Sampling Airway Surface Liquid: Non-Volatilesin the Exhaled Breath Condensate

Exhaled breath condensate (EBC) samples contain molecules that have no appreciable vapor pressure; such molecules likely derive from droplets of airway fluid. We analyzed EBC gathered from a total of 62 healthy volunteers in order to quantify the volume of airway liquid that was the source of the non-volatiles; saliva was analyzed as a reference secretion. EBC urea averaged 0.52 ± 0.12 μmol/L (n = 18), an 8,600-fold dilution from predicted blood urea nitrogen levels. Protein averaged 2.3 ± 0.3 μg/ml (n = 31), three orders of magnitude less than in saliva (1.4 ± 0.1 mg/ml, n = 15). EBC ammonia was 6.6 ± 0.6 mmol/L (1/15 that of saliva) and EBC ammonium ion was 0.90 ± 0.19 μmol/L, concentrations that are incompatible with an 8,600-fold dilution from a biological source. Thus, urea-derived dilution factors may be used to interpret EBC non-volatile molecules, but not EBC volatiles.