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To determine the relative contribution of age and absorption on phenytoin (PHT) kinetics in full-term newborns, we measured PHT apparent half-life (tSoX) 15 times in 9 infants aged 2 to 36 days. PHT was given i.v. 9 times and p.0. 6 times. The PHT t 5~~ ranged from 140 hours in a 2 day old to 3.78 hours in a 36 day old. Whereas the t correlated directly with initial concentration (Ci) in t30eX8 patients)9 days old (r=0.755, p<.05), there was no correlation in 7 younger patients. TS0% varied inversely with age, averaging 84.7 hr (SD 42.6) in patients less than 8 days old and 14.7 hr (SD 7.74) in older patients (t-4.59, p<.01). Controlling for the effect of Ci, the average t was still reduced by more than 3 times in the older group (%7 vs 25.5 hr, p<0.02). Preliminary study of PHT absorption suggests that the reduction of PHT levels seen in infants more than 1 week old is even greater than predicted by this 3-fold increase in PHT eliminatory capacity and supports the notion that PHT bioavailability is significantly reduced in infants.

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