Quality control and quality assurance
Quality assurance deals with all aspects of lab oratory practice that might influence the accuracy of the final analytical result, and is dealt with elsewhere. Quality control (QC) includes the inclusion of material spiked with a known amount of a target analyte. The independently prepared material should be included in an assay to verify that the calibration is accurate within acceptable, defined limits. However, while the routine inclusion of proper QC material should be considered essential, the practice has not been adopted widely in postmortem toxicology testing. Certified reference materials (CRMs) are becoming more widely available, with the National Institute of Standards and LGC Promochem producing drugs of abuse and therapeutic drugs in blood, hair, serum, urine and saliva at certified values. CRM for ethanol has been available for some time. Although time-consuming, spiking analytes into drug-free whole blood with storage at 40C is an acceptable practice. Ideally, if the control material is prepared in house (in-house reference material), it should be prepared by a person other than the one running the assay, and from different stock material or at the very least from a different weighing of the same powdered stock material. It is not acceptable for an analyst to spike calibrators, and then spike the same solution into separate tubes and call these the controls. Any errors made in the preparation of the standard solution or in spiking calculations would not be uncovered using this approach. It is important that, for quantitative work, acceptance criteria for controls be set and that they be realistic neither too loose nor unrealistically strict. Failure to meet these criteria should invariably result in corrective action and, as necessary, repeat of the assay. Generally, controls for drugs and other toxins should read within 20% of their nominal value. For some analytes, such as ethanol, criteria such as 10% or tighter are more appropriate. If a control is targeted close to the LOQ for the assay, 30% may be accept able for drugs and other toxins. Many laboratories work to some form of set standards. This may be a regional, national or international standards authority. This body will set the criteria for analytical results. Participation in a suitable proficiency-test programme is another vital component of a good-quality assurance programme and goes a long way to demonstrating competence, as is the accreditation of a laboratory, and inevitably enhances the quality of postmortem toxicology analyses. In such test programmes, the testing laboratory receives material from the programme organiser. It will normally be supplied with information about the sample matrix. Information may or may not be given about the expected analytes. The test laboratory applies the appropriate analyses and then reports their results to the programme organisers. If the result is within accepted limits their performance is deemed acceptable. Testing lab oratories can use their success in such performance testing schemes as an assurance of their competence.
