Calculation of plasma chloride levels using blood gas measurements

Introduction Chloride is the most important anion associated with sodium in the extra cellular fluid with a normal concentration ranging from 96 to 106 (1). The complete interpretation of acid-base disorders is impossible without knowing the serum chloride concentration (2). In the process of quantitative acid-base analysis, plasma pH is determined by three independent variables: pCO2, total concentration of nonvolatile weak acids, and the strong ion difference (SID). On the other hand, bicarbonate (HCO3) concentration is considered to be dependent on the SID. It has been suggested that chloride is the major electrolyte determining the SID and thus directly affecting acid-base disorders in the clinical setting (3). Furthermore, the serum chloride level has been associated with rates of mortality in certain groups of patients and has been suggested as a prognostic factor (4,5). Therefore, timely and accurate determination of serum chloride concentration is of great importance for decision-making in the emergency department. Several methods have been described for the measurement of serum chloride (6). The coulometric titration method uses electrolysis current to measure ion formation (7), while the ion-selective electrode uses a transducer to convert the activity of the chloride ion into an electrical potential, which can be measured by a voltmeter or pH meter (8). The colorimetric method combines chloride with other elements to form a colored solution and then measures color density to determine chloride concentration (9). A cyclic voltammetry using an Ag electrode has also been described for selective measurement of chloride ions (10). These methods have been described for years and range in their complexity, cost, and accuracy, yet even today measurement of serum chloride levels is not available in Iranian hospitals as an emergent test 24 hours a day. To overcome this problem here we describe a formula for calculation of serum chloride levels using information available in the emergency setting through analysis of blood gases.