What is the significance of a Blood Urea Nitrogen (BUN) level of 18.5 and a creatinine level of 0.56 in terms of renal function?

From the Guidelines

A BUN of 18.5 and creatinine of 0.56 are generally within normal ranges and likely not clinically significant. The BUN (Blood Urea Nitrogen) is at the upper end of normal (normal range typically 7-20 mg/dL), while the creatinine is actually on the lower side of normal (typical range 0.6-1.2 mg/dL for adults) [ 1 ]. This combination suggests normal kidney function, though the slightly low creatinine could reflect decreased muscle mass, as creatinine is produced by muscle tissue.

Key Considerations

• The BUN/creatinine ratio is approximately 33:1, which is somewhat elevated (normal ratio is typically 10-20:1), potentially indicating mild dehydration, increased protein intake, or gastrointestinal bleeding.
• According to the National Kidney Foundation practice guidelines for chronic kidney disease, serum creatinine concentration alone is not a reliable means to assess the level of kidney function [ 1 ].
• Clinical laboratories should report an estimate of GFR, as serum creatinine concentration is affected by factors other than GFR, such as creatinine secretion and generation and extrarenal excretion [ 1 ].

Clinical Implications

• Without clinical context such as age, gender, medical history, medications, and symptoms, these values alone don't warrant specific intervention.
• These values should be interpreted within the broader clinical picture, and if there are concerns about kidney function, follow-up testing or consultation with a healthcare provider would be appropriate.
• It is essential to consider the MDRD study equation for a more accurate assessment of kidney function, especially in patients with chronic kidney disease [ 1 ].

From the Research

Significance of BUN and Creatinine Levels

• A BUN level of 18.5 and creatinine level of 0.56 can be evaluated in the context of various studies that have investigated the relationship between these laboratory values and patient outcomes 2, 3, 4, 5, 6.
• The study by 3 found that a high BUN concentration at admission was associated with adverse outcome in critically ill patients admitted to an ICU, even after correction for co-founders including renal failure.
• However, the BUN level of 18.5 is below the optimal cut-off of 28 mg/dL calculated by 3, which was associated with adverse outcome.
• The creatinine level of 0.56 is within normal limits, and the study by 4 discusses the use of creatinine as a marker of kidney function, highlighting its value and pitfalls.
• Another study by 5 found that BUN was associated with in-hospital-mortality in a logistic regression analysis, and an optimal BUN cut-off was calculated by means of the Youden Index.
• The study by 6 provides a review of the available literature and an analysis of medical problem solving, costs, and complications related to electrolytes, BUN, and creatinine, but does not provide specific guidance on the interpretation of these laboratory values.

Clinical Implications

• The combination of BUN and creatinine levels can provide valuable information about renal function and patient outcomes 2, 3, 4, 5.
• However, the interpretation of these laboratory values should be done in the context of the individual patient's clinical presentation and medical history.
• The studies by 3 and 5 suggest that BUN may be a useful parameter for risk stratification in critically ill patients, but the optimal cut-off values and the relationship between BUN and creatinine levels require further investigation.