Advantages of using plasma over serum?

Advantages of using plasma

The following aspects support the preferential use of plasma versus serum in laboratory medicine:

• Time saving:

Plasma samples can be centrifuged directly after sample collection, unlike serum, in which coagulation is completed after 30 minutes,

• Higher yield:

15 to 20 % more in volume of plasma than of serum can be isolated from the same volume of blood.

• Prevention of coagulation-induced interferences:

Coagulation in primary and secondary tubes that were already centrifuged, may block suction needles of the analyzers when serum tubes are used; this can be prevented by using anticoagulants.

• Prevention of coagulation-induced changes:

The coagulation process changes the concentrations of numerous constituents of the extra-cellular fluid beyond the maximum allowable limit (8, 15). The changes are induced by the following mechanisms:

a. Increase in the concentrations of platelet components in serum as compared to plasma (e.g. potassium, phosphate, magnesium, aspartate aminotransferase, lactate dehydrogenase, serotonin, neurone-specific enolase, zinc). Release of amide-NH3 from fibrinogen induced by action of clotting factor XIII.

b. Decrease in the concentration of constituents in serum as a result of cellular metabolism and the coagulation process (glucose, total protein, platelets).

c. Activation of the cell lysis of erythrocytes and leukocytes in non-coagulated blood (cell-free haemoglobin, cytokines, receptors).

Certain constituents should only be measured in plasma (e.g. neurone-specific enolase, serotonin, ammonia) to obtain clinically relevant results.

Disadvantages of plasma over serum

The addition of anticoagulants can interfere with certain analytical methods or change the concentration of the constituents to be measured:

a. Contamination with cations: NH4+, Li+, Na+, K+.

Assay interference caused by metals complexing with EDTA and citrate (e.g. inhibition of alkaline phosphatase activity by zinc binding, inhibition of metallo-proteinases, inhibition of metal-dependent cell activation in function tests, binding of calcium (ionized) to heparin (16)).

b. Interference by fibrinogen in heterogeneous immunoassays (15).

c. Inhibition of metabolic or catalytic reactions by heparin: e.g., Taq polymerase in the polymerase chain reaction (PCR) (17)

d. Interference in the distribution of ions between the intracellular and extracellular space (e.g. Cl-, NH4+) by EDTA, citrate (8).

e. Serum electrophoresis can be performed only after pre-treatment to induce coagulation in plasma.

By: Subrata Sutradhar, B.Sc(MT)

Researcher,