Background. The purpose of this study was to investigate the clinical and ultrasonographic (US) findings affecting the nondiagnostic cytology and malignancy rate in thyroid nodules with nondiagnostic cytology upon a second ultrasound- guided fine needle aspiration (US-FNA). Materials and Methods. A total of 1124 nodules in 1081 patients (M:F = 111:970; mean age, 48.5 years) with 2 or more US-FNAs were included. Clinical and US features were compared between the nodules with nondiagnostic results and diagnostic results from their second FNA using univariate and multivariate analyses. The malignancy rate was analyzed in terms of nondiagnostic cytology. Results. Among 458 nondiagnostic results of the initial FNA, 94 nodules (20.5%) still had nondiagnostic results on the second FNA. Having cystic portion >50% (odds ratio [OR] = 2.845, P = .0001), nondiagnostic cytology of the first FNA (OR = 2.813, P<.0001), a nodule size ≥5 mm (OR = 1.742, P = .0331), or hypoechogenicity (OR = 1.512, P = .0404) were factors significantly affecting the incidence of nondiagnostic results in the second US-FNA based on multivariate analysis. The malignancy rate was 11.4% in the nodules with nondiagnostic results in the first FNA and 3.2% in the nodules with serial nondiagnostic results in both the first and second FNAs. Conclusions. The high probability of nondiagnostic results from a second FNA should be considered if a nodule shows cystic portion greater than 50%, nondiagnostic cytology on the first FNA, a nodule size ≤5 mm, or hypoechogenicity. The malignancy rate of thyroid nodules with 2 consecutive nondiagnostic results was 3.2%.
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