Skip to main content
Your session has expired. Reloading...
Other

Phenytoin Test

The phenytoin test monitors blood levels of this anticonvulsant, as prescribed by your doctor. As part of comprehensive health monitoring, this test is essential due to phenytoin's narrow therapeutic index and nonlinear pharmacokinetics.

Doctor's Assessment Included

When is this value abnormal?

Decision limits per result, in mg/l
Result Value (mg/l)
Subtherapeutic < 8
Therapeutic 8–20
Above therapeutic range ≥ 20

Dalspiegel (afname vóór de volgende gift). Betreft totaal fenytoïne. Toxisch: > 20 mg/l.

Source: UMC Utrecht

Reference ranges may vary between laboratories. When you order a test, a BIG-registered doctor assesses your personal results in context. For treatment decisions, discuss your results with your GP.

Phenytoin: what this test measures

This test measures the blood concentration of the anticonvulsant phenytoin, as prescribed by your doctor. Due to its narrow therapeutic index, even small changes in dose or metabolism can lead to significant shifts in blood levels.

Phenytoin: why this value matters

Phenytoin has a narrow therapeutic index and nonlinear pharmacokinetics, meaning that small dose increases can cause disproportionately large rises in blood levels. Regular monitoring is essential to maintain effective seizure control while avoiding toxicity.

Phenytoin: when is testing worthwhile?

Testing is recommended when starting phenytoin, after dose changes, when adding or removing interacting medications, or if seizures recur or toxicity symptoms appear. Your healthcare provider will determine the appropriate monitoring schedule.

Phenytoin: symptoms of a high or low value

Low Levels

Low phenytoin levels may indicate subtherapeutic dosing, which can be associated with an increased risk of breakthrough seizures. Consult your healthcare professional for interpretation of results.

High Levels

High phenytoin levels may indicate toxicity, which can be associated with nystagmus (involuntary eye movements), ataxia (unsteady gait), slurred speech, confusion, and in severe cases, cardiac complications. Seek immediate medical attention if you experience these symptoms.

Phenytoin: lifestyle and this value

Take phenytoin exactly as prescribed by your doctor and do not adjust your dose without medical guidance. Be aware that many medications, foods, and supplements can interact with phenytoin. Attend all scheduled monitoring appointments.

Phenytoin: frequently asked questions

Why is phenytoin monitoring so important?
Phenytoin has nonlinear pharmacokinetics, meaning small dose changes can cause large, unpredictable changes in blood levels. This makes regular monitoring essential to maintain levels within the therapeutic range.
Can food or supplements affect phenytoin levels?
Yes, certain foods, enteral nutrition, calcium supplements, and antacids can interfere with phenytoin absorption. Discuss any dietary changes or new supplements with your healthcare provider.
What is the difference between total and free phenytoin?
Total phenytoin measures both protein-bound and unbound drug, while free phenytoin measures only the active unbound portion. Your doctor may order free phenytoin levels if you have low albumin or kidney disease.

Tests with related values

These panels measure values from the same category as this marker.

Whoop

Whoop Performance Health

A 50-biomarker performance panel inspired by the WHOOP Performance Health Panel — recovery, blood production, thyroid, hormones, iron and nutrient status for people who train.

Complete Blood Count (CBC) Leukocyte Differential ALP (Alkaline Phosphatase) ALT (Alanine Aminotransferase) AST (Aspartate Aminotransferase) Albumin Total Protein Bilirubin (Total) ApoB (Apolipoprotein B) Total Cholesterol LDL Cholesterol HDL Cholesterol Triglycerides Lipoprotein(a) Urea (BUN) Creatinine eGFR (Estimated Glomerular Filtration Rate) Calcium Bicarbonate Chloride Sodium Potassium Magnesium Cortisol DHEA-S Estradiol (E2) FSH (Follicle Stimulating Hormone) LH (Luteinizing Hormone) SHBG (Sex Hormone Binding Globulin) Total Testosterone Free Testosterone TSH (Thyroid Stimulating Hormone) Glucose (Fasting) HbA1c (Glycated Hemoglobin) Insulin (Fasting) HOMA-IR CRP (C-Reactive Protein) Homocysteine Ferritin Iron (Serum) Transferrin Transferrin Saturation Vitamin D (25-OH) IGF-1 (Insulin-like Growth Factor) Creatine Kinase (CK) Reticulocytes Free T3 (Triiodothyronine) Free T4 (Thyroxine) Vitamin B12 Folate (Folic Acid)
€689,-
Whoop

Whoop

A 43-biomarker comprehensive health panel inspired by WHOOP Advanced Labs — a deep look at metabolism, cardiovascular risk, hormones, liver, kidney and inflammation.

SHBG (Sex Hormone Binding Globulin) TSH (Thyroid Stimulating Hormone) ALT (Alanine Aminotransferase) LDL Cholesterol Cortisol DHEA-S Estradiol (E2) Free Testosterone FSH (Follicle Stimulating Hormone) LH (Luteinizing Hormone) Total Testosterone Bicarbonate Calcium Chloride Ferritin Iron (Serum) Magnesium Potassium Sodium Transferrin CRP (C-Reactive Protein) Homocysteine Glucose (Fasting) HbA1c (Glycated Hemoglobin) ApoB (Apolipoprotein B) HDL Cholesterol Total Cholesterol Albumin HOMA-IR ALP (Alkaline Phosphatase) AST (Aspartate Aminotransferase) Bilirubin (Total) Total Protein Urea (BUN) Creatinine Vitamin D (25-OH) eGFR (Estimated Glomerular Filtration Rate) Insulin (Fasting) Leukocyte Differential Transferrin Saturation Lipoprotein(a) Triglycerides Basic Blood Count
€499,-
Whoop

Whoop Heart Health

A 45-biomarker heart health panel inspired by the WHOOP Heart Health Panel — an advanced look at cholesterol, lipoproteins, inflammation and kidney function.

Complete Blood Count (CBC) Leukocyte Differential ALP (Alkaline Phosphatase) ALT (Alanine Aminotransferase) AST (Aspartate Aminotransferase) Albumin Total Protein Bilirubin (Total) ApoB (Apolipoprotein B) Total Cholesterol LDL Cholesterol HDL Cholesterol Triglycerides Lipoprotein(a) Urea (BUN) Creatinine eGFR (Estimated Glomerular Filtration Rate) Calcium Bicarbonate Chloride Sodium Potassium Magnesium Cortisol DHEA-S Estradiol (E2) FSH (Follicle Stimulating Hormone) LH (Luteinizing Hormone) SHBG (Sex Hormone Binding Globulin) Total Testosterone Free Testosterone TSH (Thyroid Stimulating Hormone) Glucose (Fasting) HbA1c (Glycated Hemoglobin) Insulin (Fasting) HOMA-IR CRP (C-Reactive Protein) Homocysteine Ferritin Iron (Serum) Transferrin Transferrin Saturation Vitamin D (25-OH) Cystatin C Uric Acid
€629,-

Medical reviewer

Medical standards

Dr. Naimi oversees the medical standards behind our content and assessments.

Medical policy
CIBG Ministerie van Volksgezondheid,
Welzijn en Sport Official BIG registration BIG 49928676701 Opens in a new tab

Phenytoin

€49,-