Prospective diagnostic study with 160 patients attending 10 general practices for the first time with complaints suspicious of obstructive airway disease (OAD). Patients were referred to a lung function laboratory for diagnostic investigation. The index test was FENO measured with a portable FENO analyser based on electrochemical sensor. The reference standard was the Tiffeneau ratio (FEV₁/VC) as received by spirometric manoeuvre and/or results of bronchial provocation. Bronchial provocation with methacholine was performed to determine bronchial hyper-responsiveness (BHR) in the event of inconclusive spirometric results.

88 (55%) were female; their average age was 43.9 years. 75 (46.9%) patients had asthma, 25 (15.6%) had COPD, 8 (5.0%) had an overlap of COPD and asthma, and 52 (32.5%) had no OAD. At a cut-off level of 46 parts per billion (ppb) (n = 30; 18.8%), sensitivity was 32% (95%CI 23–43%), specificity 93% (95%CI 85–97%), positive predictive value (PPV) 80% (95%CI 63–91%), negative predictive value (NPV) 61% (95%CI 52–69%) when compared with a 20% fall in FEV₁ from the baseline value (PC₂₀) after inhaling methacholine concentration ≤ 16 mg/ml. At 76 ppb (n = 11; 6.9%) specificity was 100% (95%CI 96–100%) and PPV was 100% (95%CI 72–100). At a cut-off level of 12 ppb (n = 34; 21.3%), sensitivity was 90% (95%CI 79–95%), specificity 25% (95%CI 17–34%), PPV 40% (95%CI 32–50), NPV 81% (95%CI 64–91%) when compared with a 20% fall of FEV₁ after inhaling methacholine concentration ≤ 4 mg/ml. Three patients with unsuspicious spirometric results have to be tested with FENO to save one bronchial provocation test.