Background and objective: sRaw (specific airway resistance) is a corrected index (Raw multiplied by thoracic gas volume) that describes airway behaviour regardless of lung volume. Normal values of sRaw in adult subjects have never been formally defined. To establish sRaw interpretation criteria and to define a range of reference values, we evaluated variability, reproducibility and reliability of sRaw measurements in a group of healthy adults.
Methods: We analysed 517 subjects of both genders, aged 18–65 (group A), and to assess the reproducibility of the measurements, we investigated intra-individual variation and potential daily and weekly sRaw rhythms in a subgroup of 18 co-operative healthy subjects (group B).
Results: In group A, there was no pattern of association between any of the considered anthropometric parameters; mean sRaw was higher in men (6.24 vs 5.95 cmH2O s in females; P = 0.0128), but when the data were stratified by age, gender-related differences were only found in the group aged 46–60 (males 6.45 cmH2O s, females 6.01 cmH2O s; P = 0.0219). In group B, there was no statistically significant, time-dependent variation during the single tests, nor any circadian or weekly rhythm.
Conclusions: sRaw is a reliable parameter; therefore, we propose that the lower and upper 95% confidence limits should be considered as reference values for adults of both genders, regardless of age. The availability of reference values may be useful in clinical practice and research.