Why a Postmortem Drug Screen Cannot Replace Comprehensive Toxicology

By Okorie Okorocha, JD, MS, MS

A rapid drug screen can help a forensic pathologist decide what to investigate next. It cannot, by itself, establish every drug present, quantify those drugs, or determine whether a drug caused or contributed to death.

That distinction is especially important in postmortem toxicology. Specimen availability, decomposition, redistribution, therapeutic medications, drug metabolites, assay cutoffs, and the limited menu of a screening device all affect the evidence.

A rapid screen asks a limited question

Most point-of-care drug panels use immunoassays. Each channel is designed to respond to a drug class, representative compound, or metabolite above a defined cutoff. A negative result means that the device did not produce a positive response on that channel under the testing conditions. It does not mean that the decedent’s specimens contained no drugs.

The panel may omit the substance entirely. It may respond weakly to a particular member of a drug class. The concentration may fall below the cutoff. Postmortem specimens may also differ from the urine specimens for which a device was designed and validated.

Comprehensive methods detect drugs outside the panel

Tominaga and colleagues reviewed 2,996 forensic autopsies over 18.5 years at one institute. Their workflow used on-site immunoassay screening and GC/MS, with additional GC/MS and LC-MS/MS work in selected cases during the later years.

The rapid drug-of-abuse screen was negative in 2,440 cases. GC/MS nevertheless detected other drugs in 218 cases, including antipsychotic medications, acetaminophen, and salicylic acid. Additional LC-MS/MS testing identified low benzodiazepine concentrations in 32 cases and other medications in a fatal-abuse case.

Those figures describe one institution’s historical workflow, not the current sensitivity of every test. The broader lesson is durable: a limited immunoassay menu and a systematic chromatographic-mass-spectrometric examination answer different questions.

Alternative postmortem specimens require separate validation

Urine is not always available at autopsy. Investigators may consider pericardial fluid, vitreous humor, blood, tissue, or other matrices. A device validated for urine should not automatically be assumed to perform the same way in another specimen.

Tominaga, Michiue, and Maeda compared three on-site devices using 51 urine and 19 pericardial-fluid specimens from 66 autopsy cases. Performance differed by device and drug channel. For benzodiazepines, one product showed higher positive predictive value and specificity, while another showed higher sensitivity but more false positives. The investigators still treated the results as preliminary information before instrumental analysis.

The study excluded cases with evident decomposition and used a small, selected specimen set. It does not create a universal rule for pericardial fluid or establish that one device is always superior.

Whole blood cannot simply be placed on a urine or saliva device

Color, turbidity, proteins, and cellular material can interfere with lateral-flow and optical test systems. Toubou and colleagues developed a laboratory preparation method that acidified and ultrafiltered whole blood before applying the filtrate to an Oratect III device normally used for oral fluid.

That work was an optimized experimental method. It does not validate direct application of untreated blood to the product. It also does not turn a qualitative screen into definitive identification and quantification. A case relying on an adapted device should disclose the preparation protocol, validation data, controls, detection limits, and comparison method.

Cutoffs and drug-specific response create false negatives

Wu and colleagues compared a historical Triage urine panel with EMIT in 606 positive and 325 negative specimens. Most results agreed, but discordant samples showed why the cutoff and the compound matter. Nineteen of 27 specimens that were EMIT-positive and Triage-negative for amphetamines contained total amphetamine and methamphetamine below 1,000 ng/mL by GC/MS.

The study involved 1993 technology, and not every discordant specimen had sufficient volume for confirmation. Its appropriate use is not to assign present-day accuracy. It demonstrates that one immunoassay can be negative while another screen is positive and an instrumental method detects analyte below the first device’s decision threshold.

Reader objectivity does not solve chemical specificity

Some devices are visually read, while newer systems use an automated reader. Automated interpretation can reduce subjectivity in deciding whether a line or reaction is present. It does not expand the analyte menu or eliminate antibody cross-reactivity and cutoff effects.

The 1996 clinical evaluation by de la Torre and colleagues found that readers with different laboratory experience produced broadly comparable results after training. Performance still varied by drug class, and discordant results required chromatographic investigation. Operator competency remains important, but a trained reader cannot make an antibody detect a compound that the assay does not recognize adequately.

Detection is not causation

Even definitive identification of a drug does not automatically show that it caused death. Interpretation may require:

1. the measured concentration and the specimen in which it was measured;

2. collection site, timing, preservation, and storage;

3. postmortem interval and evidence of decomposition;

4. redistribution and matrix-specific stability;

5. parent drug and metabolite patterns;

6. tolerance, medical history, and prescribed dosing;

7. other drugs, alcohol, disease, trauma, and scene evidence; and

8. method validation, calibration, quality control, and uncertainty.

A screening result can guide testing. Cause-of-death analysis requires the complete toxicology record together with the autopsy and investigative evidence.

Records to request in a postmortem drug-screen case

The review should obtain:

1. every specimen collected, its source, volume, preservative, and storage history;

2. the rapid-screen manufacturer, model, lot, expiration date, package insert, and cutoff;

3. documentation showing whether the specimen type was validated for that device;

4. preparation steps, photographs, instrument printouts, and operator notes;

5. positive, negative, and matrix-control records;

6. the complete GC-MS, LC-MS/MS, or other definitive method and validation package;

7. calibrators, controls, chromatograms, spectra, transitions, ion ratios, and integration records;

8. quantitative results, reporting limits, dilution records, and uncertainty information;

9. chain-of-custody and specimen-identity records; and

10. the autopsy report, scene investigation, medical history, and medication records.

The scientifically defensible conclusion should match the method. A rapid postmortem screen provides preliminary, panel-limited information. Comprehensive forensic toxicology identifies and measures relevant compounds so that the results can be interpreted with the rest of the case.

References

Tominaga M, Michiue T, Inamori-Kawamoto O, et al. Efficacy of drug screening in forensic autopsy: retrospective investigation of routine toxicological findings. Legal Medicine. 2015;17(3):172-176. https://doi.org/10.1016/j.legalmed.2015.01.001

Tominaga M, Michiue T, Maeda H. Evaluation of the on-site immunoassay drug-screening device Triage-TOX in routine forensic autopsy. Legal Medicine. 2015;17(6):499-502. https://doi.org/10.1016/j.legalmed.2015.10.007

Toubou H, Namera A, Arima Y, et al. Detection of abused drugs in human blood by using the on-site drug-screening device Oratect III. Legal Medicine. 2014;16(5):308-313. https://doi.org/10.1016/j.legalmed.2014.04.004

Wu AHB, Wong SS, Johnson KG, et al. Evaluation of the Triage system for emergency drugs-of-abuse testing in urine. Journal of Analytical Toxicology. 1993;17(4):241-245. https://doi.org/10.1093/jat/17.4.241

de la Torre R, Domingo-Salvany A, Badia R, et al. Clinical evaluation of the Triage analytic device for drugs-of-abuse testing. Clinical Chemistry. 1996;42(9):1433-1438. https://pubmed.ncbi.nlm.nih.gov/8787700/

Educational information only. It is not a case-specific toxicology opinion or legal advice.