Diurnal variation and repeatability of surfactant protein A and albumin in exhaled particles

Surfactant protein A (SP-A) is considered a potential biomarker of small airway inflammation. In the present study we aimed to assess within- and between-day variation of SP-A and albumin in exhaled particles (PEx) and define the expected maximal normal variation in healthy individuals. 16 healthy non-smoking subjects were examined with PExA®, in the morning, noon and afternoon during three non-consecutive days. SP-A and albumin were quantified by ELISA and expressed as weight % in PEx (wt%). The differences on the same time points between different days were assessed and the coefficient of repeatability (CR) and 95% CI of limits of agreement (LoA) was calculated. Diurnal variation was tested with repeated two-way ANOVA. The variation of SP-A between days was low and non-significant (p = 0.75), whereas albumin in PEx varied more (p Our findings show that there is a rather small diurnal variation of SP-A in PEx. We also describe the maximal normal variation of these biomarkers over time, which may be of importance in clinical settings, when assessing the effect of exposure challenge and medical intervention.