Most young medical laboratory analysts and students often grapple with the issue of how to measure PCV(Packed Cell Volume) of a blood sample. A microhaematocrit is used in this measurement to achieve an accurate reading of the PCV
Value of test: The Packed Cell Volume (PCV), also called haematocrit, is used to calculate the mean cell volume (MCV). These red cell indices are used in the investigation of anaemia. The PCV is also used to screen for anaemia when it is not possible to measure haemoglobin, and to diagnose polycychaemia vera and to monitor its treatment. It is suitable for screening large clinic populations, e.g antenatal clinics.
Principle of Test
The packed cell volume is that proportion of whole blood occupied by red cells. Expressed as a ratio (litre/litre). Anticoagulated blood in a glass capillary of specified length, bore size, and wall-thickness is centrifuged in a microhaemtocrit centrifuge at RCF 12000-1500xg for 3-5 minutes to obtain constant packing of the red cells. A small amount of plasma remains trapped between the packed red cells. The PCV value is read from the scale of a microhaematocrit column by the height of the total column of blood.
Note: Due to trapped plasma, PCV values using a centrifugation technique are 1-3% higher than those obtained when using an electronic cell analyzer which computers the value from the MCV and red cell count (PCV=MCV x RBC).
Specimen: To measure the PCV, either well mixed well oxygenated EDTA anticoagulated blood can be used or capillary blood collected into a heparinzed capillary.
An example of a microhaematocrit centrifuge the Jouan A13 model with essential accessories. It is a robust, stable, fixed speed (1200rpm, 15290xg) microhaematocrit centrifuge with essential safety features which include a lid interlock, metallic casing and counterbalanced lid. It is fitted with a digital timer, has short acceleration and braking rates, and very low noise level when operating.
The Jouan microhaematocrit centrifuge measure 24cm wide xx 31cm deep x 22cm high, weighs 10kg, and has power requirement of 300 VA (brushed drive motor).
Availability: The A13 Jouan microhaematocrit centrifuge (cat. No. 11174544, 220/240 V, 50 Hz), complete with 24 place capillary rotor, circular haematocrit reader 100 heparinized capiliaries, plate of sealant, spare carbon brushes and fuses, costs £898(2000y). It is available from Jouan SA., Merck/BDH, and Developing Health Technology.
Important Note: When buying a microhaematocrit centrifuge, always purchase spare rotor rim gaskets. For the Jouan microhaematocrit centrifuge, spare rotor rim gaskets are available in a pack of 10 (cat. No. 41174945), priced £26 (2000y).
Capillary Tubes for Measuring PCV
These need to be plain or heparinized capillaries, measuring 75mm in length with an internal diameter of 1mm and wall thickness of 0.2-0.25mm. plain capillaries are often blue tipped and heparinized capillaries, red-tipped.
Although the end of a capillary can be heat-sealed this often distorts the end of the tube resulting in breakage, or the heat damages the red cells resulting in an incorrect PCV. Capillaries are best sealed using a plastic sealant, modeling clay, or plasticine.
Availability of capillaries and sealant: These products are available from Jouan SA Merck/BDH, Developing Heath Technology, and other laboratory equipment suppliers. Capillaries are usually sold in boxes of 200, 250, or 1000. Sealant is usually sold in a numbered plate (singly).
In trying to measure PCV, we must first understand that there are two types of microhaematocrit PCV reader, i.e an integral spiral reader which fits inside the centrifuge allowing PCV measurements to be made after centrifuging with the capillaries in place in the rotor, and a hand-held scale or graph. A hand-held PCV reader can be used to read samples centrifuged in any , whereas an integral. PCV reader can usually be used only with the centrifuge for which it has been designed.
Whenever possible, it is always advisable to perform the test in duplicate
- About three quarters fill either.
- A plain capillary with well mixed EDTA anticoagulated blood (tested within 4 hours of collection), or
- A heparinized capillary with capillary blood.
Leave about 10-15mm of the capillary unfilled.
- Seal the unfilled end, preferably using a sealant material. If unavailable, heat-seal the capillary using a small flame from a spirit lamp or pilot flame of a Bunsen burner, rotating the end of the capillary in the flame.
- Carefully locate the filled capillary in one of the numbered slots of the microhaematocrit rotor with the sealed end against the rim gasket (to prevent breakage). Write the number of the slot on the patient’s form.
Note: When an inner lid is used, position it carefully to avoid dislodging the tubes.
- Centrifuge for 3-5 minutes (RCF 12000-1500xg), using the shorter time when the RCF is 15