My, my this does go back a way. Ultrafiltration pretty well checked
out - see Clin Pharacokinetics Vol 9, Supplement, January 1984
(no, really - I just hauled it off the shelf :-)
I don't practice this black magic anymore but problems used
to be: acidic anticoagulant? shabby lot of filters? analytical
method not reliable at 1/10th usual calibration range?
On the other hand, why would anyone want a free conc if
it was expected to be usual - perhaps your answers are correct
and the pts have uremia, displacing co-medication, etc?
If using immunologic method for analysis I would get reassurance
that method doesn't x-react with unbound/low-bound metabs
which may get relatively 'concentrated' compared to bound
parent - can be important especially with uremic sp.
>
> > 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.
-
Bill Godolphin <[log in to unmask]>
Pathology, UBC, Vancouver BC Canada V6T 2B5
tel: (604) 822-7701 fax: (604) 822-7635
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