The Stewart method (strong ion gap) is superior to the anion gap in late presentation methylmalonic aciduria

摘要

Objective: To emphasize the limitations of the anion gap in a critically ill patient with methylmalonic aciduria (MMA) and to suggest an alternative method of quantification of unmeasured anions in metabolic acidosis (based on Stewart's quantitative method of acid base analysis).Methods: A 10-year-old male subsequently diagnosed with MMA presented with metabolic acidosis. Three different methods to estimate unmeasured anions were compared: this involved the anion gap (AG), the base excess (BE) and the strong ion gap (SIG). The SIG was calculated with a software program based on the Stewart-Fencl model of human plasma, and confirmed by manual calculations described by Fencl.Results: The AG, BE and SIG were measured on entry, and on days 5 and 9. While AG only became abnormal on day 9, both the BE and the SIG were abnormal on admission and reflected severe metabolic derangement on day 9, following a 24-hour infusion of total parenteral nutrition.Conclusion: The normal AG was falsely reassuring in this patient. While both the BE and the SIG were abnormal, only the SIG is specific for unmeasured anions unlike the BE, which may be increased in the presence of either hyperchloremia or unmeasured anions. The SIG alerted the physician to an inborn error of metabolism (MMA).Key messages: AG is unreliable in critically ill hypoalbuminemic patients; the SIG based on a physiochemical approach to acid-base analysis (Stewart method) is more reliable in such patients in detecting unmeasured anions; the base excess does not distinguish between a gap acidosis and a hyperchloremic acidosis and is also affected by hypoalbuminemia.