Phenytoin and phenobarbital concentrations in saliva and plasma measured by radioimmunoassay

Abstract

Saliva and plasma levels of phenytoin (DPH) and phenobarbital (PB) in a series of epileptic patients were compared by means of a radioimmunoassay (RIA) that required only 10 µ11 of saliva or plasma. There was an excellent linear relation (r = 0.98) between the logarithms of the concentrations of DPH in the two fluids. The ratio saliva/plasma was remarkably constant at 0.10 and was unaffected by varying levels of PB. The ratio was close to the fraction of DPH reported unbound in plasma at 37°. PB plasma and saliva levels were also closely related (r = 0.98 for logarithm of plasma and saliva levels). This relation was nonlinear [plasma concentration = 4.43 × (salivary concentration)^0.86], but could be approximated by the ratio plasma/saliva = 3.4. The simplicity of sample collection and the sensitivity of the RIA procedure suggest that clinical monitoring of these anticonvulsant levels may be carried out by RIA on saliva samples.