Introduction Specific patterns of exhaled volatile compounds (eVOCs) have been described in a variety of diseases including lung cancer, tuberculosis and COPD, but there are no standardised and universally acceptable methods of sample collection, processing and data analysis. Moreover, there is little if any knowledge of repeatability of eVOCs profile. As part of a larger cohort study on eVOCs in COPD (ISRCTN82911859) we aimed to test the repeatability of eVOCs profiles.

Methods 118 COPD patients and 63 healthy controls provided three consecutive breath samples (one sample every 2 min). Subjects were all fasted for 4 h fasting and rested for 20 min rest in a closed room in our hospital before testing. Breath samples were collected by slow exhalation to vital capacity through Bio-Voc breath sampler® (Markes International, UK) which collected into two-bed carbon thermal desorption tubes. They were later analysed by gas chromatography-mass spectrometry (GC-MS). We selected Isoprene and Total eVOCs minus Isoprene as markers of variability. Isoprene is a ubiquitous eVOC linked to cholesterol metabolism, with levels linked to exertion patterns. Variation in the three repeat measurements was determined by a coefficient of variation over the samples (SD as % of mean) and dividing the area under the curve (AUC) in second and third measurement by the first sample. A single researcher took all samples and a single scientist ran the GC-MS.

Results There is substantial intra-individual variation in level of total VOCs and Isoprene and total VOC over three breaths in controls and COPD. Isoprene tended to fall in repeat sampling, most strongly in the controls, while total eVOCs increased in the second breath but fell in the third.

Discussion Intra-subject variation in eVOCs profile poses important challenges and normal ranges and acceptable limits of variation need to be set as repeatability is an important characteristic for any diagnostic test. These particular changes may reflect changes in eVOCs production due to increased oxidative stress or muscle metabolism or haemodynamic changes and metabolism induced by exhalation. Further research to explore eVOCs variability and its impact on their diagnostic potential is needed.