About This Calculator

The Free Phenytoin Level Calculator is a clinical tool used to estimate the biologically active concentration of phenytoin in patients with low serum albumin or renal impairment. Phenytoin is an anti-seizure medication that is highly bound to plasma proteins, primarily albumin. Only the unbound, or "free," portion of the drug can exert its therapeutic effect. In states of hypoalbuminemia or uremia, the total measured phenytoin level may not accurately reflect the pharmacologically active free level, potentially leading to misinterpretation and incorrect dosing.

This calculator applies the Sheiner-Tozer equation to adjust the total phenytoin concentration based on the patient's albumin level and renal function, providing a "corrected" level that better approximates the true free concentration.

Outputs Explained

The primary output is the Corrected Phenytoin Level, expressed in micrograms per milliliter (mcg/mL). This value represents an estimate of what the total phenytoin level would be if the patient had normal albumin and renal function.

• Therapeutic Range: The generally accepted therapeutic range for corrected phenytoin is 10 to 20 mcg/mL.
• Sub-therapeutic: Levels below 10 mcg/mL may be insufficient to control seizures.
• Supratherapeutic / Potentially Toxic: Levels above 20 mcg/mL are associated with an increased risk of adverse effects. Nystagmus is common at levels >20 mcg/mL, ataxia at >30 mcg/mL, and mental status changes at >40 mcg/mL.
• Formula Used: The calculator indicates whether the standard Sheiner-Tozer formula or the modified version for renal failure (CrCl ≤ 20 mL/min or ESRD) was used.

How to Use the Calculator

Follow these steps to obtain a corrected phenytoin level:

1. Enter Total Phenytoin: Input the patient's total phenytoin level as reported by the laboratory.
2. Enter Serum Albumin: Input the patient's serum albumin level. Ensure the correct units (g/dL or g/L) are selected.
3. Enter Renal Function: Input the patient's creatinine clearance (CrCl) in mL/min. If unknown, you can use the integrated Cockcroft-Gault calculator to estimate it.
4. Specify ESRD/Dialysis: Check the box if the patient has End-Stage Renal Disease (ESRD) or is on dialysis. This will apply the renal failure adjustment regardless of the entered CrCl.
5. Calculate: Click the "Calculate Corrected Level" button to view the result and its interpretation.

Dosing Overview

Phenytoin dosing must be individualized. The corrected level helps guide adjustments. Due to its non-linear (Michaelis-Menten) kinetics, small increases in the maintenance dose can lead to disproportionately large increases in serum concentrations, especially when levels are near the upper end of the therapeutic range.

• Loading Dose: A loading dose (IV or oral) is often used to achieve therapeutic concentrations quickly, typically 15-20 mg/kg.
• Maintenance Dose: Typical maintenance doses are 300-600 mg/day, often divided into two or three doses for immediate-release formulations or given once daily for extended-release capsules.
• Therapeutic Monitoring: Trough levels should be drawn just before the next dose, once steady-state is reached (usually 5-10 days after initiation or a dose change). Dose adjustments should be made cautiously in small increments (e.g., 25-30 mg at a time).

Switching Formulations

Care must be taken when switching between different phenytoin formulations (e.g., IV to oral, extended-release to immediate-release). Phenytoin sodium products (like capsules and injection) are 92% phenytoin by weight, while formulations of phenytoin acid (like suspension and chewable tablets) are 100% phenytoin. Dose adjustments may be necessary to account for this salt factor. Monitoring corrected levels after a switch is recommended.

Missed Dose

If a dose is missed, the patient should take it as soon as they remember. However, if it is almost time for the next dose, they should skip the missed dose and resume their regular dosing schedule. Patients should be instructed never to take a double dose to make up for a missed one. Abrupt discontinuation of phenytoin can precipitate status epilepticus.

Safety Alerts

Phenytoin has a narrow therapeutic index and a significant side effect profile.

• Cardiovascular Risk: The FDA has a Black Box Warning regarding the risk of severe hypotension and cardiac arrhythmias with rapid IV administration. Infusion rates should not exceed 50 mg/minute in adults.
• CNS Toxicity: Dose-related CNS effects include nystagmus, ataxia, slurred speech, and confusion.
• Dermatologic Reactions: Serious and sometimes fatal skin reactions, such as Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN), have been reported. The risk is higher in patients of Asian ancestry with the HLA-B*1502 allele.
• Long-Term Effects: Chronic use can lead to gingival hyperplasia, hirsutism, coarsening of facial features, and decreased bone mineral density.

Frequently Asked Questions (FAQ)

What is the Sheiner-Tozer equation?

The Sheiner-Tozer equation is a formula used to estimate the free phenytoin concentration in patients with altered protein binding. The standard formula is: Corrected PHT = Measured PHT / ((0.2 * Albumin) + 0.1). A modified version with a different albumin coefficient (0.1) is used for patients with severe renal impairment.

Why does renal failure affect phenytoin levels?

In uremia (severe renal failure), waste products accumulate in the blood and displace phenytoin from its binding sites on albumin. This increases the free fraction of the drug, meaning the total level underestimates the active drug concentration. The calculator uses a modified formula to account for this displacement.

When should I measure a free phenytoin level directly?

A directly measured free phenytoin level is the gold standard and should be considered when there is a discrepancy between the clinical picture and the corrected level, in patients with multiple factors affecting binding (e.g., liver disease, other highly protein-bound drugs), or when a precise measurement is critical for clinical decision-making.

What is a normal *free* phenytoin level?

The typical therapeutic range for a directly measured *free* phenytoin level is 1 to 2 mcg/mL.

Why not always measure the free level directly?

Measuring free phenytoin levels is more expensive and less widely available than measuring total levels. The corrected level is a cost-effective and clinically useful estimate in most common scenarios.

Does this calculator work for children?

The Sheiner-Tozer equations were primarily validated in adult populations. While the principles apply, protein binding and drug metabolism can differ in neonates and children. Use in pediatric patients should be done with caution and correlated with clinical assessment.

What is the Cockcroft-Gault equation used for?

The Cockcroft-Gault equation is a formula used to estimate creatinine clearance (CrCl) from serum creatinine. It helps assess a patient's kidney function, which is a key input for the phenytoin correction formula.

Can other drugs affect the corrected phenytoin level?

Yes. Drugs that are also highly protein-bound, such as valproic acid or warfarin, can displace phenytoin from albumin, increasing the free fraction. This calculator does not account for these specific drug-drug interactions.

References

1. U.S. Food and Drug Administration. DILANTIN® (extended phenytoin sodium capsules) Prescribing Information. Updated 2018.
2. Tiongco FP, Agapito T, Gabriel J, et al. Phenytoin. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK555942/
3. Bajtelsmit MF, Sacco AJ, Sacco JA, Nelson LS. A review of phenytoin total and free-level monitoring. J Pharm Pract. 2015;28(1):50-55. doi:10.1177/0897190014555078
4. Patsalos PN, Berry DJ, Bourgeois BF, et al. Antiepileptic drugs--best practice guidelines for therapeutic drug monitoring: a position paper by the subcommission on therapeutic drug monitoring, ILAE Commission on Therapeutic Strategies. Epilepsia. 2008;49(7):1239-1276. doi:10.1111/j.1528-1167.2008.01561.x