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Analytical requirements , Measurement techniques

المؤلف:  Sue Jickells , Adam Negrusz (Editors)

المصدر:  Clarkes Analytical Forensic Toxicology

الجزء والصفحة:  252

2026-10-19

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Analytical requirements

Measurement techniques

A suitable analytical technique is obviously the bedrock of drug analysis, and studies to establish a correlation between plasma drug concentration and response cannot begin until a measure ment technique has been developed and validated. The final stage of establishing the clinical value of an assay is prospective randomized controlled trials that compare patients who have been monitored with a control group who have been managed without the aid of concentration monitoring. Unfortunately, there are virtually no drugs for which unequivocal evidence of the clinical benefit of monitoring has been obtained by large-scale trials. Lithium and phenytoin are the best examples. Much early pharmacokinetic knowledge was obtained from studies using colorimetry, spectrophotometry and spectrofluorimetry, but these methods have now been virtually abandoned for quantitative clinical work because of their lack of specificity. The exception is lithium, which is still frequently measured by atomic absorption or flame emission spectrophotometry, although ion-selective electrodes are displacing spectro metric techniques. Many antibiotics were once measured by bioassay, but this has poor specificity for combinations of drugs and is too slow, imprecise and labour-intensive for present day applications. Thin-layer chromatography is also little used for quantitative measurement of therapeutic concentrations. The three techniques that are most widely used for routine clinical measurement of drug concentrations are gas chromatography, high-performance liquid chromatography and immunoassays, with mass spectrometry rapidly gaining popularity. In the laboratory for TDM and Toxicology of the first author during 2005–2006, over 120 assays of drugs in serum were converted from HPLC-DAD or immunoassay into validated LC-MS/MS methods. Ion suppression seems to be the optimal analytical technique because of speed, flexibility, sensitivity, reliability and labour saving, and it requires only small sample volume. Selection of the most appropriate method for a given drug or clinical situation is not easy, and the choice depends on the avail ability of staff, expertise and equipment, the nature of the service to be provided and the range of drugs to be assayed. The widely different chemical natures of the substances to be assayed for TDM purposes means that it is rarely possible to offer a comprehensive service based on a single analytical principle. Also, compromises will usually have to be made between using the best method for each individual analyte and using techniques that allow quantification of a wide range of substances. The basic requirements of any method are that it should be specific for the substance being assayed (without interference from structurally related compounds or endogenous plasma components), capable of precise quantification, and sufficiently sensitive to detect concentrations normally found in therapy in a sample small enough for clinical work (certainly less than 1 mL plasma and ideally 10–100 μL).