Principle
- The blood mixed with anticoagulant is placed in a long capillary tube and centrifuge . the level reached by the column of erythrocytes is read with a scale reader . this method is preferable for that using a macro scale : it’s quicker and blood from the finger can be used .
Collection of specimens
- Using a blood lancet ,after cleaning the chosen area with ethanol ,draw blood by pricking either : the 3rd or 4th finger ,the lobe of the ear or the heel (infants ). The blood should flow freely or with very little pressure to the area . wipe away the first drop with filter paper .
- Apply the tip of a capillary tube containing dried heparin to the drop of blood .The blood flows to the tube by capillarity .Fill about 3-quarters of the tube .
- Plug the other end of the tube with soft wax or plastic clay .Check that it’s completely plugged to a depth of about 2mm.You can use venous blood collected on EDTA but here you use a capillary tube Not heparinized .
Technique
- Place the capillary tube in the Hct-centrifuge , make sure that the sealed end point outwards.
- Centrifuge at 3000g(usually for 10 min ), After centrifugal the tube will show 3 layers :plasma at the top ,thin layer of leukocytesin in the middle and a column of erythrocytes at the bottom.
- Hold the tube against the scale so that the bottom of the column of erythrocytes is aligned with horizontal zero line . then move the tube until the line 1.0 passes through the top of plasma column .
- The line that passes through the top of the column of erythrocyte gives the erythrocyte volume fraction (Hct\ PCV)
Low values
- Most found in anaemia
High values
- represent elevated red blood cell counts(high altitudes , chronic smokers). Dehydration produces a falsely high hematocrit that disappears when proper fluid balance is restored.
- Some other infrequent causes of elevated hematocrit are lung disease, certain tumors, a disorder of the bone marrow known as polycythemia rubra vera, and abuse of the drug erythropoietin.