Comparison between the blood urea nitrogen (BUN)-to-creatinine ratio and four urinary biomarkers of dehydration; a cross-sectional study in fasting patients awaiting surgery.

Abstract

The blood urea nitrogen (BUN)-to-plasma creatinine ratio is a conventional method for diagnosing dehydration. We examined the relationship between this method and the more recently promoted urine biomarkers of dehydration. Blood and urine sampling was performed in 921 patients who were fasting before surgery. The blood was analyzed for the BUN-to-plasma creatinine ratio which was divided into three ranges: low (<50), normal (50-100), and high (>100 mmol/L). The fractions of the cohort that belonged to these ranges were 8.9%, 76.7%, and 14.4%, respectively. The urine was analyzed for color, specific gravity, osmolality, and creatinine, and the results summarized in the Fluid Retention Index, which is a robust measure of renal water conservation. The analysis shows that none of the four urine biomarkers differed statistically between these three groups. The Fluid Retention Index was 2.3, 2.0, and 1.7, respectively (p = 0.17), which means that the degree of urine concentration varied in the opposite direction to that expected. Moreover, no linear correlations were found when individual blood and urine measurements were compared. Patients who were classified as being dehydrated according to the Fluid Retention Index had a mean BUN to creatinine ratio of 77 while the others had 76 mmol/L (p = 0.94). In conclusion, the BUN-to-ratio did not correlate with urine biomarkers of dehydration in fasting pre-surgical patients. This result suggests that the BUN-to-creatinine ratio may not reliably reflect hydration status and motivates a repeat study where the body volume deficit is known or the water consumption is monitored over time.