We measure free phenytoin by making an ultrafiltrate of serum using an
Amicon filter (Centrifree YM-30) and measuring phenytoin in the
ultrafiltrate using our usual phenytoin method on a Roche 917. We have had
complaints that the results are too high and this seems to be true (we
don't do many) because the free is a substantial proportion of the total (?
30 - 105%). However, the protein content of the ulrafiltrate is
appropriately low (0 - 1 g/L) so the membrane seems to be excluding the
protein OK.
Has anyone else had similar experiences? Any suggestions?
Thanks
John Whitfield
Clinical Biochemistry
Royal Prince Alfred Hospital
Sydney.
Phone (61 2) 9515 5246
Fax (61 2) 9515 7931
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