Introduction The interpretation of Specific Inhalation Challenge (SIC) can be equivocal, particularly for late asthmatic reactions. It has been suggested that increases in FENO 24-hours post-challenge might help separate positive from negative challenges.
Methods We reviewed all positive and equivocal SIC tests with occupational agents between March 2008 and June 2012 from our tertiary referral centre. FENO was measured pre- and 24-hours post control and active challenges using a Niox Mino handheld machine at 50 ml/sec, compliant with ERS/ATS recommendations. Post-challenge changes >20% for FENO >50 ppb, or >10 ppb for <50 ppb, were counted as per ATS guidelines for a clinically significant change (1).
Results 24 patients had complete data related to control and active challenges, which were positive in 15 and equivocal in 9 cases. 13/24 patients had raised pre-control challenge FENO (mean=31.3) after adjusting for smoking and inhaled corticosteroid use. Increases in FENO, more than the minimum clinically relevant difference, were seen after 13/24 control challenges: including 6/7 exposures to cleaning agents or hand gels and 2/6 unused metalworking fluids. 5/24 patients had a clinically significant increase in FENO after positive or equivocal challenges: including 1/4 challenges with isocyanates, 1/6 cleaning agents or hand gels, and 2/3 with used metalworking fluids. There was no statistically significant difference in mean percentage change in FENO between control and active challenges.
Conclusions The previously defined minimum clinically relevant difference for FENO was seen as commonly following control as active challenges. Measuring changes in FENO pre- and 24-hours post challenge to the diverse range of low molecular weight agents tested did not provide useful additional information for interpreting SIC responses.
Dweik RA et al. An Official ATS Clinical Practise Guideline: Interpretation of Exhaled Nitric Oxide Levels (FENO) for Clinical Applications. Am J Respir Crit Care Med 2011; 184:602–15.