Hi Paul
We've recently audited 5 years of phenytoin data looking at the effect of
albumin in about 1,000 patients (about 7,000 results). The prevalence of
hypoalbuminaemia was surprisingly high and for >20% of patients moved their
results into or above the therapeutic range i.e. changed the status of low v
normal v high. We corrected for albumin assuming 90% protein binding, this
is simple algebra and in the literature as the Shiner Tozer equation. (NB
mid of our normal range for albumin is 40g/L which changes the constant on
the bottom line from 0.2 to 0.22).
Annecdotally I see quite a few patients with clinical toxicity who are
treated to total phenytoing concentration without regard to confounders such
as albumin or coprescription of valproate. It may also partly explain the
tendency of some of our colleagues to under treat.
We no longer have a free phenytoin assay and when we did it wasn't used much
but when it's needed it's needed.
We don't currently adjust for albumin although we're considering it and
about to discuss with our neurologists. As to how far one goes with
reporting calculated values or the use of alerts for confounders, I'd be
interested in the experience of others.
Matt Doogue
Clinical Pharmacologist
Adelaide, Australia
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