Atypical squamous cells of undetermined significance: interlaboratory comparison and quality assurance monitors.

The Bethesda System recognizes "Atypical Squamous Cells of Undetermined Significance" (ASCUS) as a category of epithelial cell abnormality. Neither the acceptable rate of ASCUS nor the clinical follow-up are well defined. This study focused on interlaboratory comparison and quality assurance methods for evaluating the rate and outcome of ASCUS. Data was collected from questionnaire surveys from the College of American Pathologists Interlaboratory PAP Program and the four authors' laboratories. Most PAP laboratories (82.5%) limit the use of "atypia" terminology to abnormalities of undetermined significance. According to PAP data, the median rate of ASCUS in 1992 was 2.9%, with 10% of laboratories reporting rates greater than 9.0%. The median squamous intraepithelial lesion (SIL) rate was 2.2%, with a median ASCUS/SIL ration of 1.3. The authors' laboratories (university, independent, and hospital) revealed ASCUS rates of 1.6-9.0%, while SIL rates were 2.1-9.0%. The ASCUS/SIL ratio was less variable, 0.8-2.7. Follow-up of ASCUS patients in the authors' laboratories showed 10.3-43% with SIL, but less than 6% with high grade SIL. The ratio ASCUS/SIL may serve as a useful laboratory monitor. Peer review and follow-up studies of ASCUS serve to validate laboratory criteria and consequent clinical follow-up. Communication with clinicians is vital in ensuring optimal patient care.