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.
Reviewed by Rahul Singh, Health & Wellness Specialist
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.
References
Reviewed by Rahul Singh, Health & Wellness Specialist · Editorial policy