Phenytoin (Dilantin) Correction for Albumin/Renal Failure

Correct serum total phenytoin (Dilantin) for hypoalbuminemia and renal failure using the updated Sheiner-Tozer equation (0.275 vs 0.2 albumin coefficient by creatinine clearance).

Phenytoin (Dilantin) Correction for Albumin / Renal Failure

Corrects measured total serum phenytoin for hypoalbuminemia using the Sheiner-Tozer style equation with updated albumin coefficients (0.275 if creatinine clearance ≥20 mL/min, 0.2 if CrCl < 20 mL/min). Same approach as on MDCalc.

Measured total phenytoin

Serum albumin (g/L)

Many references apply this when albumin is at or below about 32 g/L (3.2 g/dL).

Creatinine clearance < 20 mL/min?

No (use k = 0.275)Yes (use k = 0.2)

Disclaimer: This tool is for education and clinical decision support only. It does not replace measured free phenytoin when available, drug interaction review, or professional judgment.

Phenytoin Correction (Sheiner-Tozer) Explained

Purpose

Total serum phenytoin is highly protein bound. In hypoalbuminemia and in some patients with renal impairment, measured total phenytoin can misrepresent the pharmacologically active fraction. The Sheiner-Tozer relationship estimates a "corrected" total concentration for interpretation alongside clinical status. Whenever possible, a measured free (unbound) phenytoin level is preferred over a calculated correction.

Equation

This implementation follows the updated coefficients described on MDCalc: albumin is entered in g/L but the formula uses albumin in g/dL (divide g/L by 10).

Corrected total phenytoin (µg/mL) = Measured total (µg/mL) / [(k × Albumin_g/dL) + 0.1]

k = 0.275 when creatinine clearance is ≥20 mL/min (no renal failure flag).

k = 0.2 when creatinine clearance is < 20 mL/min (renal failure flag set to Yes).

Albumin_g/dL = Albumin (g/L) ÷ 10.

Units

µg/mL is numerically the same as mg/L for total phenytoin mass concentration.
µmol/L is converted using phenytoin molecular weight 252.27 g/mol: µg/mL = µmol/L × (MW / 1000).

When correction is usually applied

References commonly apply this correction when albumin is at or below about 3.2 g/dL (32 g/L). If albumin is higher, total levels are often interpreted without this adjustment. Interpretation must always be clinical, especially with interacting drugs (for example valproic acid) that alter protein binding.

Typical total phenytoin therapeutic range

Many references cite a usual therapeutic total range of about 10 to 20 µg/mL (about 40 to 80 µmol/L). Toxicity (for example ataxia, nystagmus, altered mental status) becomes more likely above this range, but thresholds vary by patient and laboratory.

Overview

Phenytoin (brand name Dilantin among others) is a narrow-therapeutic-index antiepileptic drug used for seizure prophylaxis and treatment. Most laboratories report total serum phenytoin, which includes protein-bound and free (unbound) drug. Because phenytoin is approximately 90% bound to albumin under normal conditions, changes in albumin concentration or protein-binding capacity can make the measured total level misleading relative to the pharmacologically active free fraction.

The Sheiner-Tozer correction (with updated albumin coefficients used in contemporary clinical references) estimates a corrected total phenytoin concentration that can be interpreted against standard total-level therapeutic ranges when a measured free level is not available. This calculator also adjusts the albumin coefficient when creatinine clearance is below 20 mL/min, reflecting altered binding in advanced renal failure.

Whenever feasible, a measured free (unbound) phenytoin level is preferred over any calculated correction. This tool supports education and bedside interpretation; dosing changes require clinical judgment, toxicology expertise, and consideration of the full medication list.

Why correction is needed

Phenytoin exhibits nonlinear (capacity-limited) pharmacokinetics at concentrations within and above the usual therapeutic range: small dose changes can produce disproportionate concentration increases. Therapeutic drug monitoring therefore relies on steady-state levels, typically drawn before the morning dose (trough) when possible.

Total phenytoin falls when albumin is low because a smaller bound pool leaves more drug in the free fraction while the laboratory still reports total concentration. Clinicians may see a subtherapeutic total level in a patient who is actually adequately treated or even toxic on a free basis. Conversely, correcting or measuring free levels prevents unnecessary dose escalation driven by misleading totals.

Renal failure and uremia can displace phenytoin from albumin and alter binding proteins, so correction formulas use a lower albumin coefficient when creatinine clearance is severely reduced. Other conditions and drugs also affect binding (see below).

The Sheiner-Tozer equation

This calculator implements the updated form commonly cited in clinical decision support tools:

Corrected total phenytoin (µg/mL) = Measured total (µg/mL) ÷ [(k × Albumin_g/dL) + 0.1]

Where:

Albumin_g/dL = serum albumin in g/L ÷ 10 (for example, 28 g/L = 2.8 g/dL).
k = 0.275 when creatinine clearance is ≥ 20 mL/min (renal failure flag not set).
k = 0.2 when creatinine clearance is < 20 mL/min (renal failure flag set to Yes).

The denominator models effective protein binding. Lower albumin or the renal-failure coefficient reduces the denominator, which increases the corrected total and moves interpretation toward the active drug burden implied by the measured total.

Units and conversions

Total phenytoin may be reported as:

µg/mL, numerically identical to mg/L for mass concentration.
µmol/L, converted using phenytoin molecular weight 252.27 g/mol: µg/mL = µmol/L × (252.27 ÷ 1000).

The corrected result is displayed in both µg/mL and µmol/L for convenience. Usual total therapeutic references are often quoted as approximately 10 to 20 µg/mL (roughly 40 to 80 µmol/L), though individual targets vary by indication, comorbidity, and laboratory.

When to apply the correction

Clinical references commonly recommend applying albumin-based correction when serum albumin is at or below about 3.2 g/dL (32 g/L), the threshold for hypoalbuminemia in many institutions. If albumin is above this level, total phenytoin is often interpreted without this adjustment because binding is less distorted.

The calculator may warn when entered albumin exceeds 32 g/L because correction is frequently unnecessary, but the computation remains available for teaching and special cases. Always integrate symptoms, examination findings (for example nystagmus, ataxia), and trend of levels rather than a single corrected number.

Interpreting corrected levels against therapeutic bands

After correction, this tool classifies the result relative to a commonly cited total phenytoin range:

Corrected total (µg/mL)	Interpretation
< 10	Below usual therapeutic range (subtherapeutic total on corrected scale)
10–20	Within usual therapeutic range
> 20	Above usual therapeutic range (toxicity more likely depending on clinical context)

These cutoffs apply to the corrected total, not necessarily to the raw measured total when hypoalbuminemia or renal failure is present. Toxicity (ataxia, vertical nystagmus, sedation, confusion) can occur near the upper end of the range or with rapid rises even when a single level appears "within range."

Clinical scenarios

Hypoalbuminemia without severe renal failure

Common causes include liver cirrhosis, nephrotic syndrome, malnutrition, critical illness, and postoperative states. A low measured total with adequate seizure control may reflect sufficient free drug; correction clarifies whether the patient is subtherapeutic or appropriately treated. Dose increases based on uncorrected totals alone risk toxicity.

Advanced renal failure (CrCl < 20 mL/min)

Uremia reduces phenytoin binding to albumin. Using k = 0.2 rather than 0.275 accounts for this effect in the Sheiner-Tozer update. Patients on dialysis may have additional variability from timing of dialysis relative to the draw. Coordinate with nephrology when levels are used for maintenance dosing.

Drug interactions affecting protein binding

Valproic acid competes for albumin binding and can elevate free phenytoin despite normal or low total levels. Salicylates and some other highly protein-bound drugs may have similar effects. In these situations, a free phenytoin assay is strongly preferred; correction for albumin alone may be insufficient.

Critical illness and ICU monitoring

Albumin often falls acutely in sepsis and multiorgan failure. Phenytoin is sometimes used for seizure prophylaxis or treatment in neurocritical care. Frequent reassessment of both total and free (when available) levels, clinical signs of toxicity, and electroencephalographic or clinical seizure activity guides therapy better than isolated numbers.

Free versus corrected total phenytoin

Measured free phenytoin directly quantifies the unbound fraction responsible for central nervous system effects and hepatic metabolism saturation dynamics. Many reference laboratories report free levels with a therapeutic range often cited around 1.0 to 2.0 µg/mL (laboratory-specific ranges vary).

Corrected total phenytoin is a mathematical estimate derived from total level, albumin, and renal function. It is useful when free testing is delayed, unavailable, or not reimbursed, but it cannot fully replicate displacement interactions or non-albumin binding changes. When clinical stakes are high (status epilepticus, toxicity, pregnancy, major interaction), obtain free levels.

Phenytoin pharmacokinetics relevant to monitoring

Half-life is prolonged at higher concentrations due to saturable metabolism; time to steady state can exceed one week after dose changes.
Enzyme induction by chronic phenytoin and interacting drugs (carbamazepine, phenobarbital, rifampin, among others) increases clearance over time.
Enzyme inhibition (for example chloramphenicol, isoniazid, azole antifungals) can raise levels.
Route and formulation (IV fosphenytoin versus oral extended-release) affect how quickly levels change after loading or dose adjustment.
Albumin and renal function should be contemporaneous with the phenytoin draw when correction is used.

Signs of toxicity and subtherapeutic risk

Toxicity often begins with horizontal gaze nystagmus, then ataxia, slurred speech, and altered mental status at higher free fractions. Elderly patients and those with cerebrovascular disease may manifest toxicity at lower corrected totals.

Subtherapeutic corrected levels in a patient with breakthrough seizures may warrant dose adjustment, adherence review, interaction check, or switch to an alternative agent, always weighing the risk of further toxicity from aggressive loading.

Clinical correlation is mandatory: phenytoin is one of the classic examples of "treat the patient, not the number."

Worked example (conceptual)

A patient has measured total phenytoin 8 µg/mL, albumin 28 g/L (2.8 g/dL), and creatinine clearance ≥ 20 mL/min (k = 0.275):

Denominator = (0.275 × 2.8) + 0.1 = 0.77 + 0.1 = 0.87
Corrected total = 8 ÷ 0.87 ≈ 9.2 µg/mL (still below 10 on corrected scale, but closer to therapeutic than raw 8)

With the same measured level but CrCl < 20 mL/min (k = 0.2), denominator = (0.2 × 2.8) + 0.1 = 0.66, corrected total ≈ 12.1 µg/mL, within the usual 10–20 µg/mL band. The renal coefficient materially shifts interpretation.

Limitations and cautions

Correction is an estimate; it does not replace free level measurement when binding is altered by drugs or uremia.
Albumin assays and timing relative to nutrition, dialysis, and fluid resuscitation affect input quality.
Creatinine clearance should reflect current renal function; use a reliable estimate or measured value when setting the renal failure flag.
Therapeutic ranges differ for seizure prophylaxis versus treatment and across age groups; neonatal and pediatric targets are not identical to adult totals.
Nonlinear kinetics mean small dose changes can have large effects; recheck levels after adequate time to steady state.
Hypoalbuminemia from liver failure may coexist with impaired metabolism, adding complexity beyond binding correction.

How to use this calculator

Enter the measured total phenytoin in µg/mL (or µmol/L).
Enter serum albumin in g/L.
Indicate whether creatinine clearance is below 20 mL/min (Yes uses k = 0.2; No uses k = 0.275).
Review the corrected total in µg/mL and µmol/L and the therapeutic band (below, within, or above 10–20 µg/mL).

Use the Formula Explained tab for the equation, unit notes, and the albumin threshold at which correction is commonly applied. Obtain a free phenytoin level when interpretation remains uncertain or when valproate or other displacing drugs are present.

Disclaimer: This content is for educational purposes only and is not medical advice. Phenytoin dosing and monitoring require qualified clinicians familiar with the patient, laboratory methods, and local protocols.