Utilization of direct smears of thyroid fine‐needle aspirates for ancillary molecular testing: A comparison of two proprietary testing platforms

Background Ancillary molecular testing has been recommended for thyroid fine‐needle aspirates (FNA) with indeterminate cytologic diagnoses. Rosetta Genomics and Interpace Diagnostics have developed assays that can utilize direct smears as the testing substrate. Methods A retrospective study of indeterminate thyroid FNAs with known histologic follow‐up was performed. One Diff‐Quik‐stained smear and one Papanicolaou‐stained smear with similar cellularity (at least 60‐100 lesional cells) from each case were sent to Rosetta and Interpace, respectively, for analysis. The results were directly compared and correlated with the final histopathology. Neither company was aware of the follow‐up histologic findings in these cases. Results A total of 10 thyroid FNAs were identified from our 2015 files. The cytologic diagnoses included follicular lesion of undetermined significance (FLUS, n = 5), follicular neoplasm/suspicious for follicular neoplasm (FN/SFN, n = 4), and suspicious for malignancy (SM, n = 1). Of the seven cases with benign histology, six smears were classified as benign by the RosettaGX microRNA classifier, and one case was designated as suspicious. Five cases were negative by both ThyGenX oncogene panel and ThyraMIR microRNA classifier. One case was negative by ThyGenX and positive on follow‐up ThyraMIR, and one case was positive for KRAS mutation and positive on ThyraMIR. Both the RosettaGX and ThyGenX/ThyraMIR tests demonstrated positive results for the three histologically malignant cases. Conclusion This study demonstrates that two molecular testing platforms performed equally well using our stained direct smears. Both molecular tests revealed a 100% negative predictive rate. RosettaGX showed a 75% positive predictive value in comparison to 60% for ThyGenX/ThyraMIR.