hydrolyzed by plasma esterases, including cholinesterase. If physostigmine or cocaine are to be measured accurately the blood should be drawn into tubes containing an excess of neostigmine. Storage at –20 °C or below is recommended if the analysis cannot be performed immediately and if the stability of the analyte is unknown. However, even this may not be ideal because N- and S-oxides may be reduced to the parent compounds. Quinols such as 4-hydroxypropranolol, on the other hand, are readily oxidized.
Stabilization by addition of a reducing agent such as ascorbate or sodium metabisulfite has been advocated in some cases (Sørensen & Hasselstrøm, 2019), but is not performed routinely and in the case of olanzapine, for example, there is the possibility of reducing the N-oxide by such an addition. The monographs in the compendium by Baselt (2020) detail such stability data as there are for many compounds.
Storage at −5 to −70 °C should be accompanied by basic precautions to preserve sample integrity (Box 2.5). The requirements of the local ethics committee on the retention, storage, and safe disposal of clinical samples must be complied with.
Box 2.5 Guidance on freezer storage of samples
Do not freeze whole blood if plasma or serum is to be analyzed
Ensure that labelling is waterproof
Ensure tubes are tightly sealed and well filled, but do not overfill tubes, especially glass tubes
Do not keep too long to minimize freeze-drying effects
Keep a record of freezer contents and freezer cleaning
Keep a continuous record of freezer temperature via an electronic device
Use a spark-proof freezer if flammable materials may be stored
Fit an alarm in case of freezer failure
Implement a defined sample disposal policy in collaboration with stakeholders
2.5 Common interferences
Plasticizers, particularly phthalates, may originate from plastic bags used to store transfusion blood, infusion tubing, and from soft plastic closures for blood tubes. Such compounds are often retained on reverse-phase LC systems (Section 10.5.3) and show good absorption at 254 nm and below. Polyvinylchloride (PVC), for example, can contain up to 40 % (w/w) di-2-ethylhexylphthalate and concentrations of this latter compound of up to ca. 0.5 g L–1 have been reported after storage of plasma in PVC bags for 14 days. Phospholipids are notorious for supressing ionization in LC-MS assays (Fabresse et al., 2017). A further consideration is that post-mortem specimens may contain putrefactive bases such as phenylethylamines and indole(s) that may interfere in the analysis of amfetamines and other stimulants. Hexanal may arise from breakdown of fatty acids. GC retention data and mass spectra of a number of plasticizers, pollutants, and other substances that may be encountered in toxicological analyses have been reported (Maurer et al., 2016).
Drugs may arise from unexpected sources including food and over-the-counter (OTC) medicines. Quinine may originate from ingestion of tonic water or malaria prophylaxis, for example, caffeine from caffeinated beverages (tea, coffee, cola) and some proprietary stimulants, chloroquine, and related compounds from malaria prophylaxis, and pholcodine and other opiate analogues from cough and cold cures. Morphine is a constituent of some antidiarrhoeal preparations and is present in poppy seeds. A caffeine metabolite, paraxanthine, may be a problem in some LC theophylline assays.
Lidocaine-containing gel is commonly used as a lubricant during procedures such as bladder catheterization or bronchoscopy, and measurable plasma concentrations of lidocaine and some metabolites may be attained. Lidocaine and propofol are commonly used in association with emergency procedures, but these compounds may be misused together. Ketamine and morphine may also be given as part of emergency procedures. The alkaloids emetine and cephaeline, and their metabolites, have been detected in stomach contents, plasma, and in urine after syrup of ipecacuanha (ipecac) was given to induce vomiting, especially in children, although this form of treatment is no longer recommended practice.
Sedatives such as pethidine (meperidine) may be given prior to computerized tomographic (CT) scans, lumbar puncture, or other investigations. Neuromuscular blocking agents such as atracurium, which is metabolized to laudanosine, and vecuronium, may be encountered in samples from patients undergoing mechanical ventilation. 1,3-Propanediol (propylene glycol) is used as a vehicle in some i.v. infusions. Benzoic acid, which is metabolized to hippuric acid, is used as a preservative in some drugs and foods.
The antibiotic metronidazole is often encountered in samples from hospitalized patients. Medical imaging media may interfere in a range of assays (Lippi et al., 2014). Alcohols may originate from skin cleansing swabs. Such compounds and also drugs given in emergencies, amiodarone, anticonvulsants such as diazepam, and propofol, for example, may not be recorded on record sheets. Some compounds or their metabolites may have very long plasma half-lives. Chlorpromazine metabolites, for example, have been reported in urine many months after stopping therapy. Primary amines and even morphine, for example, can be acetylated in gastric contents if aspirin has been co-ingested (Naso-Kaspar et al., 2013).
Contamination with trace elements is a particularly difficult area (Chapter 21). Contamination with volatiles, such as solvents used in the laboratory, must be guarded against if one of the solvents in question is to be tested for in a biological or related sample. Glassware and other items must be kept clean and tested regularly for contamination via IQC procedures.
2.6 Summary
Although not in the immediate control of the laboratory, every effort must be made to ensure appropriate priority is given to sample collection and handling because if this is not done properly all subsequent effort is wasted. Care in sample collection is especially important in post-mortem and overt medico-legal work, but even in clinical work effort in providing advance information to clinicians and pathologists on sample requirements (site of collection, volume, addition of sodium fluoride, etc.) and feedback on the problems that will arise when mistakes are made can only prove beneficial.
References
1 Ameline A, Raul JS, Kintz P. Testing for midazolam and oxycodone in blood after formalin-embalmment: About a complex medico-legal case. Drug Test Anal 2019; 11: 1460–4.
2 Arora B, Velpandian T, Saxena R, Lalwani S, Dogra TD, Ghose S. Development and validation of an ESI-LC-MS/MS method for simultaneous identification and quantification of 24 analytes of forensic relevance in vitreous humour, whole blood and plasma. Drug Test Anal 2016; 8: 86–97.
3 Arslan Z, Athiraman NK, Clark SJ. Lithium toxicity in a neonate owing to false elevation of blood lithium levels caused by contamination in a lithium heparin container: Case report and review of the literature. Paediatr Int Child Hlth 2016; 36: 240–2.
4 Baselt RC. Disposition of Toxic Drugs and Chemicals in Man. Edition 12. Seal Beach: Biomedical Publications, 2020.
5 Belsey SL, Flanagan RJ. Postmortem biochemistry: Current applications. J Forensic Leg Med 2016; 41: 49–57.
6 Berk SI, Litwin AH, Du Y, Cruikshank G, Gourevitch MN, Arnsten JH. False reduction in serum methadone concentrations by BD Vacutainer serum separator tubes
