A Reestablished Reference Interval of Plasma Anion Gap Minimizing Preanalytical Errors

Baek, Eun Jung; Park, Ile Kyu

Abstract

BACKGROUND
The reference interval of anion gap established in the 1970s has been changing as the method of electrolyte measurement changes. It is also influenced by preanalytical errors. But there are only a few reliable reports that examined the effect of various preanalytical errors. Furthermore, there has been no report of values measured by the instrument being used in our laboratory. Therefore, we attempted to establish a reference interval of anion gap measured by Hitachi 7170s minimizing preanalytical errors, and analyzed the effect of a delay in electrolyte measurement and of sample exposure to the air. METHODS: The subjects were 538 healthy people who attended Hanyang university hospital health clinic with normal blood levels for albumin, creatinine and glucose. The plasma Na+, K+, Cl-, and total carbon dioxide (TCO2) were measured by Hitachi 7170s autoanalyzer (Boehringer Mannheim, Indianapolis, USA). To examine the effect of the delay between blood withdrawal and electrolyte measurement and the loss of CO2 by exposure to the room air after bottle opening, the subjects were divided into group A (124 subjects) and group B (414 subject) whose electrolyte results were reported within and after 20 minutes, respectively, of blood withdrawal and the two groups were compared with each other for the electrolytes and anion gap. Anion gap was calculated by the formula, [Na+- (Cl-+TCO2)]. RESULTS: Compared with group A, TCO2 of group B decreased by 0.9 mmol/L (P<0.001) and the anion gap increased by 0.6 mmol/L (P=0.009). The reference interval of anion gap was established based on the group A at the value of 5.4-13.4 mmol/L. CONCLUSIONS: The reference interval of anion gap determined in this study was lower than the value commonly used and should be established with a minimum delay in electrolyte measurement and sample exposure to the air.