An inconclusive DNA test means the laboratory couldn’t produce a clear yes-or-no answer from your sample. It doesn’t mean the result is negative, and it doesn’t mean it’s positive. The lab simply didn’t have enough usable genetic information to make a confident call. This happens more often than most people realize, and the reasons vary depending on whether you’re dealing with a paternity test, a forensic case, a prenatal screening, a medical genetic test, or a consumer ancestry kit.
Why DNA Tests Come Back Inconclusive
At the most basic level, DNA testing works by reading specific markers in your genetic code and comparing them to another sample or a reference database. For that to work, the lab needs a clean sample with enough DNA to read those markers clearly. When something interferes with that process, the result lands in a gray zone.
The most common technical reasons are sample degradation, contamination, and the presence of DNA from multiple people. A degraded sample means the DNA has broken down, often from heat, moisture, or age, to the point where the lab can’t read enough markers to draw a conclusion. Contamination introduces foreign genetic material that muddies the picture. And when a sample contains DNA from more than one person, the overlapping genetic profiles can make it impossible to tease out who contributed what.
Inconclusive Paternity Tests
In paternity testing, the lab compares genetic markers between the child and the alleged father. A conclusive result typically shows either a probability of paternity above 99% (inclusion) or a complete mismatch at multiple markers (exclusion). An inconclusive result falls somewhere in between, where the lab can’t confidently say the tested man is or isn’t the biological father.
One major reason this happens is spontaneous genetic mutations. When DNA is passed from parent to child, small random changes can occur at specific marker locations. These mutations create mismatches that look like exclusions even when the man actually is the father. Labs account for known mutation rates in their calculations, but when mutations stack up or occur at uncommon locations, the math can land in inconclusive territory. A study of parentage testing in Zimbabwe found 6 inconclusive results out of 91 indirect paternity cases, many involving mutations or unusual genetic patterns.
Testing close relatives as potential fathers also causes problems. If two brothers, or a father and son, are both possible candidates, they share so many genetic markers that the test may not be able to distinguish between them. In these situations, labs often recommend testing additional markers or testing the other relative directly to resolve the ambiguity. Indirect testing, where a grandparent, sibling, or aunt is tested in place of a missing alleged father, is especially prone to inconclusive outcomes because the genetic overlap is less direct.
Inconclusive Forensic DNA Results
In criminal investigations, DNA collected from a crime scene often isn’t as clean or abundant as a cheek swab taken in a clinic. Evidence may sit in heat for days, get handled by multiple people, or contain only trace amounts of genetic material. All of these factors push results toward inconclusive.
Mixed DNA profiles are particularly challenging. If a sample from a doorknob or piece of clothing contains genetic material from three or four people, separating individual profiles becomes extremely difficult. The lab may be able to say that a suspect “cannot be excluded” as a contributor, but that’s a far weaker statement than a definitive match. It means the suspect’s genetic markers are consistent with being in the mix, but the lab can’t calculate a meaningful statistical probability.
How courts handle these results varies. In a notable Massachusetts appeals case, a court ruled that inconclusive DNA evidence was admissible even without the statistical probability analysis that normally accompanies a DNA match. The majority opinion held that telling a jury a defendant “could not be excluded” still provided useful descriptive information. The dissent, however, argued that without statistical context, jurors have no way to understand what an inconclusive result actually means in terms of likelihood, and that admitting such evidence without those numbers risks misleading the jury. This tension remains unresolved across jurisdictions.
Inconclusive Prenatal Screening
Non-invasive prenatal testing (NIPT) screens for chromosomal conditions like Down syndrome by analyzing fragments of fetal DNA circulating in the mother’s blood. These fragments, released by the placenta, typically make up only 3% to 6% of the total cell-free DNA in maternal blood. When that fraction drops below about 4%, the lab can’t reliably distinguish fetal genetic signals from background noise.
Low fetal fraction is the single biggest reason NIPT fails. In one study of 303 pregnant women whose initial NIPT couldn’t produce a result, low fetal fraction accounted for about 45% of failures. Among women who still couldn’t get a result after a second blood draw, that number rose to 56%. Fetal fraction tends to increase as pregnancy progresses, so retesting a few weeks later often solves the problem.
Several factors make low fetal fraction more likely. Higher maternal body weight dilutes the fetal DNA proportion because there’s more maternal blood volume. Earlier gestational age means less placental tissue releasing DNA. Certain placental characteristics can also limit how much fetal DNA reaches the bloodstream. An inconclusive NIPT is not an indication that something is wrong with the pregnancy. It’s a limitation of the blood sample, not a diagnosis.
Inconclusive Medical Genetic Tests
When you get genetic testing for a health condition, such as screening for cancer risk genes, results come back as positive, negative, or inconclusive. In medical genetics, an inconclusive result often involves something called a variant of uncertain significance. This means the lab found a change in your DNA, but scientists don’t yet know whether that change actually causes disease or is simply a harmless natural variation.
Everyone carries thousands of small DNA differences that have no effect on health. When a test picks up a variant that hasn’t been studied enough to classify, the lab can’t tell you whether it raises your risk. This is different from a technical failure. The lab successfully read your DNA, but the scientific knowledge to interpret what it found doesn’t exist yet. As more people are tested and more research is conducted, many of these variants get reclassified over time as either harmless or disease-associated.
Consumer Ancestry Kit Failures
If your ancestry DNA kit came back with no result, the issue is almost always sample quality. These kits rely on saliva, and a surprising number of things can go wrong between spitting in the tube and the lab processing it. Common culprits include not providing enough saliva, not mixing the stabilizing fluid thoroughly, eating or drinking shortly before collection, or a loose cap that lets the sample leak or degrade during shipping.
Contamination is another frequent problem. Anything in the tube besides saliva and the stabilizing liquid, including bacteria, food particles, toothpaste residue, or chewing tobacco, can interfere with processing. If two people accidentally contribute saliva to the same tube, the mixed DNA will fail quality checks. People who have received bone marrow or stem cell transplants carry donor DNA in their blood and saliva, which can also cause failures. Chemotherapy can temporarily reduce the amount or quality of DNA in saliva. Most ancestry companies will send a free replacement kit if your first sample fails.
Chimerism: A Rare Biological Complication
In very rare cases, a person naturally carries two distinct sets of DNA in their body. This condition, called chimerism, occurs when two fertilized eggs fuse very early in development, producing one person with two genetic profiles. Different tissues (blood, skin, saliva) may carry different DNA. A chimera could test as their own sibling, fail a paternity test with their biological child, or produce confusing results on any type of DNA analysis.
Artificial chimerism happens after a bone marrow or stem cell transplant. The recipient’s blood cells eventually carry the donor’s DNA, while other tissues like hair roots retain the original profile. This means a blood sample and a cheek swab from the same person could yield completely different genetic results, and a lab unaware of the transplant history might flag the result as inconclusive or contaminated.
What Happens After an Inconclusive Result
In most cases, the next step is straightforward: retest. For paternity and forensic cases, labs can request new samples, test additional genetic markers, or bring in more relatives to clarify the picture. For prenatal screening, waiting a few weeks for fetal fraction to rise and then redrawing blood resolves most failures. For ancestry kits, a new saliva sample with careful collection technique usually does the trick.
The key thing to understand is that “inconclusive” is a statement about the quality of information available, not about you. It doesn’t mean the answer is unknowable. It means the specific test, on that specific sample, under those specific conditions, couldn’t reach the threshold of confidence the lab requires to give you a definitive answer.