PHENYTOIN LEVEL CHECK

Q. A 72-year-old man with known epilepsy and hypertension is admitted with pneumonia. His drug history includes aspirin, phenytoin, bendroflumethiazide and amlodipine. His heart rate is 67, blood pressure 170/93, sats 96 per cent on 2 L of oxygen. Neurological examination is normal. His doctor requests blood tests
including phenytoin level. What is the correct indication for this test?
A. Routine check
B. Ensure levels are not toxic
C. Confirm patient compliance
D. Ensure therapeutic level reached
E. Reassure the patient

Ans:  C

Explanation: 

 Routine measurement of phenytoin levels (A) is not good practice, they should be ordered with a question in mind. They can be helpful either for adjustment of phenytoin dose or looking for toxicity or patient compliance. Phenytoin levels are useful when adjusting the dose to avoid toxicity as phenytoin has zero-order kinetics (once elimination reaches saturation rates, it cannot be cleared any faster so a small change in the dose may result in high blood levels), but there is no reason to change this patient’s dose. There is no reason to suspect phenytoin toxicity either (B) as there are no signs or symptoms such as nystagmus, diplopia, dizziness, ataxia, confusion. However, his high blood pressure may be caused by noncompliance with his medication (C). Although target levels exist (D), they are imprecise and not applicable to all patients. Seizures may be well controlled with low levels, thus phenytoin should be adjusted according to the clinical picture and not levels. Levels are not helpful in reassuring the patient in this situation (E), although they often inappropriately reassure the doctor who requests them.