Laboratory:- Part 1 – Serum, Plasma Preparation, Specimen Storage Precautions
Serum, Plasma Preparation
For the preparation of serum or Plasma, following precautions or steps are very important to get accurate results.
Preparation of the serum and plasma:
- The sample, when collected, should be subjected to examination.
- Serum and plasma should be separated as soon as possible within 2 hours of collection.
- Don’t try to separate serum prematurely from the blood.
- Give at least 20 to 30 minutes for the clot to form.
- Centrifuge the sample to get a clear serum.
- Premature centrifugation will give rise to the microclots formation, and they may create a problem for the automation.
- The coagulation process is complete in 20 to 30 minutes in the glass and silicon tubes, but it is delayed in the plastic tube.
Avoid these samples, and possible causes are:
- Dark brown serum indicates intravascular hemolysis with the formation of methemalbumin. This may occur in severe sepsis, hemolytic crises of sickle cell anemia, and paroxysmal nocturnal hemoglobinuria.
- Dark green serum often indicates biliverdin and may be seen in severe obstructive jaundice.
- If serum was clear and after some time if turbidity appears indicates cryoglobulins, this is usually seen if the serum is kept in the fridge.
- If serum is more viscous, then check for the paraproteins. In multiple myeloma cases, serum protein may reach 13 g/dL and produce noticeable serum changes.
- If serum shows fine fibrin threads after separation may be due to heparin therapy or other anticoagulant treatment.
- Rarely may see a brown tint in the serum, which may be due to myoglobin following muscle injury or myositis.
- Serum looking more bright yellow may be due to drugs or vitamin supplements.
- Ladies on contraceptive pills may have a green tint in the serum due to ceruloplasmin.
- The jaundiced serum is yellow and should be handled with care.
- If the serum yield is small, that may indicate hemoconcentration, or another possibility is polycythemia.
The methods and precautions to get a good blood sample are:
- Improve the venepuncture procedure.
- Use plastic beads or gel tubes to get a better serum.
- Plasma, if obtained with lithium heparin anticoagulants, is a more clear sample.
- Can use serum clarifying filters to get the good serum.
- For glucose, use the preservatives which will preserve the glucose, and enzymatic methods like lithium iodoacetate.
- Avoid the exposure of the sample to high temperatures.
- Avoid the use of trauma to the sample, e.g, vigorous handling.
- Don’t keep serum or plasma for more than 30 minutes in contact with the cells.
- Potassium level increases by 2 to 8 mmol/L without hemolysis and even in the refrigerator.
- If glucose plasma is left in contact with cells at room temperature will lose 10% glucose per hour.
- Glucose serum or plasma separated immediately will remain stable for 4 hours in the fridge.
- Serum or plasma for hormones needs even more precautions; the best is to freeze the sample.
Storage of the sample:
- If there is a delay in centrifuging the sample for any reason, keep the sample at room temp. and don’t refrigerate, which may lead to hemolysis.
- If there is a delay in the test, then keep serum or plasma at 4 °C.
- If the temperature of 4 °C is not suitable for the special test, keep the serum at -20 °C.
- Centrifuge the sample with a stopper which will decrease the evaporation.
- Stopper also avoids the evaporation of volatile analytes like ethanol.
- Stopper also maintained the anaerobic conditions, which are needed for CO2 and ionized calcium.
- The use of plastic beads or silicone gel forms a barrier to separate the clot from the serum and allows the easy transfer.
- The serum or plasma should not be allowed in contact with cells for more than 30 minutes.
Difference between serum and plasma:
|Fibrinogen||0.2 to 0.4 G/dL||Nil|
|Presence (Site)||Present in the body fluid||Prepared outside the body|
|Outside the body||Always contains anticoagulant||Never anticoagulant added|