Blood tests carried out in modern day laboratories rely on extracting serum or plasma from whole blood withdrawn from patients. Unless the initially withdrawn blood is collected in an appropriate tube, the outcome may differ and could significantly affect the blood test results.
What is the difference between ‘serum’ and ‘plasma’?
Both serum and plasma are components of whole blood and it is estimated that plasma constitutes half of the blood volume. After treating whole blood with a designated technique, it is possible to separate plasma after removing the cell component (red blood cells and white blood cells). The serum is almost similar to plasma although it does not contain the clotting factors and fibrinogen that are present in a plasma sample. Thus, the plasma can be used to replenish clotting factor deficiencies in certain patients apart from its use as a medium for blood tests.
What is the technique used for preparing serum for blood testing?
In the event of preparing serum, the whole blood should be collected into a plastic tube used for such purposes (Serum-separation tube – SST). Once collected, the sample should be allowed to sediment at room temperature without any disturbances. After about 15 – 30 minutes, a supernatant will appear while the cells will clump together forming a ‘clot’.
In order to remove the clot, it is necessary to centrifuge the sample at 1000 – 2000 x g for about 10 minutes. Once centrifuged, the resulting supernatant, which is termed as the ‘serum’, should be removed carefully using a pipette.
What is the technique used for preparing plasma for blood testing?
When the idea is to prepare plasma from whole blood, the phlebotomists or health staff will make arrangements to collect the blood from the patient into a tube which is coated with ‘anti-coagulants’. When tubes are treated with anti-coagulants such as EDTA or Citrate, the whole blood will remain intact without clumping or giving rise to a clot. However, because of the need to separate the cells in order to produce plasma, the anti-coagulant treated sample should be centrifuged at 1000 – 2000 x g for about 10 minutes. The resulting supernatant needs to be carefully extracted using a pipette, immediately.
What are the measures used in handling these samples following its preparation?
In general, both plasma and serum should be handled at 2 – 8 degrees Celsius and in case it is transported or needs storage, it should be done at -20 degrees or below. However, analyzing the sample as soon as possible is the ideal technique, which will avoid many misinterpretations and false results as the sample undergoes constituency changes. At the same time, both plasma and serum should not be subjected to a ‘freeze-thaw’ cycle as it can adversely affect many of its components giving rise to false results following its analysis. Furthermore, at the time of collecting the blood sample, it is vital to mix the blood with the constituents of the tube by inverting 8 – 10 times, immediately following its transfer into the tube.