Table 4.

Parameters for assessing the optimal cut-off point for urinary cotinine (ng/ml) obtained by Youden's index, as well as the levels of 25, 50 and 100 ng/ml, for classifying pregnant women as regular or occasional smokers

Youden's index*	Cut-off point (95% CI)†	Sensitivity	Specificity	Positive PV‡	Negative PV‡	AUC of the ROC (95% CI)§
Regular and occasional smokers: 1845 non-smokers, 418 smokers
–	50	0.957	0.951	0.816	0.990
–	100	0.950	0.966	0.865	0.988	0.986 (0.982 to 0.990)
0.918	82 (42 to 136)	0.952	0.966	0.863	0.989
Results stratified by frequency of smoking
Occasional smokers: 1845 non-smokers, 37 smokers
–	25	0.892	0.870	0.121	0.998
–	50	0.649	0.951	0.211	0.993	0.947 (0.923 to 0.970)
0.772	27 (11 to 43)	0.892	0.880	0.130	0.998
Daily smokers: 1845 non-smokers, 381 smokers
–	50	0.987	0.951	0.807	0.997
–	100	0.982	0.966	0.858	0.996	0.990 (0.986 to 0.994)
0.949	115 (57 to 189)	0.982	0.967	0.862	0.996
Results stratified by SHS exposure among non-smokers
Non-exposed to SHS: 798 non-smokers, 418 active smokers
–	50	0.957	0.987	0.976	0.978
–	100	0.950	0.990	0.980	0.974	0.994 (0.990 to 0.998)
0.954	42 (27 to 57)	0.971	0.982	0.967	0.985
Exposed to SHS (1 source**): 735 non-smokers, 418 active smokers
–	50	0.957	0.935	0.893	0.974
–	100	0.950	0.958	0.928	0.971	0.981 (0.974 to 0.989)
0.910	82 (46 to 136)	0.952	0.958	0.928	0.972
Exposed to SHS (2–3 sources**): 303 non-smokers, 418 active smokers
–	50	0.957	0.894	0.926	0.938
–	100	0.950	0.924	0.945	0.930	0.977 (0.966 to 0.987)
0.877	106 (58 to 227)	0.950	0.927	0.947	0.930

*Youden's index=max (sensitivity+specificity−1).

†95% Bootstrap CI for the cut-off point associated with Youden's index.

‡Predictive value of a positive or negative result for the prevalence of smoking in the study group.

§Area under the receiver operating characteristic curve and 95% CI.

**Number of sources of exposure at work, home and elsewhere in leisure time.

SHS, second-hand smoking.