The fallacy of the BUN:creatinine ratio in critically ill patients

• Published in: Nephrology, dialysis, transplantation dialysis, transplantation, 2012-06, Vol.27 (6), p.2248-2254
• Main Authors: RACHOIN, Jean-Sebastien, DAHER, Ralph, MOUSSALLEM, Charles, MILCAREK, Barry, HUNTER, Krystal, SCHORR, Christa, ABBOUD, Mariam, HENRY, Patricia, WEISBERG, Lawrence S
• Format: Article
• Language: English
• Subjects: Acute Kidney Injury - metabolism; Acute Kidney Injury - mortality; Adolescent; Adult; Aged; Anesthesia. Intensive care medicine. Transfusions. Cell therapy and gene therapy; Biological and medical sciences; Blood Urea Nitrogen; Creatinine - blood; Critical Illness - mortality; Emergency and intensive care: renal failure. Dialysis management; Female; Glomerulonephritis; Humans; Intensive care medicine; Intensive Care Units; Male; Medical sciences; Middle Aged; Nephrology. Urinary tract diseases; Nephropathies. Renovascular diseases. Renal failure; Prognosis; Renal Replacement Therapy - mortality; Retrospective Studies; Young Adult
• ISSN: 0931-0509; 1460-2385
• DOI: 10.1093/ndt/gfr705

Acute kidney injury (AKI) is common in critically ill patients and is associated with a high mortality rate. Pre-renal azotemia, suggested by a high blood urea nitrogen to serum creatinine (BUN:Cr) ratio (BCR), has traditionally been associated with a better prognosis than other forms of AKI. Whether this pertains to critically ill patients is unknown. We conducted a retrospective observational study of two cohorts of critically ill patients admitted to a single center: a derivation cohort, in which AKI was diagnosed, and a larger validation cohort. We analyzed associations between BCR and clinical outcomes: mortality and renal replacement therapy (RRT). Patients in the derivation cohort (N = 1010) with BCR >20 were older, predominantly female and white, and more severely ill. A BCR >20 was significantly associated with increased mortality and a lower likelihood of RRT in all patients, patients with AKI and patients at risk for AKI. Patients in the validation cohort (N = 10 228) with a BCR >20 were older, predominantly female and white, and more severely ill. A BCR >20 was associated with increased mortality and a lower likelihood of RRT in all patients and in those at risk for AKI, BUN correlated with age and severity of illness. A BCR >20 is associated with increased mortality in critically ill patients. It is also associated with a lower likelihood of RRT, perhaps because of misinterpretation of the BCR. Clinicians should not use a BCR >20 to classify AKI in critically ill patients.