Investigation of occupational asthma: sputum cell counts or exhaled nitric oxide?

The measure of sputum eosinophil counts is a useful tool in the investigation of occupational asthma (OA), but processing sputum is time consuming. Measuring the fractional concentration of exhaled nitric oxide (FENO) may be an alternative in clinical practice. The aim of this study was to assess the respective changes of sputum eosinophil counts and FENO following exposure to occupational agents in the routine practice of two tertiary centers in North America and Europe.

Workers undergoing specific inhalation challenges (SICs) for possible OA in tertiary clinics in both Canada and Belgium were enrolled. Sputum cell counts and FENO were collected at the end of the control day and at 7 and 24 h after exposure to the offending agent.

Forty-one subjects had a negative SIC; 26 subjects had OA proven by a positive SIC. In subjects with positive SIC, there was a significant increase in sputum eosinophils at 7 h (9.0 [9.9]%) and 24 h (11.9 [14.9]%) after exposure compared with the baseline (2.8 [4.2]%), whereas there was a significant increase in FENO only 24 h after exposure (26.0 [30.5] ppb) compared with the baseline (16.6 [18.5] ppb). A 2.2% change in sputum eosinophil counts achieved a much higher sensitivity and positive predictive value than a 10-ppb change in FENO with similar specificity and negative predictive value for predicting a 20% decrease in FEV(1) during SICs.

Sputum eosinophil counts constitute a more reliable tool than FENO to discriminate positive and negative SICs.