It is imperative that the in vivo state of a constituent remains unchanged after withdrawal from the body fluid of a patient to obtain a valid medical laboratory result. This may not always be possible when measuring extra-cellular and cellular components of blood. Platelets and coagulation factors are activated when blood vessels are punctured, and their activation continues in sample containers that do not contain anticoagulant. Some changes of constituents can be avoided by using anticoagulants.
Blood samples may be of venous, arterial or capillary origin.
- Whole blood is defined as a venous, arterial or capillary blood sample in which concentrations and properties of cellular and extra-cellular constituents remain relatively
unaltered when compared with their in vivo state.
- Serum is the undiluted, extracellular portion of blood after adequate coagulation is complete. Historically, serum was the preferred assay material for determining extracellular concentrations of constituents in blood.
- Plasma is defined as the virtually cell-free supernatant of blood containing anticoagulant obtained after centrifugation. Today, plasma is preferred for many, but not all , laboratory investigations because the constituents in plasma reflect better the pathological situation of a patient than in serum.
More about plasma versus serum comparison.
Advantages of plasma |
Disadvantages of plasma |
Time saving
plasma samples can be centrifuged directly after sample collection, unlike serum, in which coagulation is completed after a minimum of 30 minutes |
Contamination with cations from anticoagulants
which change the concentration of the constituents to be measured (NH4+, Li+,Na+,K+) |
Higher yield
15-20% more in volume of plasma than of serum can be isolated from the same volume of blood |
Interferences due to heparin
- inhibition of catalytic reactions by heparin, e.g. Taq polymerase in the PCR
- binding of ionized calcium to heparin
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Prevention of clogged analyzer needles
which may lead to erroneous low results |
Interference by fibrinogen
in heterogeneous immunoassays and in electrophoresis |
Avoids serum increases - in the concentration of platelets components (e.g. potassium, phosphate, magnesium, ASAT, LDH, serotonin, neurone-specific enolase, zinc)
- in amide-NH3 from fibrinogen induced by action of clotting factor XIII
- in components resulting from the cell-lysis of erythrocytes and leucocytes in non-coagulated blood (free hemoglobin, cytokines, receptors)
|
Assay interference caused by metals complexing with EDTA and citrate - inhibition of alkaline phosphatase activity by zinc binding
- inhibition of metallo-proteinases
- inhibition of metal-dependent cell activation in function tests
|
Avoids serum decreases
of some constituents as a result of cellular metabolism and/or coagulation
process (fibrinogen, glucose, total protein, platelets) |
Interference
in the distribution of ions between the intracellular and the extracellular space (e.g. Cl-,NH4+) by EDTA, citrate |
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