calculate an estimate of the influence of serum Na
on its ionic activity
What the point-of-care machine actually measures is
, what it reports is
Ions interact with water by gathering a
O dipoles around themselves, thereby reducing their potential for interaction with other molecules or ions. At ion concentrations physiologically found in human plasma
is less than
by a factor of about 0.74.
This is built into the machine software as a constant correction factor for the difference between ionic activity and ionic concentration.
The difference between
is dictated by another property of aqueous electrolyte solutions:
For monovalent strong ions this simply amounts to half the sum of their values - as most of the serum ions are monovalent strong ions, normal serum
is 160mmol/l, with Na
values clearly dominating the picture. This is why our simple calculation is limited to the effects of Na
When the specimen being analysed has Na
values far from the normal range the electrolyte values reported can differ substantially from the real concentrations.
Our estimate is based on the Debye-Hückel equation, which is, though complicated enough, only really applicable to solutions with ionic strength up to 100mmol/l, while normal plasma has an ionic strength of about 160mmol/l. The result of the calculation on this page has thus to be taken with a grain of salt. Nevertheless, it gives a good idea about the kind of influence grossly abnormal serum Na
values can have on your measurements.
consult the glossary for other aspects of acid-base equilibria and the rules and mathematics behind our website: