From the Guidelines
The BUN to creatinine ratio typically increases when renal function is reduced by approximately 50% or more, corresponding to a glomerular filtration rate (GFR) below 60 mL/min/1.73m². At this level of impairment, the kidneys begin to lose their ability to clear urea at the same rate as creatinine. The normal BUN:Cr ratio is between 10:1 and 20:1, and ratios above 20:1 often indicate significant renal dysfunction. This occurs because urea reabsorption increases in the tubules when filtration is reduced, while creatinine continues to be cleared primarily through filtration. Additionally, other factors can elevate this ratio even with less severe renal impairment, including dehydration, gastrointestinal bleeding, high protein diets, or catabolic states. When evaluating kidney function, it's essential to consider both the absolute values of BUN and creatinine as well as their ratio, along with estimated GFR calculations for a complete assessment of renal status, as recommended by guidelines such as those from the National Kidney Foundation 1 and the Kidney Disease Improving Global Outcome (KDIGO) 1.
Some key points to consider:
- A GFR level less than 60 mL/min per 1.73 m2 represents loss of half or more of the adult level of normal kidney function, and below this level, the prevalence of complications of chronic kidney disease increases 1.
- The definition of chronic kidney disease is the same, regardless of age, and because GFR declines with age, the prevalence of chronic kidney disease increases with age 1.
- Other factors that can affect the BUN:Cr ratio include muscle mass, which decreases with age, and ethnic origin, which can affect serum creatinine levels 1.
- The KDIGO guidelines provide a framework for the diagnosis and management of acute kidney injury (AKI) and chronic kidney disease (CKD), including the use of serum creatinine and estimated GFR to assess kidney function 1.
Overall, when evaluating the BUN to creatinine ratio, it's crucial to consider the clinical context and other factors that may influence this ratio, and to use a comprehensive approach to assess kidney function, including estimated GFR calculations and other markers of kidney damage.
From the Research
Kidney Function and BUN/Cr Ratio
- The degree of impaired renal function required to increase the Blood Urea Nitrogen (BUN) to Creatinine (Cr) ratio is not strictly defined, but studies suggest that a decline of >25% in estimated glomerular filtration rate (eGFR) is associated with a higher BUN/Cr ratio 2.
- A BUN/Cr ratio of 20 or greater may indicate various physiological conditions, including impaired renal function, and can affect the accuracy of GFR estimates obtained using the Modification of Diet in Renal Disease (MDRD) study equation and Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) study equations 3.
- Disproportionate rises in BUN/Cr (> 20:1) may imply pre-renal azotemia, but can also be caused by increased protein catabolism or an excessive protein load 4.
Factors Affecting BUN/Cr Ratio
- Hypovolemia, congestive heart failure, septic or hypovolemic shock, and high-dose steroids can contribute to a disproportionate increase in BUN/Cr ratio 4.
- A high protein intake (> 100 g/d) can also contribute to an increased BUN/Cr ratio 4.
- Nonsteroidal anti-inflammatory drug (NSAID) use has been associated with increased levels of BUN, serum creatinine, and BUN:serum creatinine ratio in older people 5.
- A pro-inflammatory diet, as measured by the dietary inflammatory index (DII), may be associated with disease progression in patients with chronic kidney disease (CKD) 6.
Clinical Implications
- The BUN/Cr ratio can be a useful predictor of worsening kidney function and can add incremental risk prediction information relative to traditional predictive measures in outpatients with heart failure at risk for worsening kidney disease 2.
- GFR estimates for patients with a high BUN/Cr ratio should be interpreted cautiously, as they may be affected by various physiological conditions 3.