Plasma bicarbonate assays--time for a new look?

Abstract

The traditional 'electrolyte profile' still provided by many laboratories consists of the two principal cations of the plasma, sodium and potassium, the two principal anions, chloride and bicarbonate, together with some indicator of glomerular filtration rate, such as urea or creatinine, or both. As will be emphasized shortly, bicarbonate itself cannot readily be measured, but a clinically useful approximation can be obtained by measuring the total CO2 content of the plasma. This grouping of assays has its origins in the history of the clinical laboratory. Measurement of the anions long antedates the development of flame-emission spectrophotometry! in the early 1950s, which made measurements of sodium and potassium practical procedures for the hospital laboratory, capable of providing valuable clinical information with an acceptable turn-round time. The cation measurements were added to those of the anions already provided to give the profile. This focused attention on the electrical neutrality of the extracellular fluid, leading to the reporting of ion concentrations in milliequivalents rather than mass units and the development of the familiar bar-diagrams, in which columns of equal length represent cation and anion concentrations, and to the concept of the 'anion gap' which will be further discussed later. Increasing understanding of the pathophysiology of the body fluids, based on the earlier work of Gamble.i-' led to a rapid rise in demand for electrolyte assays. This was met by automation, mainly using Technicon AutoAnalyser equipment, culminating in the development of dedicated multichannel analysers, such as the SMA 6/60,