[Determination of electrolytes in serum and plasma]

In clinical chemistry two different quantities are determined for electrolytes: 1) Electrolyte concentration (total) in serum (S) e.g. S-sodium (mmol/l), S-calcium (mmol/l). 2) Electrolyte concentration (ionized) in serum water [S(W)] e.g. S(W)-sodium, ionized (mmol/kg), S(W)-calcium, ionized (mmol/kg) ad 1) For the determination of the electrolyte concentration in serum, various methods are used: Sodium, potassium: Flame atomic emission spectrometry, ion-selective electrodes after dilution of the sample, enzymatic methods; Chloride: Coulometry, absorption spectrometry after chemical reaction, enzymatic method; Calcium, magnesium: Flame atomic absorption spectrometry, flame atomic emission spectrometry (calcium), absorption spectrometry after chemical reaction, enzymatic method (magnesium). A safe and unambiguous medical interpretation of sodium and chloride ion concentration in serum is not possible without knowledge of the water concentration or of the lipid and protein concentration of the individual sample. The same holds true--even though for some other reasons--for calcium concentration in whole serum. The reference intervals of the pertinent ions are valid only for samples, which are "normal" with respect to the size of the electrolyte-free compartment and--depending on the method--the amount of complex-binding ions. ad 2) For the determination of the concentration of the "ionized" or "free" fraction of sodium, potassium, calcium, and magnesium in serum water (or the extracellular water phase of whole blood) the following method is only applicable: Ion selective electrode without dilution of the sample. A save medical interpretation of the ionized electrolyte concentration in serum water is possible without knowledge of the water concentration of the individual sample, because these quantities are independent from the size of the electrolyte-free compartment.(ABSTRACT TRUNCATED AT 250 WORDS)