{"id":"phenytoin-free","name":"Phenytoin (Free)","shortName":"Free Phenytoin","aliases":["Free Phenytoin","Dilantin, Free","Phenytoin Free Level","Phenytoin Unbound","Phenytoin, Free","Unbound Phenytoin","Free Dilantin"],"abbreviations":[],"loinc":[{"code":"3969-3","primary":true},{"code":"32109-1","primary":false},{"code":"74983-8","primary":false}],"specimen":"serum-or-plasma","units":[{"ucum":"ug/mL","display":"mcg/mL"},{"ucum":"mg/L","display":"mg/L"},{"ucum":"umol/L","display":"µmol/L"}],"referenceRanges":[{"population":"Adults","low":1,"high":2,"unit":"ug/mL","unitDisplay":"mcg/mL","note":"Therapeutic range for unbound phenytoin in people taking it, not a healthy-population interval. FDA labeling pairs it with a total level of 10 to 20 mcg/mL, and says seizure control without signs of toxicity is more likely in that range, although some people are controlled at lower levels. ARUP prints 1.0 to 2.5 mcg/mL, so some labs allow a slightly higher upper limit.","sources":[{"title":"Dilantin (extended phenytoin sodium) capsules: prescribing information","url":"https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=86dd27d1-9cee-48ae-b55d-e2d2f7dbc593","publisher":"Viatris Specialty (DailyMed)","year":2023},{"title":"Effect of hypoalbuminemia on drug pharmacokinetics","url":"https://pmc.ncbi.nlm.nih.gov/articles/PMC11882431/","publisher":"Frontiers in Pharmacology","year":2025},{"title":"Estimation of Free Phenytoin Concentration in Critically Ill Patients with Hypoalbuminemia: Direct-measurement vs Traditional Equations","url":"https://pmc.ncbi.nlm.nih.gov/articles/PMC9237157/","publisher":"Indian Journal of Critical Care Medicine","year":2022},{"title":"Phenytoin, Free and Total (test 0090141)","url":"https://ltd.aruplab.com/Tests/Pub/0090141","publisher":"ARUP Laboratories"}]}],"review":{"status":"ai-verified","label":"AI-verified against cited sources. Awaiting expert review","lastReviewed":"2026-10-03"},"summary":"Measures the active, unbound part of phenytoin (Dilantin) in your blood. It is a better guide than the total level when albumin is low or in kidney or liver disease.","url":"https://bloodtesttracker.com/markers/phenytoin-free","notice":"Information, not medical advice. Ranges differ between labs, so the range on your own report is the one your result is judged against."}