Background: IL-5 measurement in the fluid phase of induced sputum is consid
ered 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 immunoa
ssay. 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 comp
ared. 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% measur
ed 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. DT
T 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 w
ith 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.