At 2001-08-01 22:53 +0000, John Fry wrote:
>How are members dealing with the differences between the ward and laboratory
>values for electrolytes - and in some cases blood gases - due to
>differences in analytical procedures eg direct vs indirect electrodes.
This problem has been discussed in international workshops on this subject.
The problem is that the clinicians frequently do not realize the difference
between "activity" and "concentration".
If samples and technique otherwise are used lege artis, the primary output
of an ion-selective electrode closely mimics the effect of the ion on the
cell, i.e. the ion-selective sensor is a reacting identical to the ion
receptor on the cell surface.
Strictly physical, this response follows the activity expressed in molal,
i.e. f x the concentration in moles/kg of water. However the difference
between molal and moles/litre is very small and can be neglected.
By convention electrolytes are measured as a true concentration. This means
that we measure the amount of electrolyte in a defined volume of the sample
material, i.e. whole blood, plasma, serum. A significant part of such
material is not water and therefore such a measurement yields a different
result but it is expressed in the same unit as the direct measurement.
Obviously activity is the most appropriate measurement from a physiological
point of view and therefore all instruments, bedside or in the laboratory,
should report activity. Then this problem should never occur.
The most radical solution of the problem is to scrap all instruments
measuring true concentration and only report activity, but this solution
seems utopic.
The second best is to clearly express "concentration" or "activity" on the
reports and teach the clinicians the difference.
Eventually this problem is a matter of information. Good luck!
Mr Sten Öhman, PhD
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