Phenytoin is one of the most widely prescribed anticonvulsants. It is also used occasionally as a myocardial anti-arrhythmic. Phenytoin levels are measured to ensure appropriate therapeutic levels are maintained. Due to a narrow therapeutic index, plus wide individual variability in the rate of phenytoin metabolism and clearance, therapeutic drug monitoring is essential during treatment with phenytoin. Insufficient levels may lead to inadequate symptom management, while excess levels can lead to dangerous toxicity. Patients of the BC Cancer Agency may be receiving phenytoin therapy for a variety of symptoms. Therefore, the Tumour Marker Lab measures phenytoin as part of its test menu. Phenytoin is measured using flourescence polarization immunoassay (FPIA) on an Abbott TDX analyzer. The sample is serum, collected in a red top (no additive) tube, only. SST tubes may cause interference in therapeutic drug testing. Testing is performed STAT. Samples are to be sent immediately to the lab. The requisition is to include the diagnosis, sample date, and sample time. Turnaround time is 90 minutes after receipt in the TML. Please provide a contact number so results may be phoned to the correct physician or location. Therapeutic Range = 40-80 umol/L Interferences: Some cross reactivity with Fosphenytoin. However, this is not an issue if patient samples are drawn at least two hours post IV administration or 4 hours post IM administration. Uremic specimens may exhibit a positive bias with the phenytoin assay.