What I've learned from Biostats

From the beginning of our 4 week long, 1 credit Biostats course, I knew that it would be important because there will likely be some related questions on the USMLE. But, I didn't really think the class would ever be particularly clinically relevant. Well, it turns out that I have finally found the answer to something that I've always wanted to know, because of Biostats.

When I used to get strep tests, I always wondered why they had to run two tests, one of which was fast, and one which took a little while. Well, turns out that the first test they do is highly specific, but not very sensitive. This means that the test will accurately determine everyone that does not have strep. Since it finds everyone who is disease free, if it says you do have strep, then you do. But, since it's not very sensitive, it also will say that some people who have strep, don't. So, if the first test says you've got strep, it's time to start antibiotics.

If the first test said you don't have strep though, you're not totally in the clear yet. The second test is very sensitive, but not as specific. So, it's able to detect just about everyone that does have strep. Now, if this one says that you don't have strep, you really don't have strep...congratulations! Now, you just have a sore throat and no explanation. ;) Turns out, the second test they do takes longer and costs more money (it's a culture).

The same principle works on HIV testing for blood donation, but in reverse. With donation, it's obviously very important to make sure that the blood doesn't have any HIV virus floating around. So first, they do a very sensitive test. If this test says the blood is HIV free, then it's good to send off to some guy or girl in a trauma center. But, this test will also come up with a bunch of positive results that aren't really positive. So, before they call you and give you the bad news, they'll run a very specific test (but not sensitive). If this one says you've got HIV, then you've got HIV. Hopefully though, in 3 months, you get yet another call from the Red Cross asking for more blood!