Serum Urea Nitrogen, Creatinine, and Estimators of Renal Function

BACKGROUND: Renal dysfunction predicts increased mortality in cardiovascular patients, but the best renal estimator for quantifying risks is uncertain. We compared admission serum urea nitrogen (SUN) level, creatinine level, Modification of Diet in Renal Disease (MDRD) rate, and Mayo estimated glomerular filtration rate (eGFR) for predicting mortality. METHODS: In a retrospective cohort of Medicare patients (aged > or = 65 years) hospitalized for myocardial infarction (n = 44,437) and heart failure (n = 56,652), renal estimators were compared for linearity with 1-year mortality risk, magnitude of risk, and relative importance for predicting risk (percentage variance explained) in proportional hazards models. RESULTS: The SUN level, creatinine level, and Mayo eGFR had linear associations with mortality. These measures predicted steadily increased risk in patients who experienced a myocardial infarction with a SUN level greater than 17 mg/dL (> 6.1 mmol/L), a creatinine level greater than 1.0 mg/dL (> 88.4 micromol/L), and a Mayo eGFR of less than 100 mL/min per 1.73 m2; and in patients who experienced heart failure with a SUN level greater than 16 mg/dL (> 5.7 mmol/L), a creatinine level greater than 1.1 mg/dL (> 97.2 micromol/L), and a Mayo eGFR of 90 mL/min per 1.73 m2 or less. In contrast, the MDRD eGFR had a J-shaped association and failed to identify increased risks in 50.0% of patients who experienced a myocardial infarction (with an MDRD eGFR > 55 mL/min per 1.73 m2) and 60.0% of patients who experienced heart failure (with an MDRD eGFR > 44 mL/min per 1.73 m2). The SUN level and Mayo eGFR had the greatest magnitude of risks. In myocardial infarction and heart failure patients, adjusted mortality increased by 3% and 7%, respectively, per 5-U increase in SUN, and by 3% and 9%, respectively, per 10-U decrease in Mayo eGFR (P<.001), based on models including both renal measures. Of all the measures, SUN had the greatest magnitude of relative importance for predicting mortality. CONCLUSIONS: In older cardiovascular patients, SUN- and creatinine-based measures were powerful predictors of postdischarge mortality. Only MDRD eGFR was less adequate in quantifying risks for patients with mild impairment. Novel estimators, such as the Mayo eGFR, may play an important role in outcomes' prognostication for these patients.