Analysis of Clinical and Forensic Specimens for Cocaine and Metabolites

The identification of cocaine or its metabolites in clinical specimens assists patient management in clinical forensic medicine, and assists the pathologist in ascertaining cause of death.

Urine is convenient and easy to collect, and represents a longer chronological picture of drug use than does the analysis of blood or saliva. The half-life of cocaine is short, and cocaine and its metabolites remain in the urine for up to 14 days in a chronic user (Karch 1991(a) p.126). False negatives may result from bleach and salt added to specimens surreptitiously, and those individuals with cholinesterase deficiencies continue to have detectable levels for a long time.

Recent advances have been made in the analysis of hair for toxicological purposes, and Cone et al (1993 pp.55-68) describe the results of analyses of head and arm hair for cocaine and it’s metabolites. Concerns have been raised, however, about the issue of external contamination of hair shafts by smoke and sweat containing cocaine, and the implications for interpretation of positive analyses. Various pre-wash techniques have been described (Kintz et al 1995 pp.3-11 and Blank et al 1993 pp.145-156), but the matter is far from being resolved.

Analysis of samples

Specimens need to be collected and preserved in sodium fluoride (or another pseudocholinesterase inhibitor), and refrigerated. Storage in a buffer at pH 5 can also prevent cocaine metabolism prior to analysis. (Karch 1991(a)). Cocaine levels need to be interpreted with extreme care, because it is rapidly metabolised in blood, and continues to be broken down in post-mortem blood if incorrectly stored for analysis. (Wetli 1987p.2).

Commercially available test kits are available (e.g. EMIT d.a.u. – an enzyme based immunoassay kit) for screening urine for cocaine metabolites, and positive samples are confirmed using the ‘gold standard’ analytical techniques – gas chromatography/ mass spectrometry (GC/MS). (Cone et al 1990). GC/MS has a lower limit of detectability for cocaine in the blood of <0.5 mcg/L (or 0.5 mg/ml), whereas the therapeutic blood cocaine level is in the region of 3 mcg/L (or 3 mg/ml). (Corburt et al 1993 pp.136-149). Typical blood cocaine levels are found in Table 2. below.

Blood levels do not correlate well with psychological and physiological effects, unlike blood alcohol levels, as the measurement could represent a rising level associated with euphoria, or a falling level associated with dysphoria. A chronic user will also have built up tolerance to the effects of cocaine, and so a high blood concentration may not be interpreted as having any specific effect on his/her mental state at the time the sample was taken. Benzoylecgonine is not freely permeable across the blood brain barrier, and its presence at high levels in the brain may indicate cocaine metabolism in the brain, suggesting that the individual is a chronic abuser (Karch 2000 p.431).

Table 2.  Typical Blood Levels (Sources: adapted from Karch 1991(a) p.127 and Karch 1991(b) p.1)

An interesting development in the analysis of specimens for the presence of cocaine is the report by Nolte et al (1992 pp.1179-1185) of the use of the larvae of the blue bottle fly, Calliphora vicina as a ‘toxicological specimen’, after being found on a decomposed body. Cocaine was found in the larvae, and it is hoped that in the future, the pupal cases will also be capable of being utilised in the same way that hair is currently being examined.

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