Manual Packed Cell Volume Test
The packed cell volume ( PCV ) is the amount of packed red blood cells following centrifugation , expressed as percentage of total blood volume . The two methods for determination packed cell volume are the micromethod and the macromethod . In both methods , a column of blood is centrifuge in a tube of uniform bore , closed at one end. The micromethod is more convenient method in many laboratories as routine method . Therefore this test is used for screening of anaemia.
PCV values are reduced than normal in anaemia , hydration and pregnancy. Increased PCV values are found in when there is a loss of plasma as in severe burns , dehydration and also in polycythemia .
• Principle
In the micromethod , anticoagulated blood in a glass capillary tube ( haematocrit tube ) is centrifuged at high – speed to obtain the constant packing of the red cells at the bottom of the tube . The packed cell volume value is read from the scale of a microhaematocrit reader or manually calculated by dividing the height of the red cell column by the height of the total column of blood .
• Specimen
EDTA anticoagulated venous blood / capillary blood collected directly into heparinized capillary tube
• Requirements
- Microhaematocrit centrifuge
- Capillary tubes ( disposable )
- Sealant – clay
- Microhaematocrit reader
• Procedure
The microhaematocrit tube is filled by capillary action , either from a free flowing puncture wound or a well-mixed venous sample. The capillary tube should be at least 2/3 but not over 3/ 4 full .
There are two types of capillary tubes . The plain capillary tubes that without contain anticoagulant and are to be used with anticoagulated blood . The heparinized capillary tubes that contain heparin as anticoagulant and are to be used with capillary blood obtained directly from skin puncture .
The unfilled end of capillary tube seals with the clay sealant material . Then the filled tubes place in the radial grooves of the microhaematocrit centrifuge with the sealed end against the outer rim . The filled capillary tubes centrifuge for 5 minutes ( RCF 12000 – 15000 *g ).
Immediately after centrifuging , the PCV read using the hand- held microhaematocrit reader. Before read the PCV , check for any leakage around the bottom of the tube .
To read the PCV , align the base of the blood in the column ( above the sealant ) with 0 line and the bottom of the meniscus of the plasma with 100 line . The reading point is the top of the red cell column , just below the buffy coat layer ( consisting WBCs and platelets ) . The value of the PCV is taken directly from the reader .
• Limitation
- Using a blood sample containing excess EDTA causes erroneous results .
- Inadequate mixing of the venous blood sample and clot in blood sample affects to the test result.
- Storing of the specimen beyond 6 – 8 hours results in falsely increased PCV due to swelling of red blood cells .
- Haemolyzed blood sample causes falsely low results .
- When , not cleaning and maintaining the microhaematocrit centrifuge as recommended by the manufacture , It causes the errors of the result .
- A short duration and low speed of centrifugation may not achieve as complete packing as possible.
- Evaporation of plasma during centrifugation or when left for a time before being read , may be a source of error .
- Uneven upper surface of the sealer causes the erroneous results .
- Trapped leukocytes , platelets and particularly trapped plasma may cause errors .
• Reference range
Children at birth            –    44% – 54 %
Children   2 – 5 years  –  34% – 40 %
Children   6 – 12 years  – 35% – 45 %
Adult men                      –  40% – 54 %
Adult women                –   36 % – 46 %
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