On understanding screening 

It is worth devoting a few moments to the evolving science of screening for sexually transmitted infections. I entered medicine as a first year medical student in 1985. The methods of screening used then are fundamentally different from those we use now. 

In "the old days" we designed screening tests to be very sensitive. Their main priority was to pick up the presence of an organism. The price of this sensitivity was that sometimes these screening tests generated a high percentage of false positive results. We understood from the beginning that this might be the case. If this occurred, we refined and repeated the testing, using different, more specific, and usually more expensive tests to hone in on the right diagnosis. In short, screening tests were designed to be cheap, easy to perform, and sensitive to overly sensitive. Our best testing was expensive, and was reserved only for those whose screening tests were positive. It was definitely a trade-off.

I mentioned already that sexually transmitted infections fall into two basic groups: bacteria and viruses. To search for bacteria, you pretty much had to see them directly. You could make your job easier by collecting a sample from the patient's body containing bacteria and then "plating" them on a special growing medium, like a Petri dish, and letting them multiply. You could then identify them by checking various characteristics. To see viruses, which are considerably smaller, we had to look at the surrounding tissue and look for "virus effects", or the visible changes in cells attributable to the virus. We might also be able to check indirectly by looking for antibodies in the patient's bloodstream made as a response to the presence of the virus. We still use this latter method today. 

All these test suffered from inaccuracies. Sometimes a test would fail to indicate the presence of an organism that was really there. Sometimes a test would indicate that an organism was there and it really wasn't. This first problem is called lack of sensitivity. The second problem is called lack of specificity. These inaccuracies continued to prompt the development of new tests. During the same period of time our ability to understand and manipulate genes and DNA has skyrocketed. So now, when we are looking for an organism, such as a bacteria or virus, we simply look for its DNA. These types of DNA tests are called probes, or molecular tests and have a lock and key type nature which makes them very specific and very sensitive. Said another way, they are very accurate, and are not prone to over calling, or under calling a result. Furthermore, these molecular tests seem to be getting better every year, and one has to make an effort to keep up on the best kind of testing. We don't have to make trade-offs nearly so much anymore.