Plasma or serum in therapeutic drug monitoring and clinical toxicology.

The relative merits of plasma and serum in blood analysis are reviewed. The expression 'plasma concentration' is often used in the literature, although serum samples have been taken. In most cases serum and plasma concentrations of analytes are the same. The choice depends mostly on the policy of the hospital or the availability of the test tubes in the ward. Some of the advantages of plasma over serum are large volume, no delayed clotting, less risk of haemolysis. In addition, the sample is often suitable for both whole blood and plasma monitoring. Some of the disadvantages of plasma over serum are the (unknown) influence of the anticoagulant on the assay, on the protein binding and on the stability of the sample, the (unknown) influence of additives or impurities in the anticoagulants on the assay and on the concentration, the risk of the formation of small clots and dilution of the sample.