Specificity or Sensitivity?

The information on the web toward the specificity and sensitivy of hiv antibody test suggests that the specificity (combined with western blot) (99.99%) is greater than sensitivity (99.6%).

However, this seems to contradict the general belief and impression that false positive (which results from the failure to distinguish non-hiv antibody form hiv antibody) is more likely than false negative?)

The other thing is that false positive seem to get a lot of media attention while false negative is only tangentially mentioned in the public discussion on hiv testing except in the context of window period.

I have another puzzle concerning the concepualization of false negative. It seems to me there are two types of false negative. The first one occurs when human body is infected by hiv but has NOT yet produced ANY antibody. This type perhaps accounts for a large portion of false negative case in the window period.The other type is one in which the human body has not yet produce ENOUGH hiv antibody in order for the hiv test to pick up. My suspicion is that only the second type of the false negative is counted when calculating the sensitivity of the test because the the former by definition is attributable to the limitation of test in terms of sensitivity. This seems to suggest that the major part of false negatives cases (that WE label) that we thought we are able to effectively avoid by taking the test beyond the window period is in fact not the "false negative" in the technical sense which is the false undectection of the dectectable hiv antibody.

The above might explain the surface contradiction I mentioned early, the belief that false negative is less pronounce than false positive is due to the fact we attribute most false negative to our folk definiton of false negative. On the other hand, in the strictly technical sense of false negative, the rate is possibly much higher than false postive.

My conclusion is then tentatively that perhaps the case of false negative is much more prevalent than we normally conceive it to be? Which is sorta scary.

[Hao]

Sorry...correction here
"My suspicion is that only the second type of the false negative is counted when calculating the sensitivity of the test because the the former by definition is NOT attributable to the limitation of test in terms of sensitivity"

[HSci34]

I haven't totally read your post, but if it is concerning why sensitivity is less but false positives are a problem, it is important to note that the prevalence of HIV also affects a test's positive predictive value. Changes in prevalence will affect how accurate a test's positive results (and also negative results) are.