Packed cell volume (microhematocrit)

Principle

• Whole blood is centrifuged in a narrow capillary tube; red cells pack at the bottom. The PCV is the fraction of the column occupied by red cells.

Specimen

• EDTA venous blood (use plain capillary tubes) or capillary blood collected into heparinised tubes.

Reagents and equipment

• Capillary tubes about 75 mm long, sealing clay, microhaematocrit centrifuge, reading device.

Procedure

• Fill the capillary tube about two-thirds to three-quarters with well-mixed blood.
• Seal the dry end with clay; place in the centrifuge with the sealed end outward.
• Centrifuge at high speed (RCF about 10,000–15,000 g) for about 5 min.
• Read the red cell column height divided by total column height using the reader; exclude the buffy coat.
• Run duplicates; they should agree within about 0.01 L/L.

Normal values

• Typical adults: men about 0.40–0.50 L/L (40–50%), women about 0.36–0.46 L/L. Local ranges apply.

Quality control and pitfalls

• Trapped plasma (normally about 1–3%) is increased in sickle cell disease, spherocytosis, macrocytosis and thalassaemia, giving slightly high values.
• Inadequate centrifugation, leaking seal or reading the buffy coat as red cells give false highs.
• Excess EDTA (short draw) shrinks cells and lowers PCV.
• Haemolysis, poor mixing or dilution with IV fluid give errors.
• Check centrifuge speed and packing time periodically.

Clinical use

• Rapid assessment of anaemia or polycythaemia; used to calculate MCV and MCHC.