The Pap Smear: An Overview
A Pap smear is a cytological test designed to detect abnormal cervical cells. The
procedure involves scraping cells from the cervix and then smearing and fixing
them on a glass slide. The slides are sent to a cytology laboratory and evaluated
by a trained cytologist or cytotechnician who determines the cell classification
(see Table 1). Most protocols suggest that women with low-grade abnormalities
return for regular follow-up smears until the abnormality either resolves or
persists, warranting further investigation. High-grade preinvasive disease
generally is further evaluated by colposcopy (examination of the cervix with a
magnifying scope) and biopsy; precancerous lesions then are treated through
surgical removal or ablation.
Periodic screening (regardless of the screening method used) and follow-up
evaluation of women in their thirties or older is an acceptable, cost-effective
approach to preventing cervical cancer, assuming that the screening approach
used is accurate and coverage is high. (See PATH’s Fact Sheet, Natural History
of Cervical Cancer.) In general, the low sensitivity of a single Pap test makes it
necessary to screen women relatively frequently—every three to five years.
Pap Smear Screening Is Specific,
But Only Moderately Sensitive
The Pap smear generally is considered to be a very specific test for high-grade
lesions or cancer, but only moderately sensitive. Specificity is the proportion of
women correctly identified by the test as not having high-grade lesions or cancer.
Sensitivity is the proportion of women correctly identified by the test as having
these conditions. In general, it is not possible to increase Pap smear sensitivity
while maintaining a high specificity.
Several recent meta-analyses have reported quite low Pap smear sensitivities—in
the range of 50 percent but as low as 20 percent.2,3 In Zimbabwe, a study found that
Pap screening had a sensitivity of 44 percent and a specificity of 91 percent in
identifying HSIL.4
Authors of these studies note that decision makers should consider
these findings highlighting low Pap test sensitivity when establishing health policies.