The Pap test is a screening test and, like any such test, it is not 100-percent accurate. Although false positive and false negative results do not occur very often, they can cause anxiety and can affect a woman’s health.
A false positive Pap test means that a patient is told she has abnormal cells, but the cells are actually normal. A false negative Pap test occurs when a specimen is called normal, but the woman has a significant abnormality that was missed. A false negative Pap test may delay the diagnosis and treatment of a precancerous condition. However, regular screening helps to compensate for the false negative result. If abnormal cells are missed at one time, chances are good that the cells will be detected the next time.
What methods are being developed to improve the accuracy of Pap tests?
In April 1996, the Consensus Development Conference on Cervical Cancer, which was convened by the National Institutes of Health (NIH), concluded that about half of false negative Pap tests are due to inadequate specimen collection. The other half are due to a failure to identify or interpret the specimens correctly. Although the conventional Pap test is effective in the majority of cases, the conference made it clear that new methods of collecting and reading specimens are needed to reduce the number of false negatives.
The Bethesda System requires laboratories to determine whether there are enough cervical cells in the specimen to make a proper evaluation. This requirement helps improve the quality of samples and sample collection. The Bethesda System requires a sample to be categorized as “satisfactory for evaluation” or “unsatisfactory for evaluation.”
One new method of collecting and analyzing samples is called liquid-based thin-layer slide preparation. This method may make it easier to screen for abnormal cells. Cervical cells are collected with a brush or other collection instrument. The instrument is rinsed in a vial of liquid preservative. The vial is sent to a laboratory, where an automated thin-layer slide device prepares the slide for viewing. Results of this method suggest that it is comparable to, or more sensitive than, standard Pap tests for the detection of significant abnormalities.
Computer automated readers are also being used to improve the reading of Pap tests. This technology uses a microscope that conveys a cellular image to a computer, which analyzes the image for the presence of abnormal cells.