The atypia of undetermined significance/follicular lesion of undetermined significance:malignant ratio

BACKGROUND: The Bethesda System (TBS) for reporting thyroid cytopathology introduced the atypia of undetermined significance/follicular lesion of undetermined significance (AUS) category, but did not provide adequate guidance for the appropriate use of this diagnosis. In the current study, the authors sought to identify an appropriate measure for AUS use based on experience to date with TBS. METHODS: The authors reviewed 8 series, including their own laboratory experiences, with a total of 30,466 thyroid aspirates classified within TBS. RESULTS: The median AUS rate was 9.9% with a range of 3.0% to 18.0%. Use of the individual diagnostic categories within TBS varied up to 12.7-fold. The ratio of "suspicious for follicular neoplasm" plus "suspicious for malignancy" to "malignant" (M) diagnoses varied the least (1.8-fold). The AUS: M ratio provided a suitable measure of assessing AUS use, with a median ratio of 2.0 and a range of 0.5 to 4.9. CONCLUSIONS: Based on available studies, an AUS: M ratio of 1.0 to 3.0 is recommended. AUS: M ratios > 3.0 are likely because of overdiagnosis of AUS or underdiagnosis of M. AUS: M ratios < 1.0 are mostly due to low AUS rates, at the likely expense of sensitivity. Cancer (Cancer Cytopathol) 2012; 120: 111-6. (C) 2011 American Cancer Society.