--- [log in to unmask] wrote: >.....
>
> I can not believe that amongst the many participants
> in this list
> there are not several with the same problem of using
> whole blood
> for POCT in ICUs and elsewhere while using plasma or
> serum in
> the lab. A few telephone calls to colleagues
> suggested that this
> problem may be being ignored and a single reference
> range used.
> If so some correction may be needed to the whole
> blood method
> especially for sodium. Our brief experience has
> thrown up
> differences of 6-7mmol/l Na in some samples. Surely
> enough to
> alter clinical management.
>
>
Mike,
I believe the difference you allude to is not related
to whether the specimen analysed is whole blood or
plasma, but whether direct or indirect ISE is used.
The biggest differences will be seen in specimens with
relatively low plasma water percentage (e.g. in
hypertriglyceridaemia).
In effect, all plasma or serum specimens measured by
indirect ISE (or flame photometry, if anyone is still
using it) show some degree of 'pseudohyponatraemia',
compared to the measurement by direct ISE
(electrolytes measured in the plasma water phase).
When we first noticed this difference 20 years ago
(Clin Chem 1979 Vol 25, No 4, pp 643-644), some
manufacturers of direct ISE analysers started adding
'correction factors', to convert values to roughly
equivalent indirect ISE values (i.e. multiplication by
0.95!).
I don't know whether this still happens on all Direct
ISE analysers.
Stuart Robertson
Biochemistry Dept
Hull Royal Infirmary
Hull
HU3 2JZ
01482-607707
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