Plasma sodium concentration: bearer of false prophecies?

Occasionally measurement of the serumn or plasma concentration of sodium may give rise to falsely low results when carried out on a flame photometer or by an indirect reading ion selective electrode. The patients at risk are those with abnormally high serum concentrations of lipid or protein. These are, however, few: initially all low sodium concentrations as measured by flame photometry should be regarded as genuine. Measurement of the sodium concentrations of large numbers of specimens of serum or plasma became possible in routine clinical chemistry laboratories some 35 years ago with the advent of the flame photometer.' Before that, measurement of sodium was laborious, time consuming, and imprecise-and the same was true of potassium, measurement of which is equally suited to flame photometry. Indeed this technique had a virtual monopoly until suitable ion selective electrodes became available.2 As with many new techniques there are good arguments for and against change, but in this instance the physical principle of measurement differs-a fact which must be understood and taken into account by the clinical chemist. These technicalities do not usually concern or interest the clinician unless they affect the reported result to a substantial extent. Such variation may occur with measurements of serum concentrations of sodium, and both clinicians and clinical chemists need to be aware of the relevant circumstances. The measurement of sodium by flame photometry determines the number of sodium atoms in a defined unit volume of serum or plasma. The ion selective electrode, by contrast, determines the activity of sodium in the solution rather than its concentration. The concept of activity is one which is familiar to physical chemists but less so to medical scientists and clinicians. It may best be described as a measure of the number of atoms which act truly and wholly as ions in a defined unit volume of electrolytic solution. The difference between concentration and activity is reduced as the solution becomes more dilute. In other words, the difference is less in a plasma sample which has been diluted, say, 1 in 200, before measurement than in the same undiluted sample. Several factors contribute to the difference between concentration and activity, and these vary in magnitude among different specimens of plasma. The main factor of clinical relevance is the volume occupied by lipid and protein. Because lipids and proteins exclude water the volume they occupy is not 0 BRI[[SH MEL)ICAI. JOURNAL 1983. All …