Validation of an automated sensitive immunoassay for quantitation of cytokines in the sputum of cystic fibrosis patients

BACKGROUND: There is little information on the reproducibility of measurement of cytokine levels in sputum obtained from cystic fibrosis (CF) patients. Our aim was to investigate whether assay of cytokine levels in CF sputum is reproducible or is hampered by proteolytic degradation. METHODS: Interleukin-8 (IL-8) and tumor necrosis factor-alpha (TNF-alpha) levels were measured in native and spiked samples (fresh or after freezing for 15 and 23 days at -70 degrees C) obtained from nine CF patients using an immunometric assay with chemiluminescent substrate run on a IMMULITE analyzer. RESULTS: For both cytokines, linearity was >0.98 for dilutions up to 1:32. After storage, cytokine concentrations in native samples varied between -2.9% and -5.6% for IL-8 and between 0.4% and 3.0% for TNF-alpha. In spiked samples, concentrations increased by 5.8%-12.6% for TNF-alpha and decreased by 3.8%-14.3% for IL-8 after 15 and 23 days of storage. In samples spiked with cytokines, the mean recovery rates for IL-8 and TNF-alpha were 109.4% and 106.3%, respectively. CONCLUSIONS: Measurement of IL-8 and TNF-alpha in CF sputum is reproducible and is not hampered by freezing and thawing of samples.