Therapeutic Handbook - GGC Prescribing

Continued from previous page2. 'Top-up' loading dose of phenytoin for statusepilepticusIf phenytoin is already present but the patient is still not controlled, a 'top-up' loading dose may beuseful.Phenytoin sodium 'top-up' dose (mg) = (20 - measured concentration (mg/L)) x 0.7 x wt (kg)Table 2 gives the approximate increase in concentration following doses of 250 – 750 mg. Forexample, if the patient weighs 70 kg and has a measured concentration of 5 mg/L, a single dose of750 mg will increase the concentration to around 20 mg/L (5 mg/L + 15 mg/L).Table 2 – Increase in phenytoin concentration with 'top-up' dosesConcentration increase with 'top-up' doseDoseWeight50 kg 60 kg 70 kg 80 kg250 mg 7 mg/L 6 mg/L 5 mg/L 4.5 mg/L500 mg 14 mg/L 12 mg/L 10 mg/L 9 mg/L750 mg 21 mg/L 18 mg/L 15 mg/L 13.5 mg/L3. Maintenance dose of phenytoinPhenytoin typical doses are 3 - 5 mg/kg/day. The first dose should be given12 - 24 hours after the loading dose.Oral or nasogastric administration should be used, whenever possible. Only use intravenousadministration when these options are not feasible and where cardiac monitoring is available.Notes• Phenytoin sodium 100 mg capsules / tablets / injection = 15 ml (90 mg) suspension (6 mg/ml).• There are many drug interactions with phenytoin (consult the BNF Appendix 1 or your clinicalpharmacist).• Phenytoin concentrations increase disproportionately with dose; toxicity may occur if themaintenance dose is increased by more than 25 - 50 mg per day. Table 3 on the next pagemay help with dosage adjustment. Based on the patient’s current dose and the measuredconcentration (columns 1 and 2), column 3 gives a rough guide to interpretation of the result andpossible dosage adjustment.N.B. Table 3 is for maintenance dose adjustment only. For 'top-up' doses in urgent situations seeTable 2.Central Nervous SystemContinues on next pagePage 193