Table 3– The relationship between the number of volatile organic compounds (VOCs) in the support vector machine model and the prediction of exacerbations: intrasubject comparison

VOCs n	Sensitivity %	Specificity %	Correct classification %
9	100	100	100
8	100	100	100
7	100	93	96
6	100	93	96
5	100	79	89
4	100	71	86
3	100	64	82
2	100	50	75
1	100	29	64

An optimal model contained six VOCs and had a sensitivity of 100%, a specificity of 93%, a positive predicted value of 89% and a negative predicted value of 100% (bold).