Effectiveness of the Stewart Method in the Evaluation of Blood Gas Parameters

OBJECTIVES: In 1981, Peter A. Stewart published a paper describing his concept for employing Strong Ion Difference. In this study we compared the HCO(3) levels and Anion Gap (AG) calculated using the classic method and the Stewart method. METHODS: Four hundred nine (409) arterial blood gases of 90 patients were collected retrospectively. Some were obtained from the same patients in different times and conditions. All blood samples were evaluated using the same device (ABL 800 Blood Gas Analyzer). HCO(3) level and AG were calculated using the Stewart method via the website AcidBase.org. HCO(3) levels, AG and strong ion difference (SID) were calculated using the Stewart method, incorporating the parameters of age, serum lactate, glucose, sodium, and pH, etc. RESULTS: According to classic method, the levels of HCO(3) and AG were 22.4±7.2 mEq/L and 20.1±4.1 mEq/L respectively. According to Stewart method, the levels of HCO(3) and AG were 22.6±7.4 and 19.9±4.5 mEq/L respectively. CONCLUSIONS: There was strong correlation between the classic method and the Stewart method for calculating HCO(3) and AG. The Stewart method may be more effective in the evaluation of complex metabolic acidosis.