Injection Sequence Review in Forensic Toxicology

Injection-sequence review asks whether the order of testing changes how a questioned toxicology result should be interpreted. The issue becomes especially important when a low-level or borderline result follows a specimen containing a much higher concentration of the same analyte. The reviewer should examine the immediately preceding injections, intervening blanks or washes, the concentration challenge used during validation, and any repeat analysis performed in a different sequence.

This page focuses on the case-review workflow for sequence-dependent results. For the broader analytical science of residual analyte transfer, instrument surfaces, washout, validation design, and mitigation, see Carryover in LC-MS/MS Forensic Toxicology. Carryover is not the same as contamination in the broad sense. It is a sequence-dependent analytical problem: the history of what was tested immediately before a specimen may affect the result that follows. A defensible interpretation therefore considers the validation study, the run order, the concentration of preceding samples, the laboratory’s carryover controls, and what happened when a blank or low-level specimen followed a high-concentration sample.

Why carryover matters

A high-concentration specimen can leave residual analyte in an autosampler needle, injection port, tubing, analytical column, extraction device, or other part of the workflow. If the next specimen is blank or contains only a small amount of analyte, the residual signal may be mistaken for genuine analyte or may inflate the measured concentration.

The risk is especially important near the laboratory’s reporting limit, limit of detection, limit of quantitation, or decision cutoff. A signal that is trivial compared with a very high positive sample may be significant when it appears in a low-level specimen.

What validation should establish

ANSI/ASB Standard 036 establishes minimum practices for validating forensic toxicology methods and emphasizes demonstrating that a method is fit for its intended use. Carryover is one of the performance characteristics traditionally evaluated during toxicology method validation because it can affect both qualitative identification and quantitative accuracy.

A common validation design challenges the method with a high-concentration specimen and then analyzes a blank matrix specimen immediately afterward. The laboratory determines whether analyte signal appears in the blank and whether that signal exceeds the laboratory’s acceptance criterion. Older SWGTOX guidance described this directly and required laboratories to determine the highest concentration at which no unacceptable carryover was observed.

The key point is that a carryover claim is only as broad as the experiment that supports it. A validation performed only at the top calibrator does not automatically establish what happens after a specimen far above the calibration range.

High-concentration specimens can exceed the validation challenge

Case specimens sometimes contain analyte concentrations well above the validated calibration range. Laboratories may dilute and reanalyze those specimens for quantitation, but the instrument is still exposed to the original extract or injection conditions unless the workflow prevents that exposure. If a specimen greatly exceeds the concentration used in the laboratory’s carryover study, the validation record should be examined before assuming that subsequent samples are unaffected.

This is why carryover review is closely connected to dilution integrity, calibration modeling, and the laboratory’s rules for handling results above the upper limit of quantitation.

Run order is evidence

The analytical sequence can be critical. A toxicology report usually gives the final result but may not show which specimen was injected immediately before the questioned sample. The batch sequence, worklist, acquisition file timestamps, and instrument logs may answer that question.

When a low positive follows a very high positive, the reviewer should determine whether a blank, wash, quality-control sample, or reinjection occurred between them. A carryover blank that is clean is reassuring only if it was positioned where the relevant sequence risk actually occurred.

Carryover is method- and analyte-specific

Carryover behavior can vary with analyte chemistry, concentration, sample preparation, chromatographic conditions, instrument configuration, needle-wash solvents, injection volume, and matrix. A laboratory should not assume that acceptable carryover performance for one analyte proves equivalent performance for every analyte in a multi-analyte panel.

Likewise, a method can perform acceptably during validation and later develop a carryover problem because of maintenance issues, contamination, worn components, column history, or changes in the sample population. Routine quality-control practices remain important after validation.

How laboratories manage carryover

When carryover cannot be completely eliminated, laboratories typically adopt procedural safeguards. These may include stronger needle washes, additional blank injections after unusually high samples, reinjection of affected specimens, re-extraction, dilution protocols, maintenance steps, or rules that prevent reporting a low-level result until carryover has been excluded.

The specific safeguard matters more than the label. A policy that says the laboratory “monitors carryover” should be evaluated against the actual batch records to determine what was monitored, where the controls were placed, and what action limits applied.

Questions to ask when reviewing a case

What concentration was used to challenge carryover during validation?
Was the questioned specimen tested immediately after a high-concentration sample?
How high was the preceding concentration compared with the validated carryover challenge?
Was a blank or wash injection placed between the two samples?
What was the signal in that blank?
What acceptance criterion does the laboratory use for carryover?
Was the questioned result near the LOD, LOQ, reporting limit, or cutoff?
Was the sample re-injected or re-extracted in a different sequence?
Were maintenance or contamination issues documented in the batch or instrument logs?

A low-level result requires context

Carryover does not mean every low toxicology result is unreliable. It means that low-level findings should be interpreted in the context of sequence history and demonstrated method performance. If the laboratory can show that the preceding sample was within the validated challenge range, the intervening blank was acceptable, and repeat analysis reproduced the result in a carryover-controlled sequence, the evidence against carryover becomes stronger.

Conversely, when a questioned low result follows an extreme positive outside the validation challenge and the batch lacks an informative blank or repeat analysis, the laboratory record may not support a confident exclusion of carryover.

Records worth requesting

A focused carryover review usually requires more than the final toxicology report. Useful records include the complete batch sequence, calibration and quality-control results, high-positive specimen results, blank injections, carryover validation data, standard operating procedures, dilution records, reinjection records, maintenance logs, and the underlying chromatographic or mass-spectral data. See the broader toxicology laboratory records checklist for additional items.

Bottom line

Carryover is a sequence-dependent source of analytical error. The correct forensic question is not simply whether a laboratory validated carryover once, but whether the validation challenge and the actual batch conditions support excluding carryover in the specimen at issue. That requires comparing the questioned result with the preceding sample, the blank or wash sequence, the laboratory’s acceptance criteria, and any repeat testing.

Injection Sequence Review in Forensic Toxicology: High Positives, Blanks, and Low-Level Results