Validation of the fractional exhaled breath temperature measurement: reference values.

Exhaled breath temperature (EBT) is a known biomarker of inflammation and airways blood flow. As opposed to previous studies, we were able to measure temperature of separate fractions of exhaled breath (fEBT) (those from the peripheral and central airways). The aim was to validate the fEBT measurement method to determine the reference values and the influence of endogenous and exogenous factors on fEBT in healthy subjects. This cross-sectional study included 55 healthy adults in whom fEBT was repeatedly measured, two days in a row, using a FractAir^®device. Also, basal metabolic rate, level of physical activity, distance from the main road, outdoor and ambient temperature, air pressure and humidity, haematology and inflammation markers, lung function, cumulative EBT and body temperature at characteristic points on the body were measured. The results showed that fEBT from central airways was lower compared to fEBT from the periphery and that fEBTs were not related to body temperature (p> 0.05 for all). We also showed repeatability of fEBT measurements for two consecutive days. All EBT fractions correlated significantly with ambient temperature (<0.01). No associations of fEBT with other personal and external factors were found using multivariate analysis. At room temperature of 22 °C, the physiological temperature values of the first fraction were 23.481 ± 3.150 °C, the second fraction 26.114 ± 4.024 °C and the third fraction 28.216 ± 3.321 °C. The proposed reference values represent the first part of validation of fEBT as the method for the use in clinical practice.