EVIDENCE LIBRARY
Forensic Toxicology Specimen Collection and Preservation: Preanalytical Case Review
A forensic toxicology guide to specimen collection, preservatives, containers, storage, transport, sampling site, and preanalytical records that can affect interpretation.
Direct answer: a toxicology result is only as defensible as the specimen history supporting it. Before interpretation begins, the reviewer should establish what specimen was collected, from where, into what container, with which preservative or anticoagulant, at what time, under what storage and transport conditions, and whether those conditions were appropriate for the analyte actually reported. Preanalytical defects do not automatically invalidate a result, but they can change what the result is capable of proving.
This page focuses on specimen collection and preservation rather than analytical method validation. For validation records, see our Validation Package Review. For analyte-specific degradation questions, see Specimen Stability in Forensic Toxicology.
Why preanalytical review matters
Modern LC-MS/MS, GC-MS, and headspace methods can be highly selective and precise, yet the instrument only measures the aliquot presented to it. The specimen may have experienced biological change, evaporation, contamination, redistribution, hydrolysis, adsorption, microbial activity, repeated freeze-thaw cycles, dilution, or simple labeling and source ambiguity before analysis. A technically valid assay does not reconstruct an undocumented specimen history.
ANSI/ASB Best Practice Recommendation 156-23 addresses collection amounts, preservatives, containers, and storage conditions for forensic toxicology specimens. NIST lists the document in the OSAC Registry, making it a useful benchmark when evaluating whether collection and preservation practices were fit for purpose.
Core questions for a specimen-integrity review
| Issue | Records to examine | Why it matters |
|---|---|---|
| Specimen identity | Tube label, accession record, collection form, barcode history | Confirms the analyzed aliquot is traceable to the intended specimen. |
| Specimen type and source | Peripheral vs central blood, serum/plasma, urine, vitreous, tissue | Concentrations and interpretation can differ by matrix and sampling site. |
| Collection time | Draw time, death time, autopsy time, receipt time | Defines the biological and post-collection interval relevant to interpretation. |
| Container | Tube manufacturer, lot, material, closure, fill volume | Some analytes are sensitive to headspace, adsorption, contamination, or container chemistry. |
| Preservative / anticoagulant | Tube specification, expected additive, receipt observations | Important for microbial control, clot prevention, ethanol preservation, and unstable drugs. |
| Temperature history | Refrigerator/freezer logs, shipping record, evidence-room record | Stability is analyte-, matrix-, concentration-, and time-dependent. |
| Freeze-thaw history | Aliquot log, rerun dates, transfer records | Repeated cycles may affect some analytes or matrices and should be compared with validation data. |
| Condition on receipt | Leakage, clotting, hemolysis, tube fill, seal integrity | Documents whether the laboratory received the specimen in the condition assumed by the method. |
Specimen source is not a clerical detail
In postmortem work, a label such as “blood” may be insufficient. Peripheral and central blood can produce different drug concentrations because of postmortem redistribution and local contamination. A reviewer should determine whether the source was femoral, iliac, subclavian, heart, cavity, or otherwise documented. See Postmortem Blood Sampling Sites: Interpretation Guide and Postmortem Redistribution: Why C/P Ratios Are Not Correction Factors.
For living subjects, the distinction between whole blood, serum, and plasma also matters. A concentration generated in serum or plasma should not silently be treated as though it were a whole-blood concentration. Matrix-specific calibration and interpretation should be documented.
Preservatives and anticoagulants
Preservation requirements depend on the specimen and analytical purpose. Current professional best-practice literature, citing ASB 156, recommends preserved postmortem blood and refrigerated storage for toxicological investigation. For ethanol, fluoride is particularly important because microbial activity can create or destroy ethanol after collection under some conditions. The presence of a gray-top tube does not itself establish that the correct additive concentration, fill ratio, temperature history, or storage duration was achieved.
The correct question is therefore not merely “Was preservative present?” but: Was this specimen preserved and stored in a manner validated or otherwise scientifically supported for this analyte, matrix, concentration range, and time interval?
Temperature and time must be evaluated together
“Refrigerated” and “frozen” are not universal guarantees of stability. Drug stability can vary with matrix, concentration, pH, preservative, container, storage duration, and number of freeze-thaw cycles. Stability data should match the conditions actually experienced by the case specimen as closely as practicable.
This point is especially important for emerging drugs. A 2026 study of xylazine in preserved antemortem and postmortem blood found that stability differed across room-temperature, refrigerated, and frozen conditions and across concentrations. That is why a generic statement that a specimen was “kept cold” is not enough. See our Xylazine in Forensic Toxicology review for the analyte-specific discussion.
Headspace, fill volume, and volatile analytes
Volatile compounds present a distinct preanalytical problem. Container closure, headspace, repeated opening, and storage temperature can affect volatile loss. For ethanol and other volatiles, a reviewer should examine tube fill, whether the container was previously opened, the number of analyses performed, and whether the laboratory’s stability evidence addresses those conditions.
These factors are separate from instrument calibration. A perfectly calibrated headspace gas chromatograph cannot restore analyte that was lost before the vial was prepared.
Transport and evidence-room history
The laboratory accession date is not the beginning of the specimen’s history. In DUI, drug-facilitated crime, and other antemortem cases, hours or days may pass between collection and laboratory receipt. Relevant records may exist with the collecting hospital, law-enforcement agency, courier, property room, coroner/medical examiner, or toxicology laboratory.
A defensible review distinguishes documented conditions from assumed conditions. If there is no temperature record, the conclusion should normally be framed as an evidentiary limitation rather than converted into an invented temperature history.
Preanalytical problems do not all have the same consequence
The scientific consequence must be tied to a plausible mechanism. Depending on the analyte and facts, a defect may cause degradation and a falsely low result, post-collection formation and a falsely high result, increased uncertainty, inability to distinguish specimen sources, or no material analytical effect at all. The existence of a deviation is not a substitute for explaining its expected direction and magnitude.
This is why categorical statements such as “the tube was unrefrigerated, therefore the result is invalid” are often too broad. The better analysis asks what is known about the specific analyte under the specific conditions and whether the laboratory has case-relevant stability or robustness evidence.
Case-review checklist
- Identify every specimen tested and its exact anatomical or clinical source.
- Reconcile collection, accession, analysis, reanalysis, and storage dates.
- Obtain collection-kit or tube specifications when preservation is disputed.
- Determine whether the laboratory documented tube condition and fill on receipt.
- Request refrigerator/freezer logs when temperature history is material.
- Identify each aliquot, transfer, thaw, rerun, dilution, or re-preparation.
- Compare the case history with method-validation and stability studies.
- Check whether stability experiments used the same matrix, analyte concentration, preservative, container, and duration.
- Separate chain-of-custody questions from chemical-stability questions; they overlap factually but answer different scientific questions.
- State the direction of any expected bias only when supported by data.
What to request before accepting a specimen-integrity conclusion
At minimum, the useful record set may include the collection form, chain-of-custody documents, accession sheet, specimen-condition notes, tube or kit information, storage logs, aliquot history, extraction batch, instrument sequence, raw data, validation package, stability studies, SOPs in effect on the analysis date, and technical-review records. Our broader Toxicology Laboratory Records: Evidence Review Checklist organizes these materials.
Authoritative sources
- ANSI/ASB Best Practice Recommendation 156-23, Best Practices for Specimen Collection and Preservation for Forensic Toxicology.
- NIST OSAC Standards Library entry for ANSI/ASB 156-23.
- Application of professional best practices in postmortem forensic toxicology, Journal of Analytical Toxicology.
- Preanalytical factors influencing the results of ethanol analysis in postmortem specimens, Journal of Analytical Toxicology.
Technical review: Forensic Toxicology Working Group Members. This reference is intended to identify scientifically relevant records and limitations; interpretation remains case-, matrix-, analyte-, and method-specific.