1
Haematocrit (Packed Cell Volume – PCV)
determination
Objectives:
The hematocrit (PCV) is the percent of the packed red cells in a
volume of whole blood. It is a screening test for anaemia or
polycythemia.
When accurate measurements of Red cell count and Hb
concentration are available, the absolute values can be calculated.
Principle:
A volume of anticoagulated blood is placed in a glass tube which is
centrifuged so the blood will be separated into three layers:
- Red cells,
- Buffy coat (WBC and platelets) and
- Plasma.
- Ideally there should be complete separation of the three layers.
- Haematocrit is the ratio of the height of red cells column to that
of the whole blood in the tube.
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The two methods of direct measurement of the PCV which are in
current use are:
1. Macro-method using Wintrobe tubes.
2. Micro-method using capillary tubes.
The more popular one is the micro-method, as it has the advantage
of short time of centrifugation and better packing of the red cells.
Micro-Haematocrit Method
Test sample:
Heparin or EDTA venous or capillary blood.
Equipments
:
- Micro-haematocrite centrifuge.
- Plastic sealer or Bunsen burner.
Disposable materials:
- Capillary tubes 75 mm long and internal diameter of 1 mm.
Method
:
1. The blood sample should be used as fresh as possible, and
well mixed.
2. Using the capillary action, allow blood to enter the tube
stopping at 10-15 mm from one end. Wipe the outside of the
tube.
3. Seal the dry end by pushing into the plasticine two or three
times.
4. If heat sealing is used rotate the dry end of the tube in a fine
Bunsen Burner flame.
5. Place the tube into one of the centrifuge plate slots, with the
sealed end against the rubber gasket of the centrifuge plate.
6. Keep a record of the patient number against centrifuge plate
number.
7. Centrifuge (5000rpm) for five minutes.
8. Read the PCV in the micro haematocrit reader.
9. Unit: the haematocrit result is expressed in percentage.
Note: It is preferable to perform the test in duplicate.
Results:
Normal ranges:
Adult males =
40% - 52%
.
Adult females =
37% - 47%.