Daily variability of Pseudomonas aeruginosa density in cystic fibrosis sputum.

Treatment-associated differences in Pseudomonas aeruginosa (Pa) density in sputum have been used as a response biomarker in clinical trials of cystic fibrosis (CF) therapies. Although most studies have included placebo-treated groups as comparators, variability of Pa density in untreated individuals has rarely been reported. We measured day-to-day differences in Pa density in 267 sputum sample pairs collected from 13 adults with CF during days in which no changes in antibiotic therapy occurred. Although the mean sputum Pa density change across all sample pairs was modest (-0.09 log10 16S rRNA gene copies/mL), variability in day-to-day changes were substantial (SD = 1.09) with one-quarter of sample pairs having >1 log10 differences in Pa density; approximately 8 % of pairs had >2 log10 differences in density. Day-to-day variability in Pa density differed substantially between study participants (p = .001). These results will support the design and interpretation of studies using sputum Pa density change as an efficacy biomarker.