DNA testing compares specific genetic markers to determine identity, ancestry, or relationship. While this technology is a powerful source of evidence in forensic science and popular for personal curiosity, the process is not immune to error. The reliability of a result depends on factors extending beyond the actual sequencing of the DNA molecule. Understanding how and where mistakes occur is necessary for anyone relying on these complex genetic reports.
Sources of Technical and Human Error
A genetics laboratory remains susceptible to mistakes made by both humans and machinery during the complex testing procedure. One common technical issue is sample cross-contamination, where genetic material from one test subject accidentally mixes with another, often due to improper cleaning of equipment or surfaces. Since modern testing methods, like Polymerase Chain Reaction (PCR), amplify minute amounts of DNA, even a tiny speck of foreign material can skew a result.
Human error often involves administrative steps. Mislabeling of tubes, vials, or digital data files can lead to the wrong genetic profile being assigned to the wrong individual. Data entry errors, where results are incorrectly transcribed from an analysis machine into a final report, can also occur. To mitigate these risks, quality control protocols, such as using internal controls and blind sample processing, are designed to catch accidental mix-ups before a final report is issued.
Equipment malfunctions can compromise the integrity of the analysis. Sequencing machines and other specialized instruments require regular maintenance and precise calibration to function accurately. If a thermal cycler used in the amplification process is not maintaining the correct temperature, the resulting DNA fragments may be incomplete or incorrectly sized. This renders the subsequent analysis unreliable.
Statistical and Interpretive Limitations
Even when the laboratory process is executed without contamination or human mistake, the final result is often an interpretation based on probability. A reported “match” in forensic or relationship testing does not signify 100% certainty but rather a statistical likelihood that the two samples originated from the same person or are related. This probability is expressed as a random match probability, which calculates the chances that an unrelated person drawn at random would coincidentally share the same genetic profile.
The calculation of this probability relies on the reference population database used by the testing company. For consumer ancestry testing, the accuracy of an ethnicity estimate is constrained by the size, diversity, and geographical representation of the reference samples. If an individual belongs to a group that is underrepresented in the database, the algorithmic interpretation may produce a less accurate or misleading regional assignment. The use of different statistical thresholds and proprietary algorithms explains why two different ancestry companies can sometimes yield slightly different results for the same person.
Shared DNA can introduce ambiguity in relationship testing, such as paternity cases. Full siblings share a substantial amount of their DNA, meaning an alleged father’s brother could potentially show a high probability of paternity. This can lead to a false inclusion if not enough genetic markers are analyzed. To overcome this, tests must analyze a sufficient number of Single Nucleotide Polymorphisms (SNPs) or Short Tandem Repeats (STRs) to differentiate between a father and a close paternal relative.
Sample Integrity and Collection Issues
Errors can stem from issues related to the sample’s initial quality and handling. DNA degradation is a common problem, particularly with forensic evidence or samples that are not collected and stored promptly. Exposure to environmental factors like heat, moisture, and ultraviolet light can break down the DNA strands, resulting in a low quantity of usable material. Degraded samples are often difficult to amplify and can lead to incomplete or inconclusive genetic profiles.
Improper collection and storage techniques are a source of error. For at-home saliva tests, failing to fill the collection tube or not securely mixing the sample with the stabilizing solution can compromise the genetic material during transit. In clinical or forensic settings, using non-sterile collection kits or delaying processing can invite contamination from bacteria or environmental substances. Ideally, samples should be processed within 24 hours of collection or immediately stored at appropriate temperatures, such as refrigeration at 4°C, to maintain integrity.
The chain of custody is the documented process of tracking a physical sample from the moment of collection until the final report is generated. Breaks in this chain, which can involve inadequate logging or unauthorized handling, do not necessarily alter the DNA itself. However, these breaks can invalidate the legal or scientific reliability of the result by suggesting an opportunity for tampering or mix-up occurred during transport.
Understanding Conflicting Results
Receiving an unexpected or contradictory DNA test result can be confusing, but consumers can investigate the discrepancy. If a result seems questionable, the most direct course of action is to submit a fresh sample for retesting, preferably using a different laboratory. A second test using a different company’s process and equipment acts as a verification tool, helping to rule out the possibility of a technical or human error in the initial lab’s procedure.
Consumers should prioritize laboratories that hold recognized accreditations from organizations such as the American Association of Blood Banks (AABB) or the College of American Pathologists (CAP). These accreditations indicate that the lab adheres to quality control standards and proficiency testing, which reduces the likelihood of technical errors. While accreditation does not guarantee perfection, it represents a commitment to best practices in sample handling, analysis, and reporting.
When the results involve complex findings, such as non-paternity or an unexpected health risk, seeking professional interpretation is advisable. A genetic counselor can help translate the probabilistic language of the report, explain the biological and statistical limitations, and determine if the conflicting results stem from a genuine biological ambiguity. For results intended for legal use, consulting with an attorney experienced in DNA evidence is necessary to understand how factors like chain of custody or statistical thresholds might affect the case.