The use of equations to correct phenytoin for serum albumin levels is very common, but often performed out of
site from the laboratory staff by clinical pharmacists. 

Several equations exist and they are only valid for a limited range of albumin levels  (that may not be 
so low to include 24 g/L). Other equations consider the concentration of urea, which at high levels during renal failure 
can increase the free fraction of phenytoin. 

A more expensive alternative is consider testing free phenytoin levels (using small pore filters to obtain a 
protein-free filtrate that can be tested on a separate calibration curve). While more expensive, the volume of tests is often quite low.

I suggest consulting with pharmacy to select an appropriate equation to estimate free phenytoin for hospitalized
patients and consider the potential of offering testing of free phenytoin to support patients in critical care areas.  


regards, Andrew

Dr. A. Lyon
Univ. Saskatchewan

----- Original Message -----
From: "Vickery Susan (EAST KENT HOSPITALS UNIVERSITY NHS FOUNDATION TRUST)" <[log in to unmask]>
To: [log in to unmask]
Sent: Wednesday, January 16, 2013 9:49:06 AM
Subject: Correcting phenytoin concentrations for albumin




Dear Mailbase, 



We have had an enquiry from a pharmacist to see if we can provide phenytoin results that have been corrected for albumin concentration. 



The rationale being that there was a recent case where the phenytoin concentration was 10 mg/L and the albumin concentration was 24 g/L. The corrected phenytoin concentration was 20 mg/L, but the dose of phenytoin was increased because the clinician used the uncorrected value of 10 mg/L. It resulted in the patient becoming toxic. Our pharmacist feels patient safety would be improved if corrected phenytoin concentrations were reported and has provided us with the equation to do this. 



Does anybody have experience of correcting phenytoin concentrations? Are there any risks in doing this? Finally, what equations, to correct for albumin concentration, are being used? 



Thoughts and comments welcome. 



Regards, 

Susan. 



Dr Susan Vickery 

Department of Laboratory Medicine 

East Kent Hospitals University NHS Foundation Trust 
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