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Company
Drug Monitoring
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Currently under review
Pending specialist review and validation.
This blood test measures the total amount of phenytoin in your bloodstream, including drug that is bound to proteins and drug that is unbound. It is used to help your care team find a dose that is effective for seizure control and safe for you.
Phenytoin is processed by the liver and binds strongly to a blood protein called albumin. Its handling in the body can be complex, so blood levels can change with small dose adjustments or with changes in your health, diet, or other medicines. This test is part of therapeutic drug monitoring to personalize your treatment.
Checking a phenytoin level helps your clinician ensure you are getting enough medicine to prevent seizures while avoiding side effects such as imbalance, double vision, sleepiness, or confusion. Levels are commonly checked after starting treatment, after a dose or brand change, when new interacting medicines are added, or if you have symptoms that suggest too little or too much drug effect.
Monitoring is especially important if you have liver or kidney problems, low albumin, are older, are pregnant, or have had recent illness or major weight change. Test results guide decisions about dose adjustments, timing of doses, formulation choice, and whether a free (unbound) phenytoin level is needed.
Your result is interpreted alongside your symptoms, seizure control, side effects, and the timing of the blood draw. Because most phenytoin in blood is protein bound, the total level can appear lower or higher than expected if your albumin is low or if another medicine displaces phenytoin from proteins. In those situations, your clinician may order a free phenytoin level or account for albumin when interpreting the result.
For consistent interpretation, blood is often drawn just before the next dose, and the same lab and method are ideally used over time. Do not change how you take your medicine based on a single result. Your clinician may adjust the dose carefully and recheck a level after allowing time for a new steady state. Contact your care team promptly if you notice worsening seizures or signs of toxicity.
Reference intervals vary by laboratory, analyzer, methodology, population, and units. The ranges shown here are for education only. Always interpret your results against the reference interval printed on your own lab report.
Levels vary with the dosing cycle. A sample taken just before the next dose (a trough) is usually preferred for consistent interpretation.
Low albumin, kidney disease, or liver disease change how much phenytoin is protein bound. The total level may not reflect the active free drug in these settings.
Many medicines affect phenytoin levels by altering metabolism or protein binding. Examples include valproate, carbamazepine, azole antifungals, amiodarone, and some antibiotics or herbs.
Switching between brands or between immediate and extended release products can change absorption. Tell your clinician if your product or schedule changes.
Tube feedings, antacids, and some nutritional products can reduce absorption. Separating feeds from dosing or adjusting the regimen may be needed.
Genetic differences in drug-metabolizing enzymes, aging, and pregnancy can change clearance and binding. Closer monitoring and dose adjustments may be required.
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