one “indeterminate” test + one “negative” test = HIV negative?

November 23, 2008 6:02 PM

HIV testing filter: Does a negative test always trump an indeterminate one?

A few days ago I had a rapid result HIV test where they swab the inside of your mouth and give you your results in 20 minutes. My first test came back “indeterminate.” I’m not so sure this wasn’t code for “positive,” but they didn’t want to use that word and completely freak me out. They immediately gave me a second rapid result test, this time a blood test, and it came back negative. They told me that a negative test always trumps and indeterminate one, and I have nothing to worry about. Of course, I’m still worrying a little. This was at one of those free “get tested” drives on a college campus, and I never actually talked to a doctor, just the volunteers running it. They’re supposed to know what they’re talking about, right?

A few data points: I’m female and haven’t had any risky contact in the 90 day “window” prior to the test. I did have a blood transfusion in the past year – that was the primary motivating factor for getting tested in the first place. (I know the risk is small, but it’s there nonetheless.)

I understand that false positives are much more likely than false negatives on these types of tests. Statistically speaking, I’m probably fine. Aside from easing my paranoid mind, is there any reason for me to have a third test?

posted by anonymous to health & fitness (6 comments total)

Yes, negative always trumps indeterminate. No, indeterminate isn't code for anything other than indeterminate.

(I work in HIV prevention and testing. I am not your doctor.)
posted by OmieWise at 6:07 PM on November 23, 2008 [2 favorites]

To expand a little bit, rapid tests are designed to be more sensitive than standard HIV blood tests. This leads to a higher rate of indeterminate tests (although most tests that come out positive are truly positive). They are designed this way in order to preserve the surety of a negative test. You don't need a third test to confirm your negative serostatus. (Although there isn't any reason not to get one if it will put your mind more at ease.)
posted by OmieWise at 6:26 PM on November 23, 2008

This is fundamentally a question of sensitivity and specificity. Sensitivity refers to the the proportion of true positives that are correctly identified by the test, whereas specificity refers to the proportion of true negatives that are correctly identified by the test.

In the case of the rapid HIV test, which was developed largely for use in emergency rooms, sensitivity trumps specificity, i.e. it is "better" to cast a wider net for positive results and therefore identify almost 100% of positives and a proportion of negatives.

This study for the Annals of Internal Medicine typifies work done on the efficacy of the test.

Given that you had a negative test result, it is almost certain that you are not infected. However, I imagine that this was a very scary experience, and if I were you, I would certainly get another (non-rapid) test for final confirmation.

Disclaimer: IANAD.
posted by charmcityblues at 6:34 PM on November 23, 2008

The initial test was the cheesiest form of the ELISA assay possible. As one who develops these sorts of assays for a living (with microtiter plates, and robotic plate washers and multiple incubations) I can tell you that occasional background issues are just par for the course, even when you are dealing with a well characterized sample. I can only imagine how noisy they are when the sampling protocol begins, "Here, stick this in your mouth."

Again, if it eases your mind, you might look into a serious blood test (with the microtiter plate and robotic plate washer).
posted by Kid Charlemagne at 7:49 PM on November 23, 2008

while I am neither a doctor nor a mathematician, a negative is not always definitive.

Type I and Type II Errors - False Negative Rate.

for something this critical, I'd get tested again, and the most comprehensive test possible. The odds are against multiple false negatives.
posted by namewithoutwords at 5:07 PM on November 24, 2008

Even all of the science says that there are no false-negatives, if it eases your mind, have a third test. When that one comes back negative, breathe easy.

A third test to soothe your fears is totally reasonable. A fourth would be where you start going overboard into total paranoia.
posted by grapefruitmoon at 6:37 PM on November 24, 2008