Reasons for Inconclusive DNA

Contamination of Sample

• Contamination of a DNA sample could cause a test result to be inconclusive. Contamination can happen if safety precautions are not taken when collecting the sample or if there is improper handling or storage of the sample. A sample is also considered contaminated if there is DNA from more than one person present. For example, a sample taken from a victim of a gang rape could contain a mixture of DNA.

Insufficient Sample Amount

• Another reason for an inconclusive result is that the quantity of DNA collected may be insufficient to determine any results. If the samples used come from a non-standard source, the sample may be too old or degraded to produce enough DNA to analyze. For example, blood taken from a piece of clothing that was collected in a criminal investigation may not contain enough DNA to produce a conclusive result.

Paternity Testing

• Sometimes in DNA paternity tests, especially when the testing involves only one parent and the child, mutations are found in the genes of either the parent or the child, usually by a difference of one unit. Some mutations appear with a specific frequency in certain racial populations. When that frequency is figured into the testing percentage, the score will fall below 99 percent, which results in an inconclusive determination.

Identical Twins

• Although it is a rare scenario in DNA testing, if a person being tested is an identical twin, a test may come back inconclusive. For example, in a paternity test, if the suspected father of the child is an identical twin, he shares the same DNA as his twin. In this case, a DNA test could not determine which twin is the father of the child. The same situation can occur in criminal cases. If a suspected rapist has an identical twin, there would be no way to determine which one was responsible.