Table 3. Test performance.

Cut-off value (ng/ml)	Sensitivity	Specificity	PPV	NPV
3	100.0	25.85	41.08	100.0
6	93.40	50.73	49.50	93.69
9	92.45	63.90	56.98	94.24
12	89.62	68.78	59.75	92.76
15	87.74	74.63	64.14	92.17
20	79.24	83.41	71.19	88.60