Phenytoin Correction Calculator
Correct phenytoin levels for albumin and renal function using Winter-Tozer equation. Enter values for instant results with step-by-step formulas.
Formula
Corrected Phenytoin = Measured Level / (0.2 x Albumin + 0.1)
The Winter-Tozer equation adjusts the measured total phenytoin level for hypoalbuminemia. The denominator (0.2 x Albumin + 0.1) represents the expected protein binding fraction. For dialysis patients, the coefficient changes to 0.1 to account for uremic displacement from binding sites. Normal albumin is 3.5-5.0 g/dL; at normal albumin of 4.4, the denominator equals approximately 1.0 and no correction occurs.
Worked Examples
Example 1: Standard Albumin Correction
Problem: A patient has a measured phenytoin level of 7.2 mcg/mL and albumin of 2.0 g/dL. Normal renal function. Is the corrected level therapeutic?
Solution: Winter-Tozer: Corrected = Measured / (0.2 x Albumin + 0.1)\nCorrected = 7.2 / (0.2 x 2.0 + 0.1)\nCorrected = 7.2 / (0.4 + 0.1)\nCorrected = 7.2 / 0.5\nCorrected = 14.4 mcg/mL\nTherapeutic range: 10-20 mcg/mL
Result: Corrected level: 14.4 mcg/mL (therapeutic) - no dose change needed
Example 2: Dialysis Patient Correction
Problem: A hemodialysis patient has measured phenytoin of 5.0 mcg/mL and albumin of 2.8 g/dL. Calculate the corrected level.
Solution: Modified equation for dialysis: Corrected = Measured / (0.1 x Albumin + 0.1)\nCorrected = 5.0 / (0.1 x 2.8 + 0.1)\nCorrected = 5.0 / (0.28 + 0.1)\nCorrected = 5.0 / 0.38\nCorrected = 13.2 mcg/mL\nTherapeutic range: 10-20 mcg/mL
Result: Corrected level: 13.2 mcg/mL (therapeutic) - dialysis correction reveals adequate level
Frequently Asked Questions
How does hypoalbuminemia affect phenytoin levels?
Phenytoin is approximately 90 percent bound to plasma proteins, predominantly albumin, with only the remaining 10 percent being free and pharmacologically active. When albumin levels decrease below the normal range of 3.5 to 5.0 g/dL, fewer binding sites are available, causing a larger fraction of phenytoin to circulate in its unbound active form. This means a patient with low albumin may have a total phenytoin level that appears subtherapeutic at 6 mcg/mL, but after correction for albumin of 2.0 g/dL, the adjusted level could be 12 mcg/mL, which is well within the therapeutic range. Without this correction, clinicians might inappropriately increase the dose and cause toxicity.
How does renal impairment affect phenytoin binding?
Patients with significant renal impairment, particularly those with creatinine clearance below 10 to 20 mL/min or those on hemodialysis, have altered phenytoin protein binding due to the accumulation of uremic toxins that compete with phenytoin for albumin binding sites. This results in a higher free fraction of phenytoin even if albumin levels are normal. The modified Winter-Tozer equation for renal patients uses a different coefficient: Corrected Phenytoin = Measured / (0.1 x Albumin + 0.1) for dialysis patients. This accounts for the additional displacement of phenytoin from albumin by uremic compounds. Whenever possible, free phenytoin levels should be measured directly in renal patients for the most accurate assessment.
What is the therapeutic range for phenytoin?
The standard therapeutic range for total phenytoin is 10 to 20 mcg/mL, while the free phenytoin therapeutic range is 1.0 to 2.0 mcg/mL. Some neurologists accept a slightly wider range of 10 to 20 mcg/mL for seizure prophylaxis and 15 to 20 mcg/mL for active seizure management. Toxicity symptoms typically begin above 20 mcg/mL with nystagmus, progress to ataxia above 30 mcg/mL, and can include lethargy and coma above 40 mcg/mL. The free level is the better predictor of both efficacy and toxicity since it represents the active drug fraction. It is important to note that some patients may have adequate seizure control at levels below the standard therapeutic range.
When should free phenytoin levels be ordered instead of total levels?
Free phenytoin levels should be ordered in any clinical situation where protein binding may be altered, making total levels unreliable. This includes patients with hypoalbuminemia below 3.5 g/dL, renal failure with creatinine clearance below 20 mL/min, hepatic dysfunction with impaired albumin synthesis, pregnancy where albumin dilution occurs, critically ill patients in the ICU with multiple binding alterations, and patients taking medications that displace phenytoin from albumin such as valproic acid. Free levels are the gold standard measurement but are more expensive and take longer to result than total levels. When free levels are unavailable, the Winter-Tozer correction provides a reasonable estimate.
How does valproic acid interact with phenytoin binding?
Valproic acid is one of the most clinically significant drugs that interact with phenytoin protein binding. Valproic acid competes with phenytoin for albumin binding sites, displacing phenytoin and increasing its free fraction from the normal 10 percent to as high as 15 to 20 percent. This displacement means that total phenytoin levels may decrease or appear low while free levels remain therapeutic or even become toxic. The standard Winter-Tozer equation does not account for this interaction, and patients on concurrent valproic acid should have free phenytoin levels monitored directly. Additionally, valproic acid inhibits phenytoin metabolism through CYP2C9, adding a pharmacokinetic interaction on top of the binding displacement.
How should phenytoin doses be adjusted based on corrected levels?
Phenytoin dose adjustments must account for its nonlinear (Michaelis-Menten) pharmacokinetics, where small dose changes can produce disproportionately large changes in serum levels near saturation. If the corrected level is subtherapeutic, doses should be increased cautiously, typically by no more than 30 to 50 mg per day for maintenance dosing. If supratherapeutic, the dose should be held until levels decrease and then restarted at a lower maintenance dose. Loading doses can be given for acutely subtherapeutic patients using the formula: Loading Dose = (Target - Current Corrected Level) x Volume of Distribution x Weight. The volume of distribution for phenytoin is approximately 0.7 L/kg. Levels should be rechecked 5 to 7 days after dose changes to allow for steady state.