Being Positive About Positivism

Book reviewed in this article:
Matthew H. Kramer, In Defense of Legal Positivism: Law Without Trimmings     

A survey of extraction techniques for drugs of abuse in urine

Sixty nine participants in the United Kingdom national external quality assessment scheme for drugs of abuse in urine reported details of their sample extraction technique by questionnaire. Laboratories were categorised by differences in technique and their analytical test results compared for samples containing D-amfetamine 0.4 (4) and 0.8 (3) mg/l, morphine 0.4 (4) and 0.8 (4)mg/l, and benzoylecgonine 0.15/0.2 (2) and 0.45/0.5 (4) mg/l. Values in parentheses are numbers of samples. For amfetamine, there was no significant difference in the frequency of true positive results between liquid-liquid or solid phase extraction and the Toxi-Lab A system at 0.8 mg/l. Toxi-Lab A gave significantly fewer positives when operating below its specified threshold at 0.4 mg/l. Paradoxically, laboratories using >5 ml urine volume performed less well. Acidification of the extract before volume reduction gave significantly more true positives. For extraction of morphine, solid phase systems significantly outperformed both liquid-liquid and the Toxi-Lab A system at both 0.8 and 0.4 mg/l. No significant differences between extraction techniques were demonstrated for analysis of benzoylecgonine.     

External quality assessment of laboratory performance in analysis of toxicological cases

A mean of 44 members of the United Kingdom national external quality assessment scheme (UKNEQAS) for toxicology reported analytical findings on 10 toxicological cases circulated between December 1995 and February 2000. Material distributed usually consisted of a 5ml blood and a 20ml urine sample simulated by quantitative addition of drugs and their metabolites to material donated by volunteers and patients. The samples were accompanied by a brief outline of the circumstances surrounding the case. Laboratories were requested to report their analytical findings, list methods of analysis, and provide interpretation of their findings. The mean overall success rate for identification of drugs or their pharmacological group was 76%, failure being largely by laboratories providing an immunoassay-based screening service for a fixed range of drug groups. The latter laboratories indicated that cases would be referred to regional toxicology centres for further investigation or confirmation. The coefficient of variation of measurements was <7% for routine analytes, such as ethanol and paracetamol, but 26-44% for tricyclics and opiates. There were 3% false positive reports. The quantity and content of interpretative comment provided by the laboratories was very variable. A number provide nothing in addition to the analytical result.