Atypical squamous cells of undetermined significance in liquid-based cytologic specimens: results of reflex human papillomavirus testing and histologic follow-up in routine practice with comparison of interpretive and probabilistic reporting methods.

Background: Human papillomavirus (HPV) DNA testing for high-risk types after Papanicolaou (Pap) smear interpretations of atypical squamous cells of undetermined significance (ASCUS) is a sensitive method for identifying women who harbor underlying high-grade squamous intraepithelial lesions (HSIL). To the authors' knowledge, the application of HPV testing to ASCUS smears in routine practice with comparison of probabilistic and interpretive models of cytologic reporting has not been reported.

Methods: HPV DNA testing was performed reflexively on 216 liquid-based Pap smears that initially were interpreted as ASCUS. According to the interpretive model, ASCUS interpretations were modified and reported as either low-grade squamous intraepithelial lesions (LSIL) or squamous intraepithelial lesions (SIL) when HPV positive and as reactive when HPV negative. Using the probabilistic model, ASCUS interpretations were maintained and simply reported with the HPV test result. Histologic follow-up data were obtained.

Results: Of the 216 women with ASCUS cytology, 142 (65.7%) were positive for high-risk HPV types. Of the 142 HPV-positive ASCUS smears, 101 (71.1%) were modified to an interpretation of LSIL (96 cases) or SIL (5 cases). Histologic follow-up of 55 of the 101 HPV-positive smears in the interpretive group and 26 of the 41 HPV-positive smears in the probabilistic group yielded similar percentages of lesions (18 lesions [32.7%] and 9 lesions [34.6%], respectively). However, there was a preponderance of low-grade lesions in the interpretive group (89%) but a nearly equal distribution of low-grade and high-grade lesions in the probabilistic group (56% and 44%, respectively); overall, 22% of the lesions were high-grade. Of the 74 HPV-negative ASCUS smears, 71 (96%) were modified to reactive and all 5 with histologic follow-up were judged as negative.

Conclusions: Colposcopy with tissue studies was virtually restricted to HPV-positive cases, regardless of the reporting model used, suggesting that clinicians are basing colposcopy triage on the HPV test result rather than the definitiveness of the cytologic interpretation. This observation, the similar yield of lesions in both groups, and the significant risk of high-grade lesions argue against application of the interpretive model to HPV-tested ASCUS cases.