calculate an estimate of the influence of serum Na+ on its ionic activity

What the point-of-care machine actually measures is ionic activity, what it reports is ionic concentration.
Ions interact with water by gathering a cloud of H2O dipoles around themselves, thereby reducing their potential for interaction with other molecules or ions. At ion concentrations physiologically found in human plasma ionic activity is less than ionic concentration 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 ionic activity and ionic concentration is dictated by another property of aqueous electrolyte solutions: ionic strength.
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 ionic strength is 160mmol/l, with Na+ values clearly dominating the picture. This is why our simple calculation is limited to the effects of Na+ only.

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.

serum Na+ [mmol/l]

consult the glossary for other aspects of acid-base equilibria and the rules and mathematics behind our website:     Glossary