ABC Forensic DNA common mistakes
ABC Forensic DNA · Understand the traps and how to avoid them.
Starts sampling on the most visually obvious stain and never examines the item under alternate light sources or samples contact areas such as collars, cuffs, waistbands, or ligature zones.
The vivid stain anchors attention and feels like 'the evidence'; under turnaround pressure the analyst stops screening once one candidate is found and assumes one item carries one stain.
What to do instead
Use a written screening sequence: ALS/white-light examination first, sketch and photograph every candidate area with a scale, then collect in a documented priority order that includes contact areas and a nearby unstained substrate control, recording why each area was or was not sampled.
Consumes an entire stain, swab head, or cutting in extraction instead of splitting it, leaving nothing for re-testing, re-amplification, or a defense request.
The instinct that more material yields a stronger profile, reinforced by training items where the whole swab was used and no one asked for a retest.
What to do instead
Apply a split-sample rule: photograph with a scale, sample or cut approximately half, and retain the remainder refrigerated with a location note; state on the worksheet how much was consumed and what remains available.
Places an oversized fabric cutting or a thick, dyed substrate into the digestion, producing a dark, viscous extract that later performs poorly.
The assumption that more substrate means more DNA, with little appreciation for how much dye, detergent, and humic material co-extracts and inhibits downstream enzymes.
What to do instead
Follow the laboratory substrate sizing table (a small punch for fabric, scrapings for dried stains), select extraction chemistry validated for that substrate, and record substrate type and size on the extraction worksheet so the choice is auditable.
Reads only the DNA concentration from the quant run, ignores a suppressed or failed internal control, and loads the default template volume into amplification.
The quant value is treated as the assay's only output, and an inhibited sample with a plausible-looking concentration is easy to wave through.
What to do instead
Interpret quantitation as two results, quantity and quality: when the internal control is inhibited or the value sits at or below the lowest standard, dilute and re-quantitate, concentrate, or switch to an inhibitor-tolerant master mix, and document the decision before amplifying.
Calls a small peak that appears at the same position as a much taller peak in a different dye channel as a genuine allele.
Spectral pull-up mimics a real peak at exactly the size an analyst is scanning for, and the review is often done on peak tables rather than on the raw data across all channels.
What to do instead
Before calling, compare the candidate to the tallest peak in every channel at that scan position; pull-up appears in a fixed height ratio and disappears on dilution or re-injection. Re-inject diluted product and apply the laboratory's documented pull-up criterion.
Treats a split peak or a peak migrating one base below the ladder allele as a separate allele, or miscalls a single allele as a heterozygote.
Incomplete 3' adenylation and off-ladder mobility shift create shoulders and doublets, and at compressed viewing scale the artifact looks like a second allele.
What to do instead
Review raw data at a scale that shows peak morphology, apply the laboratory's +A/-A and off-ladder rules, and re-inject before finalizing; an off-ladder designation requires a documented interpretation decision rather than an automatic bin call.
Reports a combined probability of inclusion or a random match probability for a multi-contributor mixture as though the profile were single-source.
Software output is convenient and familiar, CPI sounds conservative, and the distinction between an inclusion probability and a likelihood ratio conditioned on known contributors is often muddled under time pressure.
What to do instead
Establish the number of contributors and decide conditioning first; when a person of interest is a known contributor, compute the likelihood ratio conditioned on that proposition. Reserve CPI for profiles where it is valid under the laboratory's interpretation model, and state every assumption in the report.
Assigns allele designations to peaks below the laboratory's stochastic threshold and then feeds those designations into the statistical calculation.
The analytical threshold and the stochastic threshold are routinely conflated; any visible peak feels callable, and low-template mixtures are exactly where such peaks appear.
What to do instead
Check the validated stochastic threshold before making any call. Peaks below it are not interpretable in a binary model; handle them with a probabilistic genotyping or drop model that explicitly accounts for dropout, and never include them in a CPI-type calculation.
Releases a report in which the negative amplification control or reagent blank shows a peak, attributing it to 'background' without opening a nonconformance or investigating the batch.
Deadline pressure plus the belief that a small control peak cannot matter; controls are run routinely and are sometimes reviewed superficially at sign-off.
What to do instead
Treat any control peak as a trigger: hold the batch, review reagent lots, extraction negatives, and recent sample history, re-amplify from retained extract, and document the investigation and its outcome; do not report affected data until the source is resolved.
Writes 'the DNA matches the suspect' in the report, or states a conclusion in testimony that goes beyond the written statistical result and its stated limitations.
Case narrative and courtroom framing push toward plain-language certainty, and the conclusion paragraph is often drafted less carefully than the numbers behind it.
What to do instead
Draft conclusions from a validated template covering profile type, number of contributors, statistical weight with its assumptions, and explicit limitations; have the wording reviewed against the data before release, and in testimony stay within the report rather than improvising beyond it.