Dear John
The difference between the Direct ISE on the wards and the lab-based
equipment is something that has bothered us for years. We have a Direct
ISE in the lab on which we check our suspect sodium and potassium results
on. We also use this for lipaemic and, high and low protein samples. In
most circumstances the two results do not differ as the Direct ISE has been
"fiddled", but major differences start to arise when the total proteins are
> 95g/L or <50g/L or in lipaemic samples. Here the fiddled Direct ISE
result is probably giving the most useful clinical information (and relates
better to the measured osmolality), therefore we report this result with a
comment about how the sodium has been measured. Pseudohyponatraemia has
been well reported and pseudonormonatraemia has also been reported. We
believe that pseudohypernatraemia is also present in a small minority of
samples, especially from ICU patients with low proteins. It is in these
patients that problems start to arise, as there is a fond belief on some
Wards that the lab must be correct, and the result from the POCT Direct ISE
must be wrong. Clearly the POCT result is of more clinical use.
I beleive there is only one major analyser that uses a Direct ISE module
(and therefore should have better correlation with the POCT equipment) but
this uses 65µL of sample and so is not ideal for paeds. I have asked
several companies whether they would consider developing dual Direct
(requiring a low sample volume) and Indirect ISE's for sodium, and to date
only one company has shown any interest. We could then report activity and
concentration at the same time with appropriate reference ranges, with no
need for fiddle factors. I am an eternal optimist.
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