Massive and disproportionate elevation of blood urea nitrogen in acute azotemia

In renal failure, blood urea nitrogen and serum creatinine usually rise in tandem; the normal BUN : Cr ratio is 10-15 : 1. Disproportionate rises in BUN : Cr (> 20 : 1) often imply pre-renal azotemia but may be caused by increased protein catabolism or an excessive protein load. In this study we looked at intensive care patients who acutely developed markedly increased BUN (≥ 100 mg/dL) with only modest elevation of Cr (≤ 100 mg/dL) with only modest elevation of Cr (≤ 5 mg/dl) for possible causes of the disproportionate azotemia. There were 19 such cases collected over 6 months, nine women and ten men, with mean age 69.2 ± 4.4 years (13/19 > 75 years). Peak BUN was 156 ± 11 mg/dL; peak Cr 4.3 ± 0.5 mg/dL. Eleven patients expired. Mean serum albumin at the time of consultation was 2.7 ± 0.2 g/dL; mean total lymphocyte count 1.0 ± 0.1/mm3. Of possible factors causing the azotemia, nine patients had documented hypovolemia; eight had congestive heart failure; six were in septic or hypovolemic shock, and two received high-dose steroids. As contributing factors, eight patients had Salb < 2.5 g/dL; eight were given a high protein intake ≥ 100 g/d; two had HIV, and two others had gastrointestinal bleeding. Infection was present in 14 patients; seven had sepsis (bacteremia with hypotension). All patients had at least one of these factors present and 16/19 had two or more. Fractional Na excretion was < 1% (consistent with pre-renal azotemia) in only four of the 11 patients in whom it was measured. We conclude that severely disproportionate BUN : Cr is frequently multifactorial and is most common in the elderly, perhaps due to their lower muscle mass, and in ICU patients given a high protein intake. It is often not indicative of uncomplicated renal hypoperfusion, although low renal perfusion (hypovolemia, shock, or heat failure) is common. Mortality is high due to the severe illnesses, especially infection, worsened by decreased renal function and hypercatabolic state.