IV. Exhaled NO in Clinical Situations

Measurement of exhaled NO provides a useful method of differentiating asthma from other conditions. The diagnostic value of NO measurements and the ability of the method to differentiate between subjects with airway symptoms and patients with true asthma were analysed by Dupont et al. [Dupont LJ et al. 2003] The sensitivity and specificity of the method is dependent on the selection of an appropriate cut-off point. The study involved 240 consecutive, non-smoking, steroid-naïve patients and found that a cut-off point for exhaled NO of 16 ppb (at a flow rate of 200 mL/s), gave a specificity for the diagnosis of asthma of 90% and a positive predictive value of > 90%. [Dupont LJ et al. 2003] With a cut-off of 20 ppb (flow rate 200 mL/s), a specificity of 100% was seen. In a study by Heffler et al. [Heffler E et al. 2006] involving 48 patients with asthma-like symptoms, in which asthma was diagnosed on the basis of 12% improvement in FEV1 after salbutamol or a methacholine PD20FEV1 < 800 μg, none of the asthmatic patients had exhaled NO values < 25 ppb (at 50 mL/s) and all the patients with exhaled NO > 100 ppb (n = 5) were diagnosed with asthma. The sensitivity and specificity of exhaled NO for detecting asthma, using 36 ppb as cut-off point, were 78% and 60% and the positive and negative predictive values were 54% and 82%, respectively.