Failed Quality Control in Forensic Toxicology: Batch Review

Quick answer: A failed quality-control result does not automatically mean that every result in an analytical batch is wrong, but it does mean the laboratory must determine the scope and cause of the failure before affected case results are released. The correct response depends on what failed, which analytes and concentration ranges are implicated, whether the problem is qualitative or quantitative, and what the validated procedure requires.

This guide focuses on the technical review of failed QC in forensic toxicology. For a broader overview of how calibration, controls, blanks, and instrument performance are used to accept a run, see Batch Acceptance in Toxicology Testing. For the surrounding quality framework, see Quality Control in Forensic Toxicology and Technical and Administrative Review in Forensic Toxicology.

What does a failed QC actually mean?

Quality control is concurrent evidence about whether a method is operating within predetermined performance limits during the batch that produced the case result. A failed QC therefore raises a question about analytical fitness for that run. It does not, by itself, identify the cause or prove that every unknown specimen is unreliable.

The scope can be narrow or broad. A single analyte may fail while others remain acceptable. A low-concentration control may fail while mid- and high-range controls remain within limits. A quantitative criterion may fail even though the data still satisfy the laboratory’s validated qualitative identification rules. Conversely, a blank, carryover, calibration, or internal-standard problem can affect the interpretability of multiple specimens or an entire sequence.

Start by identifying exactly what failed

A defensible investigation begins with the specific acceptance criterion, not the generic statement that “QC failed.” Useful questions include:

Was the failure in a calibrator, positive control, negative control, blank, internal standard, or instrument-performance check?
Was the problem qualitative, quantitative, or both?
Did it affect one analyte, one transition, one concentration level, one matrix, or the entire method?
Was the failure isolated to a single injection, or did it persist after reinjection or re-extraction?
Did the laboratory follow a written decision rule for acceptance, rejection, reintegration, repeat analysis, or partial reporting?

The laboratory’s procedure and validation data control the answer. An analyst should not invent a new acceptance rule after seeing the case results.

For the larger reliability framework connecting validation scope to routine batch performance, see Validation Package Review in Forensic Toxicology.

Calibration failure: the curve may no longer support the reported number

Quantitative results depend on an accepted calibration model. ANSI/ASB Standard 054 addresses routine calibration acceptance, including how laboratories establish criteria for calibrator performance and when individual calibrators may be excluded. A calibration problem can therefore undermine the mathematical basis for quantitative results even when the instrument produced visually plausible peaks.

Case review should identify which calibrator failed, why it was excluded or retained, whether the remaining points still met the validated model requirements, and whether the questioned case concentration remained inside the accepted reportable range. A high-end calibrator failure may have different implications from a low-end failure near the reporting limit.

Positive-control failure: ask whether identity, quantity, or both failed

Positive controls are expected to demonstrate that the method can recover and identify the target analyte under the conditions of the batch. In quantitative testing, they also provide evidence that measurement performance remains within established limits.

A control can sometimes meet qualitative identification criteria while failing a quantitative acceptance limit. Current professional best-practice literature notes that, where the laboratory’s validated rules allow it, data may in some circumstances remain usable qualitatively even when quantitative acceptability is not met. That is not a blanket permission to downgrade every failed quantitative batch to “positive.” The laboratory must have a defensible, pre-existing decision rule, and the qualitative identification criteria must themselves be satisfied.

Negative-control or blank failure: contamination and carryover move to the front of the inquiry

A target analyte appearing in a negative control or blank may indicate contamination, carryover, mislabeled material, preparation error, or an interference. Sequence position is critical. If a high-concentration specimen or standard immediately preceded the affected blank or case sample, the reviewer should examine whether the method’s carryover controls were adequate and whether subsequent injections demonstrated that the system was clean.

For a detailed sequence-based review, see Carryover in LC-MS/MS Forensic Toxicology and Injection Sequence Review in Forensic Toxicology.

Internal-standard failure can be analyte- and sample-specific

Internal-standard response is often used to detect unusual extraction recovery, injection problems, ion suppression or enhancement, or other sample-specific analytical behavior. An abnormal internal-standard response should be evaluated against the laboratory’s validated acceptance rule and the behavior of the paired analyte.

The critical question is not simply whether an internal standard was present. It is whether the internal standard was suitable for the analyte, behaved as expected in the questioned specimen, and provided the correction that the quantitative model assumes. Stable-isotope-labeled standards can improve quantitative reliability, but they do not automatically correct every matrix effect, interference, degradation problem, or carryover event.

See Stable-Isotope Internal Standards in Forensic LC-MS/MS and How Matrix Effects Are Evaluated in LC-MS/MS Forensic Toxicology.

Does one failed analyte invalidate the entire batch?

Not necessarily. Modern multi-analyte methods may contain dozens or hundreds of compounds. A failure can be analyte-specific, transition-specific, concentration-specific, or global. The laboratory should determine the scientifically affected scope rather than assuming either extreme—that one failure invalidates everything or that the failure can simply be ignored.

For example, a poor low-level control for one analyte may call into question results for that analyte near the lower reporting range without affecting unrelated analytes whose controls met all criteria. By contrast, a preparation error, mislabeled calibrator, broad extraction failure, severe instrument malfunction, or contamination event may have batch-wide implications.

Can part of a batch be reported?

Sometimes, but only when the validated method and laboratory quality system support a scientifically separable conclusion. Partial reporting can include unaffected analytes, unaffected concentration ranges, or a qualitative result where quantitative criteria failed but identification criteria remained valid. The decision should be documented, technically reviewed, and traceable to written policy rather than improvised after the fact.

When a laboratory reports part of a batch after a QC problem, the case file should make clear what failed, what was repeated, what remained acceptable, and why the released result was considered unaffected.

Root-cause review should follow the data

Common areas to investigate include:

preparation or dilution errors;
calibrator or control preparation, lot, storage, expiration, or labeling;
matrix mismatch or unusual matrix effects;
pipette or volumetric-equipment performance;
extraction efficiency or hydrolysis problems;
instrument sensitivity, tuning, contamination, or maintenance;
chromatographic deterioration or co-elution;
carryover from high-concentration samples;
integration or peak-selection errors;
internal-standard failure or inappropriate internal-standard assignment; and
transcription, sequence, or sample-identification errors.

The investigation should distinguish a true analytical problem from an isolated processing or injection event. Repeating the same injection until a passing result appears is not a substitute for understanding why the original criterion failed.

Reinjection, re-extraction, and recollection answer different questions

Reinjection tests whether the existing extract produces reproducible instrumental behavior. Re-extraction repeats sample preparation and can address extraction, preparation, or extract-specific problems. Recollection, when possible, creates a new specimen and addresses problems that may originate before or during collection. These actions are not analytically interchangeable.

A reviewer should determine which step was repeated, whether the procedure authorized that response, whether the original failed result remained in the audit trail, and how the laboratory reconciled discordant results.

Records needed to evaluate a failed-QC batch

the analytical procedure and QC acceptance criteria in effect on the testing date;
the complete calibration data and any excluded calibrators;
positive, negative, blank, and process-control results;
internal-standard responses for controls and case specimens;
the injection sequence and carryover checks;
chromatograms, integrations, and raw transition data;
control and calibrator preparation records, source materials, lots, and expiration information;
instrument maintenance, tuning, and troubleshooting records;
repeat-injection, re-extraction, dilution, or alternate-method results;
the deviation, nonconformance, or corrective-action record; and
technical-review documentation explaining why results were accepted, rejected, repeated, or partially reported.

What a failed QC does not prove

A QC failure is evidence that a predefined analytical expectation was not met. It does not automatically prove contamination, analyst misconduct, an incorrect case result, or total batch invalidity. Those conclusions require investigation of the specific failure and its relationship to the case data.

The opposite is also true: an accredited laboratory, a passing final report, or a later successful repeat does not erase the need to evaluate a documented failure. The original event remains part of the scientific record and may be important to understanding how the laboratory reached its final conclusion.

Frequently asked questions

If one QC sample fails, must the whole run be discarded?

No universal rule applies to every failure. The laboratory must apply its validated acceptance criteria and determine the scientifically affected scope. Some failures are analyte- or range-specific; others can affect the entire batch.

Can a result be reported qualitatively when quantitative QC fails?

Potentially, if the qualitative identification criteria remain satisfied and the laboratory’s validated procedure permits that disposition. The decision should be documented and technically reviewed.

Does repeating the analysis cure the original QC failure?

No. A repeat can provide additional evidence, but the original failure should remain documented and its cause and impact should still be evaluated.

Why does the injection sequence matter?

Sequence position can reveal carryover or contamination risk, especially when a low-level case specimen follows a high-concentration standard, control, or unknown.

Primary technical sources

Editorial review: The Trillest: A Working Group of Forensic Toxicology and Pharmacology Experts from a Variety of Scientific Backgrounds & with a variety of Opinions. Reviewed September 2026.

Scientific education only. Case-specific conclusions require review of the laboratory’s complete procedures, validation, quality records, raw data, and the facts of the matter.