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Background:Several thyroid societies have endorsed ultrasound (US) malignancy risk stratification systems for thyroid nodules and the recently released European Thyroid Imaging Reporting and Data System (EU-TIRADS) needs large prospective studies for validation. Objective:The purpose of our study was to evaluate the performance of EU-TIRADS in identifying thyroid nodules for fine-needle aspiration biopsy (FNAB) and its ability to reduce the number of unnecessary biopsies. Methods:This was a single-center prospective study. From August 2017 to September 2018, 783 consecutive patients with 1,000 thyroid nodules underwent US examination and US-guided FNAB. A total of 741 patients (median age 50 years; range, 15–87 years; 649 females, 92 males) with 942 nodules (median largest diameter 14 mm; range, 4–96 mm) met the following inclusion criteria: (1) nodules with benign or malignant cytology – categories II and VI of the Bethesda System for Reporting Thyroid Cytopathology (BSRTC); (2) nodules with non-diagnostic and indeterminate cytology (BSRTC I, BSRTC III, and BSRTC IV), or suspicious for malignancy (BSRTC V), if postoperative histology was present; (3) nodules classified as BSRTC I and BSRTC III with a repeat FNAB and conclusive cytology. Results:Of 942 nodules, 839 (89.1%) were benign and 103 (10.9%) were malignant. Nodules were classified as follows: EU-TIRADS 2 – 4.8%, EU-TIRADS 3 – 37.4%, EU-TIRADS 4 – 25.2%, and EU-TIRADS 5 – 32.6%. The malignancy rate in categories 2 to 5 was 0, 0, 3.8, and 30.6%, respectively. Sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and accuracy of EU-TIRADS with a cut-off set at category 5 were 91.3, 74.6, 30.6, 98.6, and 76.4%, respectively. Diagnostic performance other than sensitivity and NPV was superior in nodules ≥10 mm. FNAB number would be reduced by 53.4% if FNAB criteria were strictly applied. When the indication for FNAB was applied as test positivity, the estimated sensitivity, specificity, PPV, and NPV of EU-TIRADS were 69.9, 56.3, 16.4, and 93.8%, respectively. Conclusion:EU-TIRADS provides effective malignancy risk stratification that can guide the selection of thyroid nodules for biopsy. The application of the guidelines criteria for FNAB in the clinical practice might reduce significantly the number of unnecessary FNAB.