The effect of sampling procedures on the urate and lactate concentration in oral fluid

This study was aimed at evaluating the influence of sampling procedure on the determination of uric acid and lactate in oral fluid. Samples of non-stimulated and stimulated oral fluid were collected from 22 healthy volunteers. Different frequencies of stimulation were obtained by moving a polyester swab within the mouth at 50, 100 and 150 min–1. Three oral fluid samples were consecutively collected from a subgroup of 5 volunteers at a constant stimulation (70 min–1) and at a self-selected pace to evaluate reproducibility. The urate concentration in oral fluid decreased with the increase of the stimulation and oral fluid flow rate (r = -0.98, p = 0.01). Also, the lactate concentration was much (p = 0.03, two tailed) lower in samples collected under a mild stimulation (50 min–1) than in samples collected without stimulation. Nevertheless, it progressively increased at higher stimulations (100 and 150 min–1). A transfer process mediated by membrane carriers (i.e. urate transporter and organic anion transporters) was hypothesized to explain these results. Finally, a reduced variability (relative standard deviation below 10%) of the urate concentration was obtained when oral fluid was sampled at constant stimulation (70 min–1), but it increased remarkably (20–50%) in case of sampling at self-selected pace. Nevertheless, expressing the salivary excretion of urate as a function of time (μgmin–1), the variability of sampling procedure at self-selected pace was lower than 15%.