Biomarker validation - room air variation during human breath investigations

Abstract The engineering of ion mobility spectrometers has made significant advances in recent years with new levels of sensitivity being achieved which facilitate their use, for example, in the medical analysis of trace components in human breath. In the study reported here direct measurements of 10 mL of exhaled air were carried out using ion mobility spectrometry (IMS) and the influence of the ambient air composition on the validation of potential biomarkers carefully considered. Changes in the IMS-signals observed within a room used for clinical studies were analysed in the absence of clinical staff. A time series of one measurement every hour of ambient air was referenced against clean bottled synthetic air for comparison using two different IMS instruments equipped with multi-capillary columns (MCC). 26 different peaks were identified and selected for the signal intensity monitoring during 1 week. Some peaks show no changes, while certain peaks varied with time in a seemlingly random manner. Correlated peak intensity changes were observed as well as changes apparently relating to variations of the ventilation system during day and night cycling. It is important to fully understand these variation if incorrect conclusions are to be avoided during, for example, the important hunt for validatable biomarkers. These results will be applicable not only to IMS studies but to all experimental design strategies in this fields regardless of the selected instrumentation. As an example it is possible to cite a particular time series where a particular peak observed within the IMS-Chromatogram shows an analyte intensity decrease within the room air between the evening and the next morning of 75%. On a subsequent day the increase between the morning and the evening was observed by a factor of about 3. During 4 days, setting the mean value to 100%, an increase of up to 173% and the decrease of up to 21% were observed. In case of measurements near to the detection limit of the method of investigation, the influence of the surrounding inhaled air must be considered carefully. The daily changes of the concentrations of specific analytes in the ambient air should be taken into account in addition to the measurement of the analyte concentrations taken from the exhaled air of a patient. Recommendations will be made covering more robust validation strategies which include near-simultaneous background ambient air measurements alongside synthetic air measurements in clinical studies. Care should be taken to avoid unnecessary changes to the ambient room air during measurements such as caused by doors opening during the entire measurement time. Ambient air measurements should be included with sampling on the patient and the surrounding air, before and after measurement of the patient breath.