Rare atypical squamous cells of undetermined significance (ASCUS): a clinically significant diagnosis?

To determine the clinical significance of rare atypical squamous cells of undetermined significance (ASCUS) in cervical screening, we studied 748 ASCUS cases prospectively noted to have rare abnormal cells. Comparing the rare ASCUS (RASC) group (defined as five or fewer abnormal cells) statistically to cases diagnosed as within normal limits (WNL), ASCUS unqualified as to number of cells low-grade squamous intraepithelial lesion (LGSIL), and high-grade SIL (HGSIL), we found that the probability of the RASC patients having an abnormal cytology (ASCUS/SIL) or biopsy (dysplasia) result within 1 yr was greater than that of the WNL group, but less than that for ASCUS unqualified, LGSIL, or HGSIL. When only ThinPrep specimens or cases with subsequent definitive SIL/dysplasia were considered, the RASC group was not significantly different from the WNL group. We conclude that RASC increases the risk of a subsequent abnormal cytology/biopsy result in conventional smears, but only when the threshold for abnormality is a subsequent ASCUS. It did not predict dysplasia (SIL/CIN) in those conventional samples. RASC did not have the power to predict any subsequent abnormality and did not appear to be clinically significant in ThinPrep samples.