Table 5. Ability of the three risk stratification systems to identify nodules whose FNAB were unnecessary, and to discriminate between benign and malignant nodules.

RSS	Avoided unnecessary biopsies (%)	FNR % (95% CI)	Sensitivity %(95% CI)	Specificity %(95% CI)	PPV %(95% CI)	NPV %(95% CI)	AUC
ACR TIRADS	122/425 (28.7)	62.3 (55.9, 68.6)	37.7 (31.4, 44.1)	61.9 (55.1, 68.8)	53.4 (45.6, 61.2)	46.2 (40.2, 52.3)	0.69 (0.64, 0.75)
K-TIRADS	63/425 (14.8)	60.1 (53.7, 66.5)	39.9 (33.5, 46.3)	32.0 (25.4, 38.5)	40.4 (34.0, 46.9)	31.5 (25.0, 38.0)	0.66 (0.60, 0.71)
R-TIRADS	107/425 (25.2)	25.4 (19.7, 31.1)	74.6 (68.9, 80.3)	54.3 (47.3, 61.3)	65.4 (59.6, 71.2)	64.8 (57.5, 72.2)	0.79 (0.74, 0.83)

FNAB, fine needle aspiration biopsy; ACR, American College of Radiology; K, The Korean Society of Thyroid Radiology; R, revised; TIRADS, Thyroid Imaging Reporting and Data System; AUC, area under the receiver operating characteristic curve; CI, confidence intervals; FN, false negative; FNR, false negative rate; PPV, positive predictive value; NPV, negative predictive value; RSS, risk stratification system.