How Accurate is a DNA test?
The accuracy of a DNA test depends on several factors. The quality of the testing and the quality of the sample are important. Other important factors are the method of DNA typing, how many DNA loci are being tested for, and who is being tested versus the objective of the DNA testing.
Quality of the DNA Testing
Laboratories that do DNA testing should be ISO 17025 accredited. This accreditation assures the highest standards and integrity of the DNA testing and the laboratory. ISO 17025 labs have frequent, rigorous inspections to validate that the laboratory is following the DNA testing standards on a consistent basis as set forth in ISO 17025. The lab techs doing the testing must to be properly trained and consistently follow established procedures that have been determined to give consistent high quality results.
Quality of the DNA Sample
DNA can be extracted from all kinds of body cells as it is present in all cells except red blood cells. Nevertheless, because DNA is present in white blood cells, tiny amounts of blood can be used for DNA testing. DNA samples can be stored for years provided that they are stored properly. Samples can degenerate in quality if the sample has not been properly stored.
DNA laboratories such as ones used for paternity testing recommend that buccal swabs (cells from inside the cheek) used for collection be allowed to dry after collection of the sample for at least an hour before being placed in a paper bag (which is breathable) in order to prevent moisture build up and bacterial contamination. Blood samples are to be collected in blood collection tubes with the EDTA anticoagulant and refrigerated not frozen. Forensic laboratories maintain DNA samples frozen. If DNA is not being collected directly from a person but from an object contaminated with blood or body fluids, the quality of the specimen may be compromised by environmental conditions or other substances such as dirt, grease and some fabric dyes.
DNA Typing Process
The DNA typing process has evolved considerably over the years. There are two methods of DNA testing done today. One is called STR (short tandem repeat) and the other is called mitochondrial. STR evaluates at least five variable regions of the DNA on the chromosomes. The recommendation for paternity DNA testing is to test at least fifteen loci before making an interpretation. This testing process measures the length of the DNA but not the actual sequence.
Less common is mitochondrial DNA testing. Mitochondrial DNA is only inherited from the mother. The sequence of a person’s mitochondrial DNA matches the mother’s and can be used as one type of DNA testing. One key positive regarding mitochondrial DNA testing is this type of testing is less subject to inaccuracies due to degradation of the sample than the STR method.
DNA Analysis Comparisons
A DNA profile in itself is of no use. The profile has to be compared to other DNA profiles in order for a conclusion to be made. The validity of the comparisons can vary; therefore, incomplete or flawed comparisons will compromise the conclusion of the DNA results. For example, the analysis of DNA results uses statistics to derive “random match probability” or RMP. The database used for comparison has to be adequate to make a statistically valid conclusion. Additionally, not all cases are straightforward; thus, the person interpreting the unusual cases must thoroughly understand statistics and probabilities.
The most common kinds of DNA analysis comparisons are for those of criminal suspects or for paternity testing. In forensic analysis, the DNA profile of a suspect in question has to be compared to other suspects. If there are no suspects, the comparison is made to a DNA database of criminals.
For paternity testing, there is no 100% certainty for inclusion as there is always the possibility that the alleged father matches the alleged child by chance; the odds of the match being by chance vary by ethnic origin. However, it’s possible to determine 100% exclusion of an alleged father being related to a child provided that the lab has properly tested the samples and no mixing of different people’s samples has occurred.
Quality DNA testing is done in an accredited laboratory. The sample used for testing must have been properly collected and stored. Interpretation of results is only accurate if valid comparisons are made using appropriate statistics and probabilities.