In the evaluation of thyroid nodules, fine needle aspirate (FNA) cytology can classify most thyroid nodules as benign or malignant; a minority of results, however, remain indeterminate. The Bethesda System for Reporting Thyroid Cytopathology includes 3 categories for indeterminate cytology (categories III, IV and V). Most indeterminate nodules are either sampled again or prophylactically removed. Over the last decade, molecular testing has improved the diagnostic accuracy of thyroid FNA cytology. Most recently, a new 112-gene test (ThyroSeq v3 Genomic Classifier (GC)) was developed. However, the diagnostic accuracy of this new test has not been investigated. In this prospective cohort study, 286 cytologically indeterminate thyroid nodules from 256 patients were tested using the multigene GC test to assess the diagnostic accuracy of this test in evaluating cytologically indeterminate thyroid nodules for cancer/noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP). Among the 286 samples evaluated, the FNA cytology diagnosis was Bethesda III in 172, Bethesda IV in 101, and Bethesda V in 13 cases. Notably, 10% of the samples failed a pre-sequencing or post-sequencing step due to low sample cellularity or low thyroid cell marker expression. Researchers found that the sensitivity of the test was high for Bethesda III and IV nodules combined at 94% (95% CI 86% to 98%). Specificity of the test was slightly lower at 82% (95% CI 75% to 87%) for Bethesda III and IV nodules combined. With a cancer/NIFTP prevalence of 28% across Bethesda III and IV nodules combined, the negative predictive value (NPV) was 97% (95% CI 93% to 99%), and the positive predictive value (PPV) was 66% (95% CI 56% to 75%). Importantly, of the 3% of samples that were found to be false-negatives, all were found to be low-risk follicular carcinoma tumors, without vascular invasion or metastasis. In summary, results from this study demonstrate the strength of the ThyroSeq GC test in ruling out thyroid cancers and NIFTP, as well as its potential role in preventing diagnostic surgeries and repeat cytology testing in patients with indeterminate thyroid nodules.
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