Induced sputum: Validity of fluid-phase IL-5 measurement

Background: IL-5 measurement in the fluid phase of induced sputum is considered to be important in the assessment of asthma, but the validity of these measurements is uncertain. Objective: We investigated the validity of sputum IL-5 measurements through a series of spiking experiments and examined the effect of dithiothreitol (DTT) on these measurements. Methods: Induced sputum from 26 asthmatic subjects was spiked with IL-5 and processed, and the percentage of recovery was measured by means of immunoassay. In 6 of the 26 samples the effect of adding albumin to the processing fluids was studied. In 3 separate samples radiolabeled IL-5 was added, and the recovery measured by means of gamma counting and immunoassay were compared. In addition, the effect of DTT on the immunoassay was examined. Results: The mean +/- SD recovery of spiked IL-5 was 26.1% +/- 14.6% measured by means of immunoassay; adding albumin increased the recovery to 47.7% +/- 8.0% (P < .001). The mean recovery measured by means of gamma counting was 84.8% +/- 5.7% (P < .001); adding albumin had no effect on recovery. DTT had no significant effect on IL-5 measurement. Conclusion: The validity of IL-5 measurement by means of current methods is poor. The discrepancy in recovery as measured by gamma counting compared with immunoassay suggests that there is a problem with the recognition of IL-5 epitopes by immunoassay in induced sputum. This cannot be attributed to DTT but may be due to other interfering substances present in sputum, such as sputum proteases, soluble receptors, or autoantibodies.