How Common Are False Positive Tests: PAP Smears

It is more difficult to talk about the false positive rate for Pap smears because there are two types of abnormal results. First, roughly 10 percent of Pap smears need to be repeated because of inflammatory changes, generally related to infection. Here, age is again an important variable: adolescents have a false positive rate of more than 15 percent, whereas for women over 60 it is less than 5 percent.

Second, around 5 percent of women undergoing a Pap smear will be told they have a cellular abnormality that is potentially worrisome for cancer. This number also varies by age. Among adolescents the proportion of smears diagnosed with ASCUS (atypical squamous cells of unknown significance) or SIL (squamous intraepithelial lesion) has been reported to be as high as 14 percent. Since almost no women with ASCUS and SIL have or will develop invasive cervical cancer, virtually all of these test results can be considered false positives. However, many of these abnormalities end up being treated, so the problem can also be characterized as unnecessary diagnosis.(Though to be fair, the treatment is fairly simple, often consisting of freezing part of the cervix, much as is done for pre-cancers of the skin.)

So there you have it: our four major screening tests-all of which can, and do, render false positive results. (If you want a rule of thumb for the frequency of false positive cancer tests, put it somewhere between 5 and 10 percent.) In fact, a positive screening test is much more likely to be a false positive than a cancer, generally speaking. So if you receive a worrisome screening test result, just remember: chances are good it is incorrect.