Atypical squamous cells

The TBS category of “atypical squamous cells (ASC)” derives from the concept that a significant number of cases show changes that, while abnormal, are morphologically insufficient for a definitive interpretation of either LSIL or HSIL. ASC is the most common abnormality reported on cervical cytology specimens. These equivocal specimens have a higher risk for follow-up findings of SIL than do negative cases and therefore it is important to include the ASC category in the Bethesda reporting system to maintain the sensitivity of the Pap test. In Bethesda 2014, ASC continues to be further subdivided into “Atypical squamous cells – undetermined significance (ASC-US)” and “Atypical squamous cells – a high grade squamous intraepithelial lesion cannot be excluded (ASC-H).” ASC-US comprises the predominance of ASC cases, and is broadly defined as those cases showing features suggestive of LSIL. ASC-H represents only a small minority of cases and includes cases suggestive of HSIL. This conceptual scheme is consistent with the dichotomous categorization of the more definitive categories of LSIL and HSIL, and relates to our current understanding of HPV biology and cervical carcinogenesis. This chapter includes numerous examples of each ASC entity and many of the reactive changes from which it should be distinguished.