Collection and handling of clinical blood samples to assure the accurate measurement of cocaine concentration.

The measurement of serum cocaine concentration is difficult because it is rapidly metabolized in vivo and in vitro. Previous investigators have used sodium fluoride (0.5-3.0%) in an attempt to stabilize its concentration in samples of whole blood, but these concentrations of sodium fluoride are not readily available outside the analytical laboratory. This study was performed to assess the stability of cocaine in whole blood when stabilized in a commonly available concentration of sodium fluoride (0.25%), with or without concomitant refrigeration. Whole blood was enriched with cocaine to a final concentration of 900 ng/mL and added to flasks containing sodium fluoride (0, 0.25, 0.5, and 1.0%) or 0.25% sodium fluoride plus potassium oxalate (gray-top Vacutainer tubes), after which it was stored at 4 degrees C or at room temperature. Whole blood cocaine concentrations were measured at 0, 24, and 48 h by gas chromatography. Sodium fluoride at all concentrations, with or without refrigeration and potassium oxalate, effectively inhibited cocaine degradation, with 86-91% of the drug present after 48 h. In contrast, substantial degradation of cocaine occurred in the samples stored without sodium fluoride, regardless of temperature. Thus, the use of commercial gray-top Vacutainer tubes effectively stabilizes cocaine in blood during procurement, transport, and short-term storage.