Phenytoin (Total)
Measures the total amount of phenytoin (Dilantin), a seizure medicine, in your blood. Your prescriber uses it to check your dose.
AI-verified against cited sources. Awaiting expert reviewAlso called PHTLOINC 3968-5
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What it is
Phenytoin (Dilantin) is a medicine that prevents seizures. This test measures the total amount in your blood, including the large part attached to a protein called albumin and the small free part. Only the free part is active. Because the body handles phenytoin in an unusual way, a small change in dose can cause a big change in the level.
Why it matters
Too little phenytoin may leave seizures less controlled, and too much can cause side effects such as eye flicking, unsteady walking and drowsiness. A level helps your prescriber fine-tune your dose, check that you are taking it as prescribed, and watch for the effects of other medicines or illness. If you have low albumin, kidney or liver disease, or are pregnant, the total level can be misleading, and a free phenytoin test may be more useful.
If it’s low
A low level means less phenytoin is in your blood than the usual range. Some people are controlled at lower levels, so your prescriber looks at the number together with your seizures and side effects. If your albumin is low, a low total level can still carry a usual amount of active, free phenytoin.
Common causes
- Missed or late doses
- A recent dose reduction, or not yet at a steady level after a change
- Medicines that speed up phenytoin's breakdown
- Low albumin, which lowers the total level more than the free level
What can help
- Tell your prescriber if you have missed doses or have trouble taking the medicine as planned
- Ask whether a free phenytoin level is a better measure for you
If it’s high
A high level means more phenytoin is in your blood than the usual range. Side effects tend to appear in steps as the level rises: eye flicking on side gaze around 20 mcg/mL, unsteady walking around 30, and slurred speech and drowsiness above 40. People vary a lot. Because small dose changes can cause large jumps in level, a high result can appear quickly.
Common causes
- A recent dose increase or a switch between phenytoin forms
- Medicines that slow phenytoin's breakdown, since many medicines interact with it
- Liver disease, or inherited differences in the CYP2C9 enzyme that clears it
- Taking extra doses by mistake
What can help
- Ask your prescriber how this result fits with how you have been feeling
- Tell your prescriber about every medicine and supplement you take
- Ask whether a free phenytoin level would give a clearer picture
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
For adults: 10–20 mcg/mL. Source: Viatris Specialty (DailyMed), ARUP Laboratories, Scientific Reports. Labs treat above 30 mcg/mL as urgent.
- Adults (this one applies)10–20 mcg/mL
Sources: Viatris Specialty (DailyMed), ARUP Laboratories, Scientific Reports
What can affect your result
- Albumin level, kidney disease, liver disease and pregnancy, which change how much phenytoin is free
- Dose changes: a small increase can raise the level a lot
- Other medicines, since many interact with phenytoin
- Inherited CYP2C9 variants, which lower clearance
- Phenytoin form: free-acid products contain about 8% more drug than sodium-salt products
- Timing of the draw relative to your dose
- Recent fosphenytoin (Cerebyx), which can falsely raise some tests until it converts
- After fosphenytoin (Cerebyx), wait until it has converted to phenytoin, about 2 hours after the end of an IV infusion or 4 hours after an IM injection. Before that, common immunoassays can read falsely high; chromatographic methods stay accurate, and EDTA tubes limit conversion in the tube.
- Total phenytoin understates the active (unbound) drug when albumin is low, in kidney or liver disease and during pregnancy, because less is bound to protein. A free phenytoin level avoids this.
- Phenytoin is broken down by a saturable process, so a small dose change can produce a large change in level.
- Free-acid forms (Infatabs, suspension) contain about 8% more phenytoin than sodium-salt products such as the capsules, so switching forms can change levels.
Trough levels (drawn just before the next scheduled dose) show the usual level and confirm you are taking it as prescribed. Peak levels are drawn at the expected peak time and show the threshold for dose-related side effects. Capsules peak about 4 to 12 hours after a dose. At least 7 to 10 days (5 to 7 half-lives) to reach a steady level after starting or changing the dose. The average half-life is about 22 hours, with a range of 7 to 42 hours.
When to talk to a clinician
Contact your prescriber promptly for new unsteady walking, eye flicking, slurred speech, marked drowsiness or confusion, or if seizures become more frequent. A new skin rash, fever or swollen glands while taking phenytoin also needs prompt attention. Talk with your prescriber before changing or stopping phenytoin. If someone may have taken too much, can't be woken, or has trouble breathing, call 911 or your local emergency number.
Questions you could ask
- How soon after a dose change should my level be rechecked?
- What level are you aiming for in my case?
- Could another medicine or my liver be raising my level?
- Should we check a free phenytoin level?
- Could missed doses or low albumin explain this result?
- Should we check a free phenytoin level?
Questions people ask
Why might my doctor also order a free phenytoin level?
Most phenytoin in the blood is attached to albumin, and only the free part is active. If your albumin is low, or you have kidney or liver disease or are pregnant, the total level can look low while the free level is usual or high. A free level measures the active part directly.
Why can a small dose change cause a big change in my level?
Your liver's ability to break down phenytoin can become saturated at higher levels. Past that point, a small increase in dose can push the level up a lot. That is why dose changes are made carefully and the level is usually rechecked after a week or more.
How do I convert mcg/mL to µmol/L?
Multiply mcg/mL by 3.96, so 10 mcg/mL is about 40 µmol/L and 20 mcg/mL is about 79 µmol/L. To go the other way, multiply µmol/L by 0.2523.
Show the science and sources
Units
- 1 mcg/mL = 1 mcg/mL
- 1 mcg/mL = 1 mg/L
- 1 mcg/mL = 3.964 µmol/L
Open the Phenytoin unit converter
For clinicians and researchers
About 90% of circulating phenytoin is bound to albumin. FDA labeling gives 10 to 20 mcg/mL total, corresponding to 1 to 2 mcg/mL unbound, as the range where seizure control without clinical toxicity is most likely, and says to base monitoring on the unbound fraction in renal or hepatic disease, hypoalbuminemia and pregnancy. Hepatic metabolism through CYP2C9 (and, less, CYP2C19) is saturable at high levels, so the half-life rises and small dose increments can cause disproportionate rises in concentration. The mean half-life is about 22 hours (range 7 to 42), and a steady state needs at least 7 to 10 days. A 2022 study of critically ill patients with low albumin found that the Sheiner-Tozer albumin correction does not predict free phenytoin reliably and recommended measuring the free level directly. Immunoassays cross-react with fosphenytoin until conversion is complete (about 2 hours after IV infusion, 4 hours after IM), so chromatographic methods are preferred early on. A 2026 survey of 411 US hospitals found a median critical limit of 30 mcg/mL for total phenytoin (range 20 to 45). 1 mcg/mL equals 3.964 µmol/L (molecular weight 252.27).
LOINC 3968-5 (primary), 14877-5, 74976-2, 97193-7. Specimen: serum-or-plasma.
Sources
- Drug Critical Limits for Urgent Physician Notification, Clinical Pharmacology & Therapeutics (2026)
- Dilantin (extended phenytoin sodium) capsules: prescribing information, Viatris Specialty (DailyMed) (2023)
- Cerebyx (fosphenytoin sodium) injection: prescribing information, Pfizer (DailyMed) (2025)
- Identification and characterization of critical values in therapeutic drug monitoring: a retrospective analysis, Scientific Reports (2024)
- Phenytoin, Free and Total (test 0090141), ARUP Laboratories
- Estimation of Free Phenytoin Concentration in Critically Ill Patients with Hypoalbuminemia: Direct-measurement vs Traditional Equations, Indian Journal of Critical Care Medicine (2022)
- Effect of hypoalbuminemia on drug pharmacokinetics, Frontiers in Pharmacology (2025)
- Phenytoin (PubChem CID 1775), PubChem, National Library of Medicine
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.