Packed cell volume is the percentage of volume of blood occupied by the red cells
Enumerate the methods used for determination of PCV
Macrohematocrit method (Wintrobes method)
Microhematocrit method
Electrical method
Indirect method
Describe the Wintrobes method in detail for estimation of PCV
Apparatus required
Wintrobes tube –
It is 110mm long, narrow, thick walled tube with 3mm internal bore
Graduated from 0 to 10 cm with graduation on both sides in ascending and descending order on 2 sides of tube
Scale with the markings from 0 to 10 from above downwards is used in ESR determination and from below upwards is used for PCV determination
Pasteur pipette
centrifuge
Procedure –
2 ml of venous blood is collected and mixed with double oxalate (ammonium oxalate and potassium oxalate) or EDTA powder in the proportion of 1.5mg/ml
Blood is drawn into Pasteur pipette and introduced in the Wintrobes tube from the bottom to 0 or 10 mark above
Place the Wintrobes tube in the centrifuge machine and other Wintrobes tube filled with water on the opposite side so as to balance it.
Centrifuge the tube at the speed of 3000rpm for 30 minutes
After 30 minutes stop the centrifuge, take out the tube and note the readings
Calculation –
Hematocrit = [ Height of RBC’s in mm/Height of RBC and plasma] X 100
Describe the Microhematocrit method (Capillary tube method)
Apparatus
Microhematocrit tube (Capillary tube) – 75mm long with 1mm diameter which contains Heparin and show a red ring at the end of the tube
Microhematocrit centrifuge device
Plastic seal to seal one end of tube
Microhematocrit reader
Procedure
Clean the finger with 70 % alcohol and let dry
Prick the finger
Place the tip of the capillary tube onto a drop of blood on finger
Fill the capillary tube with blood upto 2/3 to 3/4th of the tube
Seal the other end of the tube which was not in contact with blood by plastic sealent or clay
Place the tube in the microhematocrit centrifuge and spin it for 5 minutes at the speed of 10,000rpm
Using a special reading device note down the readings
Name the zones separated after centrifugation
Top layer – Plasma (48 – 52%)
Normally amber or pale yellow colour
yellow – jaundice
Pink or red colour indicates – hemolysis
creamy white – hyperlipidemia
Brown coloured – meth hemoglobinemia
Cloudy (increased viscosity) – Multiple myeloma
Intermediate zone – Buffy coat – Zone of platelets and leukocytes (2% – 3% or 1mm thick)
Greyish – white tan layer
Smears prepared from buffy coat can be used to diagnose
Sub leukemic leukemia
LE cells
Detection of plasma cells
Hemoparasites
Lower most zone or bottom layer – Zone of packed RBC’s (45% – 50%)
What is the normal PCV
Males – 40 -50%
Females 37 – 47%
New born – 55 – 60%
What other indices can be calculated by using PCV
Mean corpuscular volume (MCV) = PCV/RBC X10
Normal value – 92 ± 9 fl
Mean corpuscular hemoglobin concentration (MCHC) = Hb/PCVX100
What are the indications for PCV estimation
Screening test for anaemia / polycythemia
For calculation of other red cell indices
To counter check the value of Haemoglobin in the reported sample (PCV= Hgb in gms/dl X3)
What are the clinical implications of PCV
PCV is affected by the number of RBC’s, their size and plasma volume
High PCV –
increased number of RBC’s
increase in size of RBC
Decrease in plasma volume
Low PCV
decrease in number of RBC’s
decrease in size of RBC
Increase in plasma volume
Causes of increased PCV
Polycythemia vera
High altitudes
Hypoxia conditions
Lung and heart diseases
Dehydration
Burns (due to loss of plasma)
Causes of decreased PCV
Anemia
Conditions with increased WBC’s
long term illness, infection
leukemia
lymphoma
Hemodilution or overhydration
Acute kidney disease – lower erythropoietin production leads to less RBC’s production by bone marrow
Pregnancy
What are the sources of error during PCV estimation
An increased amount of anticoagulant decreases the hematocrit as a result of erythrocyte shrinkage
Time and speed of centrifuge should be correct
Improper sealing of PCV tube leads to loss of blood leading to decreased PCV
Microhematocrit centrifuge should never be forced to stop by applying pressure to the metal cover plate. This causes RBC layer to “sling” forward and result in a falsely elevated value.
Describe the electrical method for the measurement of PCV
This method is based on the principle that plasma is a conductor where as the red cells are non – conductors
Thus a signal passed through a column of blood will be impeded to a degree proportional to the volume occupied by the red cells.
This principle is applied in estimation of PCV by some types of automated blood counting apparatus.
The method appears to be fairly reliable in most cases. But errors can be produced by gross variations in plasma proteins and electrolytes and also by a too high concentration of EDTA
What is the indirect method of measuring PCV
In automatic cell counters PCV can be derived from red cell count and mean corpuscular volume
PCV obtained by this method is 1.5 to 3% lower than the microhematocrit value.
Accuracy of Coulter S PCV is influenced by calibration of the instrument which may not be set correctly and by incorrect dilution of the specimen for counting
References
Praful B. Godkar, Darshan P. Godkar.Textbook of medical laboratory technology 2007. Second Edition
Sabitri sanyal, Aparna Bhattacharya.Clinical pathology A practical manual 2017. Third edition.
By
Dr.Shyam Sundara Rao (Professor of Pathology, Narayana Medical College, Nellore)
Dr. V.Shanthi (Professor of Pathology, Narayana Medical College, Nellore)