Phenytoin and phenobarbital concentrations in saliva and plasma measured by radioimmunoassay.

Saliva and plasma levels of phenytoin (DPH) and phenobarbital (PB) in a series of epileptic patients were compared by means of a radioimmunoassat (RIA) that required only 10 mul 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 degrees. PB plasma and saliva levels were also closely related (r = 0.98 for logarithm of plasma and saliva levels). This relation was nonlinear [plasma ocncentration = 4.43 X (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.