The gray zone squamous lesions: ASC-US / ASC-H

The unequivocal and easily recognizable entities of LSIL and HSIL pose no diagnostic problems for a trained eye. However, when the defining morphologic features are either qualitatively or quantitatively insufficient, it is then that the borderline category of “Atypical Squamous cells” (ASC) may have to be used. Scant and suboptimal preparations (mainly in conventional smears) are the common causes that hinder confident decision-making. The binary classification of the ASC category has been retained in The Bethesda System 2014. It includes ASC of undetermined significance (ASC-US) when the atypia is seen in mature cells and ASC-cannot rule out high-grade lesion (ASC-H) when borderline changes are seen in less mature, smaller metaplastic cells or smaller basaloid cells. There are many criticisms of the ASC category. The major one is its subjective and inconsistent applications and the low interobserver and intraobserver reproducibility. However, studies have shown that if we eliminate ASC-US, the LSIL rate will increase. If ASC-H is eliminated, the chances of detecting true lesions are reduced. Hence, there are strong reasons to retain the ASC category. The usual problems leading to the categorization of such cells as atypical are hyperchromasia beyond that acceptable as reactive change; abnormal chromatin pattern that is not overt dyskaryosis; minor variations in nuclear shape; and membrane outlines. Qualifying the atypical cells precisely in one of the categories has bearing on the clinical management and follow-up of the patient. Surveillance of women under the ASC-US category is either by repeat smear at 6 months and 1 year or by reflex human papillomaviruses DNA testing. Women with a Pap smear interpretation of ASC-H are directed to undergo immediate colposcopy. This article describes in detail the morphologic features of the ASC category, doubts about the correct interpretation of the chromatin pattern of the cells in question, and the differential diagnosis between normal, reactive, or inflammatory conditions, and LSIL/HSIL.