Agreement in surfactant measurements of sputum induced with hypertonic and normal saline

OBJECTIVE: The contribution of surfactant to the airway epithelial barrier is increasingly being recognized. Dipalmitoyl phosphatidycholine (DPPC), the major constituent of surfactant, is affected by lung injury. Hypertonic saline-induced sputum is a convenient and reliable method of assessing airway cells and mediators. However, the influence of hypertonic saline on DPPC content of sputum is unknown. In this study the level of DPPC in induced sputum obtained using hypertonic saline is examined and compared with that obtained using normal saline. METHODOLOGY: Sputum was induced on two occasions using nebulized hypertonic saline and normal saline, in 15 well non-smoking adults, in random order on different days. DPPC content (microg/mL of sputum) was measured using a standard spectrophotometric method. RESULTS: The mean of log10 DPPC level in sputum obtained using hypertonic saline was 1.88 microg/mL sputum (95% CI 1.53, 2.13) and that obtained using normal saline was 1.83 (95% CI 1.62, 2.14). The mean difference of the two methods was -0.03 (95% CI -0.27, 0.33). Bland and Altman plot showed an equal distribution around the mean and all points were within the mean +/- 2 SD. CONCLUSION: We conclude that DPPC concentration of sputum can be easily measured in induced sputum and that the use of hypertonic saline does not influence the DPPC levels.