False Positives, Flase Negatives, and “WTF” Positive Reactions

When criminalists run presumptive forensic tests there is a risk of obtaining results that may not support certain conclusions drawn from those results.  In plain English; what you get may not be what you got. This is where analysis, interpretation, and integration with other findings comes into play. It’s part of the investigative process that all of us are used to.  Actually, a false positive is not really “false” in terms of the reaction, merely the reaction one might expect. As a writer, however, keeping these terms in mind can be very helpful when developing a scene.

A false-positive reaction occurs when a chemical reagent reacts positively to a substance other than the one being tested for. As an example, if a CSI sprays the blood reagent Luminol at a crime scene and sees a small area that glows he or she might believe or presume that the reagent is reacting with blood. In fact, there are other substances that Luminol may react with. This scenario might require other tests before one could testify that it is, in fact, blood.

Similarly, a test may result in a false-negative finding. Again, the result may not be truly negative in as much as it is negative to the questions being asked. Fore example, if you were to test a stain for blood with a reagent not sensitive enough to detect the trace amount present then you would get a negative result, but it would still be blood. Sometimes a sample is simply  “contaminated” or may have been cleaned and therefore unable to yield a result to the exam (other than negative that is).

Different test have different failure rates. Some tests are very specific with very low rates of false results. Other exams (like a nitrate (NO3) test for gunshot residue) may be much less specific as Nitrates can be found in a large number of commercial products like fertilizers, tobacco, and cosmetics (to name a few). Actually, if you want to add some suspense into a scene you may use a false-result exam to either arrest or free a suspect that deserves neither.

Lastly, we have the “WTF” positive reaction. As you might imagine, this is not a scientifically accepted term. Every once in a while however we may get a reaction with absolutely no idea how it occurred. There are literally thousands of potential substances that we may encounter at crime scenes that have absolutely nothing to do with the crime being investigated. Such is the nature of the world. This is why forensic scientists utilize confirmatory exams and incorporate other findings into a crime scene reconstruction.