Atypical squamous cells (ASC) are abnormal cells found on a Pap test, but the reason for the abnormalities is not known. The causes of ASC include infection and inflammation.

ASC is further classified as:

• ASC of undetermined significance (ASC-US).
• ASC but cannot exclude high-grade squamous intraepithelial lesion (HSIL) (ASC-H).

When a woman has ASC-US abnormalities, her health professional will probably recommend watchful waiting or human papillomavirus (HPV) testing. ASC-H abnormalities have a higher risk of being identified as more severe changes and are evaluated with colposcopy.