Certified Reference Materials in Forensic Toxicology

Quick answer: a certified reference material (CRM) is a reference material characterized by a metrologically valid procedure and accompanied by documentation that states one or more property values, the associated uncertainty, and metrological traceability. In forensic toxicology, CRMs can support calibration, method validation, identity confirmation, and quality assurance—but simply purchasing or using a CRM does not automatically make a laboratory result traceable, accurate, or fit for purpose.

The distinction matters when reviewing quantitative toxicology results. The defensibility of a reported concentration depends not only on the label on a reference standard, but on how the laboratory prepared calibrators and controls, documented lot information, handled storage and expiration, accounted for uncertainty, and connected the material to the actual measurement procedure.

Reference material, CRM, and NIST SRM are not interchangeable terms

A reference material is a material sufficiently homogeneous and stable for an intended use in a measurement procedure. A certified reference material adds a higher level of characterization: the certified property value is established by a metrologically valid procedure and is accompanied by a certificate providing the value, its uncertainty, and a statement of metrological traceability.

A NIST Standard Reference Material (SRM) is a specific class of CRM certified by NIST. SRMs provide at least one certified property value that is metrologically traceable to an internationally recognized higher-order reference system and are intended for uses such as calibration or validation of measurement procedures.

Why CRMs matter in forensic toxicology

Forensic toxicology laboratories often measure drugs, metabolites, alcohols, poisons, or other analytes across wide concentration ranges. A defensible quantitative result requires an appropriate calibration system. Reference materials may be used to prepare primary or working solutions, calibrators, controls, or other materials used to establish or verify method performance.

CRMs can contribute to:

assignment of known analyte concentrations to calibration materials;
method validation and verification;
assessment of bias and accuracy;
confirmation of analyte identity where appropriate reference materials are used;
quality-control programs and inter-lot comparisons; and
the documented calibration chain supporting metrological traceability.

A CRM does not automatically make a result traceable

NIST expressly cautions that buying or using an SRM does not, by itself, make a user’s measurement results traceable to NIST. Traceability is a property of a measurement result. The laboratory must establish and document the chain connecting the result to specified references, including the relevant calibration steps and associated uncertainties.

That means a reviewer should distinguish between a vendor statement that a standard is “NIST traceable” and evidence that the reported toxicology result itself has a defensible metrological chain. The relevant question is how the laboratory used the material within its measurement system.

Certificate review matters

A CRM certificate can contain information that is directly relevant to forensic review. Depending on the material, useful items may include the certified analyte value, concentration basis, uncertainty, traceability statement, lot number, preparation or certification date, expiration or period of validity, storage conditions, minimum sample size, matrix, and instructions or limitations on intended use.

For a quantitative drug assay, the certificate should be evaluated alongside the laboratory’s own preparation records. A correct certified concentration can still be implemented incorrectly through dilution error, unit conversion error, evaporation, degradation, use outside the stated validity period, or preparation into a matrix that behaves differently from the case specimen.

CRMs, calibrators, and controls serve different functions

A reference material is not the same thing as a calibrator or quality-control sample. A CRM may be used to prepare either, but the roles are different. A calibrator establishes the relationship between instrument response and concentration. A control challenges the measurement process independently of the calibration model and helps determine whether the analytical run is performing acceptably.

Using the same stock source for both calibrators and controls can reduce independence if an error in that source or preparation affects both in the same direction. Forensic laboratories therefore need documented quality-control procedures governing material selection, preparation, and use. ANSI/ASB Standard 054 addresses minimum quality-control requirements for forensic toxicology laboratories, including materials used to prepare calibrators and controls.

Matrix and preparation can be as important as certification

A CRM may have excellent certified properties and still be used in a way that does not adequately model the analytical behavior of the case specimen. Matrix effects, extraction recovery, ion suppression or enhancement, protein binding, pH, and endogenous interferences can alter method performance.

That is why the provenance of the stock solution should be reviewed together with the laboratory’s approach to matrix-matched calibration, extraction, internal standards, and validation. Certification of the source material does not eliminate matrix-specific analytical problems.

Lot changes deserve documentation

When a laboratory changes lots or vendors of reference material, the transition should be controlled. Lot-to-lot differences, concentration reassignment, solvent changes, or preparation differences can affect downstream calibration. The records should make it possible to identify which lot was used for the analytical batch associated with a particular case.

For case review, useful records may include certificates of analysis, lot numbers, receipt and opening dates, storage logs, dilution worksheets, preparation records, expiration information, vendor documentation, and any lot-verification or comparison data.

Uncertainty belongs in the chain

A CRM certificate typically reports uncertainty for the certified value. That uncertainty is not necessarily the same as the uncertainty of the final toxicology result. Additional uncertainty can arise from volumetric preparation, calibration modeling, extraction, instrument response, repeatability, matrix effects, and other steps in the laboratory process.

Accordingly, the uncertainty stated on the CRM certificate should be treated as one component of the broader measurement system rather than as a substitute for evaluating the uncertainty of the reported result.

Questions to ask when reviewing a forensic toxicology case

What reference material or stock solution was used to prepare the calibrators?
Was it a CRM, an SRM, or another type of reference material?
What analyte, concentration, solvent, matrix, and lot are identified on the certificate?
What uncertainty and traceability statement accompanies the certified value?
Was the material within its stated validity period and stored as required?
How were intermediate and working solutions prepared, and are the dilution calculations documented?
Were calibrators and controls prepared independently enough to detect preparation error?
Was the calibration matrix appropriate for the case specimen?
Can the laboratory identify the exact lot used in the analytical batch?
How does the reference-material chain connect to the laboratory’s claimed uncertainty and traceability of the final result?

Authoritative references

Bottom line

A certified reference material can provide a well-characterized anchor for a forensic toxicology measurement, but it is only one link in the analytical chain. The reliability of the reported concentration depends on the certificate, preparation records, calibration design, matrix, quality controls, lot traceability, and the laboratory’s documented measurement process. A strong forensic review follows that chain from the reference material all the way to the reported result.