Increased detection of interleukin-5 in sputum by addition of protease inhibitors

The measurement of interleukin (IL)-5 in sputum is problematic, with interf ering factors affecting immunoassay. The authors investigated whether sputu m proteases could he acting as interfering factors by studying the effect o f protease inhibitors (PI) on sputum IL-5 measurement. Induced sputa from 20 subjects with asthma were divided into aliquots, proc essed with and without protease inhibitors (in low and high concentrations) and the levels of IL-5 (spiked and endogenous) measured by enzyme immunoas say were compared. The concentration of sputum IL-5 was significantly increased by PI, with me dian (interquartile range) levels processed with no, low and high PI concen trations being 0 (0), 41.8 (75.6) and 66.1 (124.4) pg.mL(-1), respectively. There was also a significant increase in percentage recovery of spiked IL- 5. Although high concentrations of PI reduced cell viability, there was no effect on total or differential cell counts and low concentrations of PI ha d no effect on cell counts or viability. Levels of endogenous interleukin-5 in sputum of asthmatic subjects can be s ignificantly increased by the addition of protease inhibitors, and samples which would be regarded as negative for interleukin-5 without protease inhi bitors may instead have considerable amounts of interleukin-5 detected.