In which scenario is serum preferred over plasma for laboratory testing?

• A. Tests where clotting factors may interfere with the assay
• B. Tests requiring clotting factors
• C. Tests requiring anticoagulants
• D. Tests measuring fibrinogen

Answer: (A)

When blood is allowed to clot and then is separated, the liquid that remains (serum) has had its clotting factors used up. That means serum contains little to no fibrinogen and other clotting proteins, whereas plasma, obtained with an anticoagulant present, still contains those factors. If an assay can be affected by the presence of clotting factors, using serum reduces potential interference and yields cleaner results. So serum is preferred for tests where those clotting components might skew the measurement.

If an analysis requires the clotting factors to be present (for example, to assess factor levels), serum would not be suitable and plasma would be the better choice. Tests that measure fibrinogen specifically require plasma, because fibrinogen is removed during serum formation and would not be accurately measured in serum. Similarly, if you need to preserve anticoagulant conditions in the specimen, you collect with an anticoagulant and use plasma.