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.

Phenytoin Free Fraction Estimator in Renal Failure

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.

Interpretation and notes

A corrected level that lands in the 10 to 20 range suggests an appropriate free concentration even when the raw total looks low. Because renal failure widens the free fraction, the renal-adjusted equation returns a higher corrected number for the same inputs, which is why the toggle matters. This estimate assumes a stable patient and standard binding behaviour; a directly measured free phenytoin assay is always preferred when available, particularly in critical illness. This calculator is educational and does not replace pharmacist or physician judgement.