.

Specimen and preparation

• Specimen: Whole blood (EDTA)
• Tube: Lavender top
• Preparation: Avoid hemoconcentration from a long tourniquet time. Mix well before testing.

Reference range

Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.

Causes of high values

• Dehydration, burns, diarrhea
• Polycythemia vera
• Chronic hypoxia or high altitude
• Secondary erythrocytosis

Causes of low values

• Anemia of any cause
• Hemorrhage
• Overhydration or pregnancy (dilution)
• Bone marrow suppression

Method

Calculated by analyzer (RBC × MCV) or microhematocrit centrifugation

Good to know

Roughly three times the hemoglobin in g/dL when cells are normocytic. SI form is a fraction (e.g. 0.45 L/L).

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