in the assay to correct for systematic errors in the analysis
In quantitative work, assay performance is monitored by the systematic analysis of internal quality control (IQC) samples, independently prepared standard solutions of known composition prepared in the same matrix as the samples and not used in assay calibration. Plotting the results for the IQC samples on a chart allows the day-to day performance of the assay to be monitored and gives warning of any actual or impending problems. When new batches of calibration and IQC samples are prepared it is prudent to ensure comparability of the results obtained with those given by an earlier batch, or with the results obtained using external QC material.
An important parameter in the routine use of a method is measurement uncertainty (MU). This is expressed as ±1.96 CV %, in other words the 95 % confidence interval of the sum of the IQC CV values given by an assay over a defined period (Section 3.2.3.4). An especial issue concerns accuracy at a limit defined in legislation such as drink- and drugs-driving legislation. Here it is good practice to allow 3 SDs error (99.9 % of measurements will be either at, or above the limit) before reporting a positive result as a ‘not less than’ concentration.
Participation in appropriate external quality assessment (EQA)/proficiency testing (PT) schemes is also important. In such schemes, (sometimes lyophilized) plasma, serum, whole blood, urine, or hair specimens are sent to a number of participating laboratories. After reconstitution in deionized water if appropriate, the specimens are analyzed as if they were real samples and the results are reported before the true or target concentrations are made known (Section 3.6.2).
1.4 Applications of analytical toxicology
Requests for toxicological analyses include (i) emergency and general hospital toxicology, including ‘poisons screening’, and (ii) more specialized categories such as screening for substance misuse, forensic toxicology, TDM, and occupational/environmental toxicology. However, there is considerable overlap between all of these areas (Smith & Bluth, 2016).
1.4.1 Clinical toxicology
The specialized nature of analytical toxicological investigations and the expense of modern equipment dictate that facilities are concentrated in centres that are often remote from the patient. Frequently, routine clinical chemical tests will be performed at one site, whilst more complex toxicological analysis will be performed by a different department, possibly at a different location. The toxicology laboratory will usually undertake a range of analyses in addition to emergency toxicology.
Despite physical separation, the importance of direct liaison between the physician treating the patient and the analytical toxicologist cannot be over-emphasized (Flanagan et al., 2013; Thompson et al., 2014). Ideally, this liaison should commence before any specimens are collected because some analytes, toxic metals for example, require special precautions in specimen collection (Section 21.2). At the other extreme, residues of samples held in a clinical chemistry laboratory or by other departments, for example in the emergency department refrigerator or in the histology department, can be invaluable if the possibility of poisoning is raised in retrospect (Vuori et al., 2013).
Toxicology screening is normally performed using immunoassays and/or temperature programmed capillary GC-MS or gradient elution LC-MS. Proponents of STA sometimes overstate the case for absolute reproducibility of retention data. In practice many factors (clinical and circumstantial evidence, availability of a particular poison, past medical history, occupation, number of peaks present on the chromatogram, selective detector responses, MS fragmentation patterns, and so on) are considered before reporting results. For example, Grapp et al. (2018) positively identified drugs by accurate mass measurement (±5 ppm for [M+H]+; ±10 ppm for [M-H]–), LC retention time (±0.35 min), isotopic pattern match (less than 10 m/z root mean square error [RMS, ppm]), isotope match intensity (less than 20 % RMS) and the presence of at least two fragment ions.
The range of analyses that can be offered by specialized laboratories, sometimes on an emergency basis, usually encompasses several hundred poisons. ‘Poisons screens’ must use reasonable amounts of commonly available samples (20–30 mL urine, 2–5 mL plasma). If any tests are to influence immediate patient management, the (preliminary) results should be available within 2–3 h of receiving the specimens (ideally 1 h in the case of paracetamol). In some cases, the presence of more than one poison may complicate the analysis and examination of further specimens may be required.
A quantitative analysis carried out on whole blood or plasma is usually needed to confirm poisoning unequivocally, but this may not be possible if laboratory facilities are limited, or if the compound is particularly difficult to measure. It is important to discuss the scope and limitations of the tests performed with the clinician concerned and to maintain high standards of laboratory practice, especially when performing tests on an emergency basis. It may be better to offer no result rather than misleading data based on unreliable tests. Clinicians often treat poisoned patients on the basis of suspicion and history rather than await the results of a laboratory test, but may change their approach once they have the result. The treatment of paracetamol poisoning is an example.
Circumstantial evidence of the compound(s) involved in a poisoning episode is often ambiguous and thus, on the rare occasions when an analysis for ‘poisons’ is indicated, it is advisable to perform a ‘poisons screen’ routinely in all but the simplest cases. Similarly, the analysis should not end after the first positive finding because additional, hitherto unsuspected compounds may be present. An exception is sub-lethal carbon monoxide poisoning, which can be difficult to diagnose even if carboxyhaemoglobin measurements are available – circumstantial evidence of poisoning may prove invaluable in such cases. Of course, a positive result on a ‘poisons screen’ does not of itself confirm poisoning because such
