Unmeasured anions identified by the Fencl-Stewart method predict mortality better than base excess, anion gap, and lactate in patients in the pediatric intensive care unit.

OBJECTIVES: This study was undertaken to compare three methods for the identification of unmeasured anions in pediatric patients with critical illness. We compared the base excess (BE) and anion gap (AG) methods with the less commonly used Fencl-Stewart strong ion method of calculating BE caused by unmeasured anions (BEua). We measured the relationship of unmeasured anions identified by the three methods to serum lactate concentrations and to mortality.

DESIGN: Retrospective cohort study.

SETTING: Tertiary care pediatric intensive care unit in an academic pediatric hospital.

PATIENTS: The study population included 255 patients in the pediatric intensive care unit who had simultaneous measurements of arterial blood gases, electrolytes, and albumin during the period of July 1995 to December 1996. Sixty-six of the 255 patients had a simultaneous measurement of serum lactate.

MEASUREMENTS AND MAIN RESULTS: The BEua was calculated using the Fencl-Stewart method. The AG was defined as (sodium plus potassium) - (chloride plus total carbon dioxide). BE was calculated from the standard bicarbonate, which is derived from the Henderson-Hasselbalch equation and reported on the blood gas analysis. A BE or BEua value of < or =-5 mEq/L or an AG > or =17 mEq/L was defined as a clinically significant presence of unmeasured anions. A lactate level of > or =45 mg/dL was defined as being abnormally elevated for this study. The presence of unmeasured anions identified by significantly abnormal BEua was poorly identified by BE or AG. Of the 255 patients included in the study, 67 (26%) had a different interpretation of acid base balance when the Fencl method was used compared with when BE and AG were used. Plasma lactate concentration correlated better with BEua (r2 = .55; p = .0001) than with AG (r2 = .41; p = .0005) or BE (r2 = .27; p = .025). Mortality was more strongly related to BEua < or =-5 mEq/L (relative risk of death = 10.25; p = .002) than to lactate > or =45 mg/dL (relative risk of death = 2.35; p = .04). In logistic regression analysis, mortality was more strongly associated with BEua (area under the receiver operating characteristic curve = 0.79; p = .0002) than lactate (receiver operating characteristic curve area = 0.63; p = .05), BE (receiver operating characteristic curve area = 0.53; p = .32), or AG (receiver operating characteristic curve area = 0.64; p = .08) in this patient sample.

CONCLUSIONS: Critically ill patients with normal BE and normal AG frequently have elevated unmeasured anions detectable by BEua. The Fencl-Stewart method is better than BE and similar to AG in identifying patients with high lactate levels. Elevated unmeasured anions identified by the Fencl-Stewart method were more strongly associated with mortality than with BE, AG, or lactate in this patient sample.