Sampling airway surface liquid: Non-volatiles in 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 mumol/L (n=18), an 8,600-fold dilution from predicted blood urea nitrogen levels. Protein averaged 2.3+/-0.3 mug/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 mumol/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.