Some Wisconsin Crime Laboratory DNA analysts are now using the term "adventitious DNA" when unexpected/inappropriate DNA is found in controls during their tests, instead of calling it what it is -- contamination. They must use a thesaurus in the lab and search for the most obscure terms they can find to hide their mistakes! Because the definitions I find for adventitious are more like "unexpected" or "fortuitous" or "extrinsic." Yes, contamination may also fit that definition to some degree, but when DNA is found in a control (that is, a sample with a known value) it may reflect poorly on the lab and/or analyst.
It is important to understand that it is only when unexpected DNA is found in a control sample that we can be sure that there is some type of contamination affecting the test. Controls are used in lab tests to be sure the instruments are working properly, or to exclude the possibility that something outside of the evidence sample could be causing contamination. So a "negative" (or blank) control is run along with the sample of DNA. The test should show that negative control has zero human DNA. If it doesn't -- if instead some human DNA is appearing in the negative control -- then the analyst knows the test has become contaminated, and the result should be thrown out the test started over.
But contamination could also occur with the crime scene evidence samples and you wouldn't necessarily know it. Those evidence samples have already been found to contain some amount of DNA, but it is from an unknown source. The lab analyst may assume that DNA showing up in the crime scene sample came from the perpetrator, when it may have come from contamination of the sample, and therefore is completely unrelated to the crime. That DNA may mistakenly be considered proof of a perpetrator's DNA. That's why labs should obey strict procedures to prevent, or eliminate as much as possible, any contamination occurring during or before the test is run.
Therefore, the simple truth is that finding ANY amount of DNA in a negative or blank control should not only "unexpected," but it may also indicate sloppy lab or analyst practices. Since it's a "control", it's something that is prepared and run by the analyst, so one can't blame the police for sloppy evidence collection on this. It means that despite all their careful and detailed procedures about how to avoid contamination during a test, someone still messed up and interjected human DNA into a negative control.
Even if the "official" case report discloses that there was "adventitious" DNA found in a control, it won't usually disclose whose DNA was found in the negative control. The attorney has to ask for the internal records generated during the DNA test in that case, and the attorney has to understand how to read those records.
-Was it the analyst's own DNA? That implicates the analyst him or herself; meaning their sloppy practice got their own DNA into the test samples.
-Or was it another analyst in the lab? Same as before for not cleaning up the lab testing area properly.
-Or was it DNA that matches DNA found in another case being handled by that lab? This is a very dangerous type of cross-contamination that could also have occurred with the evidence sample, with no way of telling.
-Or was it DNA from a reference sample (such as a known sample from the victim or potential known suspect) used in the case? This would indicate that either the analyst didn't follow the protocol that prohibits DNA tests of reference samples on the same day as the evidence samples are being done, or the analyst did follow the protocol, and yet the prior reference sample test done on a prior day left ambient DNA somewhere in the lab to contaminate other tests days or weeks later! This has been known to happen in Wisconsin crime labs, and it seriously undermines the credibility of most lab results that were obtained in any case tested in the lab.
DNA tests do not happen in the real world like they do on TV. Crime labs should be honest when they know contamination occurred during a DNA test, and avoid using silly terms like "adventitious." Call it what it is -- contamination -- and throw the test out and start over.