Why does phenytoin need a correction?

Phenytoin is highly protein bound to albumin, but only the free, unbound fraction is pharmacologically active. When albumin is low the bound portion falls, so the same free level corresponds to a lower total level. A normal total can then mask a therapeutic or toxic free concentration.

What is the Winter-Tozer equation?

It estimates the total phenytoin a patient would have at normal albumin. The standard form is measured total divided by the quantity 0.2 times albumin plus 0.1. The 0.2 reflects normal binding and the 0.1 is the residual free fraction, giving an albumin-normalised value to compare against the usual 10 to 20 range.

How does renal failure change it?

Uraemia reduces phenytoin binding to albumin beyond what low albumin alone explains, so the binding coefficient drops from 0.2 to 0.1. The renal-adjusted equation is measured total divided by 0.1 times albumin plus 0.1, which yields a higher corrected value reflecting the greater free fraction.

What target range applies to the corrected value?

The corrected concentration is interpreted against the usual total phenytoin target of about 10 to 20 micrograms per millilitre, since it represents what the total would be at normal albumin and binding. Free phenytoin itself targets roughly 1 to 2 micrograms per millilitre.

Is this a substitute for a free phenytoin assay?

No. When available, a directly measured free phenytoin level is more reliable than any equation, especially in unstable or critically ill patients. The Winter-Tozer estimate is a practical tool when only total phenytoin and albumin are known.

What is the Phenytoin Free Fraction Estimator in Renal Failure?

Apply the Winter-Tozer equation with a renal-failure adjustment to estimate the albumin-corrected phenytoin concentration when hypoalbuminaemia and low GFR coexist. Helps pharmacists dose epilepsy in CKD. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Phenytoin Free Fraction Estimator in Renal Failure

Name: Phenytoin Free Fraction Estimator in Renal Failure
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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Phenytoin is one of the trickiest drugs to monitor because only its free, unbound fraction is active, yet most laboratories report the total concentration. In patients who are both hypoalbuminaemic and in renal failure, the total level badly understates the active drug, and an uncorrected reading can lead to dangerous overdosing or undertreatment.

How it works

The Winter-Tozer equation normalises the measured total phenytoin to what it would be at a normal albumin of 4.4 g/dL:

corrected = measured / (0.2 x albumin + 0.1)        (normal renal function)

In significant renal impairment, uraemia further loosens protein binding, so the binding coefficient is halved:

corrected = measured / (0.1 x albumin + 0.1)        (CrCl below ~10-25 mL/min)

The corrected value is then read against the familiar total phenytoin target of roughly 10 to 20 micrograms per millilitre.

Why phenytoin binding falls in renal failure — and why it matters

Under normal circumstances approximately 90% of phenytoin is bound to albumin, leaving roughly 10% free and pharmacologically active. Two independent mechanisms reduce this binding in renal disease:

Low albumin: patients with CKD frequently have hypoalbuminaemia from multiple causes. Less albumin means fewer binding sites, so for a given total phenytoin, more is free.
Uraemic binding inhibitors: accumulated organic acids and other uraemic solutes displace phenytoin from albumin binding sites even when albumin is present in normal quantity. This is the mechanism the renal-adjusted coefficient (0.1 instead of 0.2) captures.

The combined effect means a patient with an albumin of 2.5 g/dL and severe CKD may have a free fraction of 20–25% rather than the normal 10%, so a measured total of 8 micrograms/mL might correspond to a free level well within or even above the therapeutic range.

Worked example

A patient with CrCl 8 mL/min and albumin 2.2 g/dL has a measured total phenytoin of 7 micrograms/mL.

Using the renal-adjusted equation:

corrected = 7 / (0.1 × 2.2 + 0.1)
           = 7 / (0.22 + 0.1)
           = 7 / 0.32
           ≈ 21.9 micrograms/mL

The corrected value of approximately 22 micrograms/mL sits above the conventional 10–20 target, suggesting the patient may already be in the toxic range despite a seemingly low raw total. This is an illustrative calculation; clinical context and ideally a free phenytoin assay should guide dosing.

Target ranges and assay comparison

Parameter	Target range
Total phenytoin (corrected or normal albumin)	10–20 micrograms/mL
Free phenytoin (direct assay)	1–2 micrograms/mL

When both total and free levels are available, the directly measured free level is always preferred. The Winter-Tozer estimate is most useful in settings where a free assay is unavailable or delayed.

Interpretation and notes

Because renal failure widens the free fraction, the renal-adjusted equation returns a higher corrected number for the same inputs — which is the point: it reveals that the raw total was understating the active drug. This calculator is educational and does not replace pharmacist or physician judgement. Always confirm management with a clinician who can assess the full clinical picture.