Can saliva replace plasma for the monitoring of methadone?
The aims of this study were to determine the relationship between saliva and plasma methadone concentrations and the influence of variability in saliva pH. Saliva and plasma samples were taken before the daily dose of methadone in 60 patients undergoing methadone maintenance treatment (MMT). Saliva pH was measured immediately after sampling, and concentrations of (RS)-, (R)-, and (S)-methadone in saliva and plasma were assayed by LC/MS. In addition, unbound (R)- and (S)-methadone concentrations were measured in plasma samples by ultrafiltration. Plasma binding and pH differences between plasma and saliva were then used to estimate methadone saliva/plasma ratios and to compare them with observed values. Saliva pH ranged from 5.1 to 7.6 (mean +/- SD, 6.7 +/- 0.5). Plasma and saliva concentrations correlated weakly [(RS)-, r = 0.14, P = 0.007, n = 44; (R)-, r = 0.10, P = 0.04, n = 43; (S)-, r = 0.22, P = 0.002, n = 43], and the mean saliva-to-plasma methadone concentration ratios were 1.1
