False positive cases in thyroid cytopathology - the experience of a single laboratory and a systematic review

Fine needle aspiration (FNA) is fundamental in the pre-operative assessment of thyroid nodules. However, the shining success of thyroid FNA is occasionally eclipsed by false positive (FP) cytological diagnoses. We describe our experience regarding FP cytological diagnoses and present a literature review concerning FP rates.

We retrospectively analyzed 2031 consecutive cytology reports. FP was defined as a case with "suspicious for malignancy" or "malignant" cytology, whose surgical follow-up failed to confirm the initial diagnosis of malignancy. We used the PubMed database for the literature review using specific search terms ("thyroid", "cytology", "false positive").

Out of 2031 FNA reports, 93 cases with "suspicious for malignancy" or "malignant" cytology underwent surgical excision. Histopathology did not confirm malignancy in 10 cases (0.6% FP rate). Nodules with FP cytology were significantly larger (P = 0.0024) than those with with TP (true positive) cytology. A review of 25 publications shows comparable results to ours with some slight differences. The majority of studies achieve a FP rate below 3%. Most FP cytological diagnoses turn out to be non-neoplastic lesions (nodular hyperplasia, adenomatous nodule, lymphocytic thyroiditis), which combined account for 80.5% of all histological diagnoses.

Thyroid nodules with FP cytology were larger than nodules with TP cytology. Most studies manage to adhere to the recommended FP rates, or values close to them, with some outliers. Nevertheless, the mean FP rate values are very close to the value recommended by The Bethesda System for Reporting Thyroid Cytopathology (TBSRTC). The FP rate value depends on how one defines true negative cases, which can be either histologically proven benign cases, via a benign clinical course, or by assuming that cytologically benign cases remain as such.