Exhaled nitric oxide (FE_NO) has been used as a marker of asthmatic inflammation in non-occupational asthma, but some asthmatics have a normal FE_NO. In this study we investigated whether, normal FE_NO variants have less reactivity in methacholine challenge and smaller peak expiratory flow (PEF) responses than high FE_NO variants in a group of occupational asthmatics.

We measured FE_NO and PD₂₀ in methacholine challenge in 60 workers currently exposed to occupational agents, who were referred consecutively to a specialist occupational lung disease clinic and whose serial PEF records confirmed occupational asthma. Bronchial responsiveness (PD₂₀ in methacholine challenge) and the degree of PEF change to occupational exposures, (measured by calculating diurnal variation and the area between curves score of the serial PEF record in Oasys), were compared between those with normal and raised FE_NO. Potential confounding factors such as smoking, atopy and inhaled corticosteroid use were adjusted for.

There was a significant correlation between FE_NO and bronchial hyper-responsiveness in methacholine challenge (p = 0.011), after controlling for confounders. Reactivity to methacholine was significantly lower in the normal FE_NO group compared to the raised FE_NO group (p = 0.035). The two FE_NO variants did not differ significantly according to the causal agent, the magnitude of the response in PEF to the asthmagen at work, or diurnal variation.

Occupational asthma patients present as two different variants based on FE_NO. The group with normal FE_NO have less reactivity in methacholine challenge, while the PEF changes in relation to work are similar.