Why Identical Touch-DNA Samples Produce Different Results Across Laboratories

Imagine sending the same trace-DNA material to several competent forensic laboratories. Each laboratory follows a validated workflow, uses established instruments and reports its results in good faith. Yet the measured DNADNA, or Deoxyribonucleic Acid, is the genetic material found in cells, composed of a double helix structure. It serves as the genetic blueprint for all living organisms. Read Full Definition quantities differ by a hundredfold. Change the substrate as well, and the overall spread becomes still larger. Even the apparent proportions of two contributors may change according to the measurement and interpretation method.

That is the central finding of a new multi-laboratory study from the TransAct project. Published in Forensic ScienceDiscover the fascinating field of Forensic Science, the application of scientific principles to legal matters. This post delves into its many disciplines, from DNA analysis to crime scene investigation, its importance in the justice system, Read Full Definition International: Genetics, the research examined how factors unrelated to the transfer event itself can influence dataInformation in analog or digital form that can be transmitted or processed. Read Full Definition on DNA transfer, prevalence, persistence and recovery—collectively known as TPPR.

From a laboratory perspective, this is not an abstract reproducibility problem. TPPR data may be used to assess questions such as whether a person handled an item, whether DNA could have arrived through an innocent interaction, or how likely a profile would be after a claimed activity. If the data depend strongly on the laboratory method that produced them, their relevance to another laboratory’s casework cannot simply be assumed.

Key takeaway: A DNA result does not arise from the transfer event alone. It is the combined product of deposition, substrate, sampling, extraction, quantification, amplification and interpretation.

What Did the TransAct Study Test?

Broderius and colleagues designed the study to separate biological transfer from laboratory processing. Eight forensic laboratories received standardised mock touch-DNA samples prepared from controlled mixtures of three components:

• Skin cells, representing cellular material commonly associated with handled items;
• Buccal cells, representing another source of nucleated human cells; and
• Cell-free DNA, representing extracellular DNA that can be present in biological deposits.

The samples were placed on defined substrates and processed by the participating laboratories. Because the starting material was standardised, differences in the reported results could be examined without relying on naturally deposited fingermarks, which vary substantially between donors and contact events.

This design is important. Natural touch samples are affected by pressure, duration, recent handwashing, perspiration, personal habits, the surface touched, previous contacts and the biological material already present on the hands. Those variables can conceal the contribution of the laboratory workflow. The TransAct approach deliberately reduced that biological noise so the analytical component could be seen more clearly.

Readers who want a broader introduction can first review Touch DNA Analysis: Advantages, Limitations, and Best Practices. The new study takes the next step by asking what happens after the trace enters the laboratory.

The Main Findings

The scale of variation is the headline, but the deeper message is methodological. “DNA quantity” is often discussed as though it were an inherent attribute of a stain. In practice, the number reported by a laboratory is an estimate produced by a defined chain of procedures.

Touch DNA Is Not a Single Biological Material

The term touch DNA is convenient, but it can encourage an overly simple mental picture: a person touches an item, skin cells remain, and the laboratory measures them. Real deposits can contain corneocytes, nucleated cells from other biological sources, cell-free DNA and material transferred to the hand from elsewhere. A recovered profile therefore need not represent only cells shed directly from the skin during the final contact.

This is one reason that detecting a person’s DNA and explaining its presence are different tasks. A DNA comparison may strongly support that a person contributed to a profile while saying little, by itself, about the activity responsible for deposition. Simply Forensic’s guide to the hierarchy of forensic propositions explains the separation between sub-source, source and activity-level questions.

Why Can the Same Starting Material Produce Different Results?

1. Sampling determines what enters the workflow

A swab, moistened swab, double-swab procedure, adhesive device or direct cutting does not recover every biological component with equal efficiency. Surface texture, porosity and chemistry affect what remains attached and what can be released. The sampled area, pressure, number of passes and moisture conditions can further change recovery.

Consequently, the laboratory never analyses “all DNA on the item.” It analyses the fraction captured by a particular collection method. Two procedures can therefore begin with the same substrate yet deliver different amounts and compositions to extraction.

2. Extraction changes recovery and concentration

Extraction workflows differ in lysis chemistry, incubation, agitation, purification mechanism, transfer steps and final elution volume. Some procedures prioritise purity; others prioritise maximum recovery or automation. DNA may be retained on the substrate, lost during transfers, adsorbed to plastic, remain bound to purification material or be eluted into different volumes.

This creates an important distinction between concentration and total yield. A concentrated extract can report more DNA per microlitre while containing less DNA overall than a larger-volume extract. Meaningful comparisons require both the measured concentration and the volumes used throughout the workflow.

3. qPCR is a measurement system, not a direct molecule counter

Forensic qPCR assays estimate amplifiable target DNA through assay-specific primers, probes, standards and analysis settings. Results can be influenced by target choice, calibrationThe act of checking or adjusting (by comparison with a standard) the accuracy of a measuring instrument. Operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement Read Full Definition, inhibition, degradation, replicate variation and the relationship between the quantification target and the STR system used later.

The TransAct finding that measured concentrations differed according to method does not mean that qPCR is unreliable. It means that a qPCR result must be understood within its validated system. A numerical value produced by one workflow is not automatically interchangeable with the same number produced elsewhere.

4. Substrate and method can interact

Glass, fabric, plastic, metal and paper retain and release biological material differently. A collection method effective on a smooth non-porous surface may perform differently on a porous or chemically reactive substrate. This is why a transfer study conducted on one item cannot be applied mechanically to another merely because both were “touched.”

5. Amplification decisions shape the visible profile

The amount of extract added to PCR, target input strategy, amplification cycle number, STR kit and analytical threshold influence the allelesAlleles are different forms of a gene resulting from mutations or variations in the DNA sequence or gene expression. They can be dominant (expressed with one copy) or recessive (expressed only with two copies). Read Full Definition ultimately observed. Low-template samples are particularly susceptible to stochastic effects, including allele dropout, peak imbalance and variable representation of minor contributors.

Even when laboratories begin with different qPCR estimates, they may compensate through their validated amplification strategy. That can improve profile generation while making a simple comparison of reported starting concentrations even less informative.

6. Interpretation adds another analytical layer

The study compared mixture-related information obtained from the ratio of Y-chromosomal to autosomal qPCR targets with estimates produced using EuroForMix. These measurements do not ask precisely the same question. A qPCR ratio reflects assay targets before STR amplification; probabilistic genotyping evaluates information in the resulting profile under a statistical model.

Disagreement therefore should not automatically be framed as one method being wrong. It shows why a “male-to-female ratio” or an inferred mixture proportion must be linked to the stage, targets and model from which it was derived.

Why This Matters for TPPR Research

TPPR describes four related but distinct questions:

• Transfer: What biological material moves during a specified contact or activity?
• Persistence: How long does that material remain detectable under stated conditions?
• Prevalence: How commonly is DNA already present on people, objects or environments before the activity of interest?
• Recovery: What proportion of the available material is detected after sampling and laboratory analysis?

These data become especially important when the parties agree whose DNA was detected but dispute how it arrived. Consider competing propositions such as:

• The person of interest handled the item during the allegedAlleged is a term used in legal contexts to describe an accusation or claim that has been made against an individual but has not yet been proven or confirmed. Read Full Definition offence.
• The person of interest did not handle it during the offence, and their DNA arrived through an earlier innocent contact or indirect transfer.

The value of the DNA findings at this level depends on the probability of observing them under each proposed activity. Those probabilities may be informed by experiments. But if the experiment and the casework laboratory differ substantially in sampling efficiency, extraction yield, detection sensitivityIn the context of laboratory equipment or analytical techniques, the term "sensitive" describes the capability of a machine or method to detect even very small amounts or concentrations of a substance. Sensitivity is a quantitative Read Full Definition or interpretation, the experimental probability may not transfer cleanly.

This concern is not theoretical. The earlier ReAct project, involving 23 laboratories and more than 2,700 samples, also reported substantial inter-laboratory variation in recovered DNA quantity and showed that those differences could affect likelihood ratios addressing activity-level propositions. Simply Forensic previously examined that broader problem in Making Sense of DNA Patterns.

This Is Not a Ranking of “Good” and “Bad” Laboratories

The study should not be misread as a proficiency-test league table. It did not establish that the laboratory producing the highest yield was necessarily the best, or that a lower yield made another laboratory’s casework invalid. A method may be fit for its validated purpose while still producing data that are not numerically interchangeable with another method.

Nor does the study show that every reported STR conclusion would differ. Variation in recovered quantity is relevant because it can change detection and contributor representation, especially near low-template limits, but profile outcomes depend on the complete validated workflow.

The most defensible interpretation is narrower and more useful: TPPR data require method context. Before importing an external dataset into an activity-level assessment, the analystA designated person who examines and analyzes seized drugs or related materials, or directs such examinations to be done; independently has access to unsealed evidence in order to remove samples from the evidentiary material for Read Full Definition must determine whether the conditions and analytical process are sufficiently comparable to the case.

A Method-Comparability Checklist for TPPR Evidence

The following checklist is designed for laboratories, researchers, lawyers and students reviewing a transfer or persistence study.

1. Define the actual question. Does the study address transfer, persistence, prevalence, recovery—or a combination? Is the case question at sub-source, source or activity level?
2. Compare the biological material. Were the deposits naturally generated, deliberately seeded or constructed from controlled cell mixtures? Do they resemble the likely case deposit?
3. Compare donor and contact conditions. Review donor number, recent activities, handwashing, contact duration, pressure, repetitions and time since contact.
4. Compare the substrate. Match material, texture, porosity, condition, prior use and relevant environmental exposure.
5. Review the sampling method. Identify swab type, wetting solution, number of swabs, area sampled, collection pressure, storage and time to processing.
6. Review extraction details. Check lysis conditions, platform, purification chemistry, transfer steps, concentration procedure and elution volume.
7. Review quantification. Identify the qPCR kit, instrument, standards, replicate policy, inhibition and degradation metrics, and whether concentration or total yield is being compared.
8. Review amplification and detection. Compare STR kit, template input, cycle number, instrument, injection conditions and analytical thresholds.
9. Review mixture interpretation. Determine how contributor number, mixture proportion, dropout and probabilistic-genotyping assumptions were handled.
10. Check controls and contaminationContamination - The unwanted transfer of material from another source to a piece of physical evidence. The inadvertent touching of a weapon, thereby adding fingerprints to it is an example of evidence contamination. Read Full Definition safeguards. Assess substrate controls, reagent blanks, elimination procedures and unexpected-profile management. See Simply Forensic’s forensic DNA contamination-control guide.
11. Check replication and uncertainty. A single successful transfer event cannot define a general probability. Examine replicates, confidence intervals, missing data and donor-to-donor variation.
12. Decide whether bridging is required. If important methods differ, can the casework laboratory perform a limited comparison or sensitivity analysis before relying on the external data?

What Laboratories Should Do Differently

Preserve detailed metadata

Published tables should not be separated from the methods that generated them. Sampling materials, volumes, extraction steps, assay settings and interpretation rules need to travel with the data. The documentation templates supplied by the TransAct authors are therefore more than administrative appendices; they are part of the scientific result.

Use local data or bridging studies where the method matters

A laboratory does not need to repeat every published TPPR experiment. It should, however, understand whether its own process lies within the conditions represented by the external data. Targeted bridging experiments can test whether key recovery or detection differences are likely to alter the proposed use.

Do not pool incompatible datasets merely to increase sample size

A larger dataset is not automatically stronger if it combines measurements with materially different meanings. Harmonisation may require common outcome categories, method-stratified analysis or models that explicitly include laboratory-dependent recovery.

Test the sensitivity of the conclusion

Where several scientifically reasonable datasets or parameter choices exist, analysts should examine whether the activity-level conclusion changes across them. A result that remains stable is easier to defend than one that depends on a single external study or narrow assumption.

State limitations in the report

Reports and testimony should identify when probabilities are informed by studies using different methods or conditions. Transparent qualification is not a weakness; it allows the court to understand the scope of the inference.

Questions Lawyers and Courts Should Ask

• Does the cited research address the same activity and competing explanation as the case?
• Were the substrate, time interval and contact conditions comparable?
• Were the research samples processed using methods comparable to the casework laboratory?
• Is the expert using a detection rate, a transfer probability or a likelihood ratio—and are those being confused?
• How sensitive is the conclusion to a different but credible TPPR dataset?
• Does the evidenceEvidence is any form of proof, such as objects, materials, or scientific findings, presented to establish or disprove a fact in a legal proceeding. It is used to reconstruct events and link or exclude individuals Read Full Definition establish the contributor, the biological source, the activity, or only one of those levels?

A practical illustration of why case context matters appears in The 2-Hour Window: Can a Kiss Explain Away DNA on a Cigarette Butt?. An alternative transfer account cannot be assessed from the DNA profile alone; it requires data that are relevant to the proposed activities and generated under sufficiently comparable conditions.

An Analyst’s Perspective

For me, the study reinforces four principles that should guide the use of trace-DNA evidence.

1. Sensitivity is not the same as interpretive certainty. Detecting more DNA can reveal more information, but it also increases the need to explain background presence and transfer.
2. Numbers retain the history of the method that produced them. A concentration, mixture proportion or recovery rate should never be detached from its workflow.
3. Source-level strength does not answer an activity-level dispute. A strong association with a contributor may coexist with genuine uncertainty about when and how the DNA was deposited.
4. Standardisation should improve comparability without erasing legitimate validated approaches. The goal is not necessarily one universal laboratory method. It is transparent documentation, appropriate calibration and defensible limits on data reuse.

The TransAct study does not reduce confidence in forensic DNA analysis. It sharpens the question we should ask of every result: confidence in what conclusion, under which method, and at which level of inference?

Frequently Asked Questions

Does this study mean forensic DNA results are unreliable?

No. It demonstrates that numerical results can be method-dependent. A laboratory may have a properly validated casework process while producing quantities that differ from another validated process. The main concern is the uncritical transfer of research probabilities or numerical expectations between methods.

Why did the laboratories receive mock touch DNA rather than natural fingerprints?

Natural deposits vary greatly between people and contacts. Standardised mixtures allowed the researchers to reduce that variability and examine the contribution of sampling and laboratory methodology more directly.

What is the difference between DNA concentration and total DNA yield?

Concentration describes the estimated DNA amount per unit volume, such as nanograms per microlitre. Total yield considers both concentration and the volume of extract. Different elution volumes can therefore produce different concentrations even when the recovered totals are similar.

Can a laboratory use published TPPR data from another laboratory?

Potentially, but applicability must be assessed. The biological scenario, substrate, sampling, extraction, detection and interpretation conditions should be compared, and important differences may require bridging data or sensitivity analysis.

Do higher DNA yields always produce better evidence?

No. Higher recovery can improve profile completeness, but it may also recover more background DNA and increase mixture complexity. Evidential value depends on the propositions and the quality of interpretation, not quantity alone.

What should a court understand about a touch-DNA match?

A match or likelihood ratio addressing the possible contributor evaluates a different question from how the DNA was deposited. Activity-level conclusions require consideration of transfer, persistence, prevalence, recovery and the case circumstances.

References and Further Reading

1. Broderius K, Buckel I, Kulstein G, et al. Methods matter: On the impact of non-transferential variables on reported data of studies on nucleic acid transfer, prevalence, persistence and recovery (TPPR). Forensic Science International: Genetics. 2026;103632.
2. Gill P, Fonneløp AE, Hicks T, et al. The ReAct project: Analysis of data from 23 different laboratories to characterise DNA recovery given two sets of activity level propositions. Forensic Science International: Genetics. 2025;76:103222.
3. Gill P, Hicks T, Kokshoorn B, van Oorschot RAH, Taylor D, Parson W. Minimum FSI: Genetics requirements for publishing data on DNA transfer and recovery, given activities. Forensic Science International: Genetics. 2026;80:103330.
4. Taylor D, Kokshoorn B, Biedermann A. DNA Commission of the International Society for Forensic Genetics: Assessing the value of forensic biological evidence—Part II, evaluation considering activity-level propositions. Forensic Science International: Genetics. 2020;44:102186.