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Blood and plasma concentrations

المؤلف:  Sue Jickells , Adam Negrusz (Editors)

المصدر:  Clarkes Analytical Forensic Toxicology

الجزء والصفحة:  p17-18

2026-09-13

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Blood and plasma concentrations

Many drugs show differences in concentration between whole blood and plasma (or serum). This occurs because the uptake of drugs into red blood cells (erythrocytes) can be limited by the physiochemical properties of the drug and its ability to move through cellular membranes. For example, THC is almost absent from red blood cells, and hence the plasma concentration is almost twice that of whole blood, assuming a haematocrit of 0.5. In contrast, chloroquine has a much higher red blood cell concentration than plasma concentration (plasma: whole blood ratio about 0.3). If this ratio is known, then the blood concentration can be estimated from a plasma concentration, or vice versa. Toxicologists should be wary of performing this calculation for haemolysed specimens, since haemolysis liberates the contents of red blood cells into the plasma (serum), and thereby affects this equilibrium. This applies particularly to postmortem specimens.