‘Atypical squamous cells’ (ASC) is used when cytological findings are considered suggestive but not diagnostic of a squamous intraepithelial lesion (SIL) (Figure 31). The term ASC was retained in the 2001 Bethesda System because of the wide recognition that these cells imply a significant risk for an underlying high-grade cervical intraepithelial lesion (SIL). In various studies, the prevalence of CIN 2 or 3 in women with ASC has varied between 10% and 20% (Wright et al., 2002). The ASC category roughly correlates with the ‘borderline dyskaryosis’ category used in the United Kingdom. However, neither the WHO terminology nor the CIN terminology incorporates a category similar to ASC.
The 2001 Bethesda System also clearly separate s A S C from reactive/reparative changes and an interpretation of ASC should not be made whenever a cytopathologist identifies minor cytological abnormalities. The term ASC should be used only when the cytological findings are suggestive, but not diagnostic, of SIL. Currently, approximately 4–5% of all cervical cytology specimens are classified as ASC in the USA (Jones & Davey, 2000).
The ‘atypical squamous cell’ category is formally subdivided into two subcategories: ‘atypical squamous cells – of undetermined significance’ (ASCU S or ASC-US) and ‘atypical squamous cells – cannot exclude a high-grade SIL’ (ASC-H). This subdivision was felt to be important because women with ASC-H (Figure 32) are at considerably higher risk for having CIN 2 or 3 and of being high-risk HPV DNA-positive than are women with ASCUS (Genest et al., 1998; Sherman et al., 1999, 2001; Selvaggi, 2003).
Information from the US National Cancer Institute ASCUS–LSIL Triage Study (ALTS) clinical trial indicates that the risk that a woman with AS C-H has CIN 2 or 3 is over twice that of a woman with ASCUS (Sherman et al., 2001). Moreo ver, the prevalence of high-risk HPV DNA- positivity among women with ASC-H is almost as h igh as that of women with a high-grade squamous intraepithelial lesion (HSIL) cytologic a l result. Therefore the recommended management of women with ASCUS and ASCH differs (Wright et al., 2002).