From the Guidelines
Blood Urea Nitrogen (BUN) and creatinine are crucial laboratory tests for assessing kidney function, with elevated levels indicating impaired kidney function, and their interpretation should consider factors such as muscle mass, dehydration, and high-protein diets. These tests measure waste products in the blood: BUN measures urea nitrogen, a waste product from protein breakdown in the liver, and creatinine measures a waste product from normal muscle breakdown. When kidney function declines, these waste products accumulate in the blood, causing elevated levels. Normal BUN ranges from 7-20 mg/dL, while normal creatinine ranges from 0.7-1.3 mg/dL for men and 0.6-1.1 mg/dL for women, though reference ranges may vary slightly between laboratories 1. The BUN-to-creatinine ratio (normally 10:1 to 20:1) provides additional diagnostic information. Elevated levels of both substances typically indicate impaired kidney function, with higher values correlating with more severe kidney damage. However, BUN can also increase due to dehydration, high-protein diets, or gastrointestinal bleeding, while creatinine is more specific to kidney function but can be affected by muscle mass 1. Clinicians should not use serum creatinine concentration as the sole means to assess the level of kidney function, as it is affected by factors other than GFR, such as creatinine secretion and generation and extrarenal excretion 1. Instead, estimated GFR (eGFR) derived from serum creatinine using formulae that include age, sex, and ethnicity, such as MDRD and CKD-EPI, should be used to monitor renal function over months and years 1. These tests are often performed together as part of a basic metabolic panel and are crucial for diagnosing kidney disease, monitoring kidney function in chronic conditions, and adjusting medication dosages for drugs cleared by the kidneys. Key considerations in interpreting these tests include:
- Muscle mass: Creatinine is affected by muscle mass, with higher levels in individuals with more muscle mass 1.
- Dehydration: BUN can increase due to dehydration, while creatinine may not be affected as much 1.
- High-protein diets: BUN can increase due to high-protein diets, while creatinine may not be affected as much 1.
- Gastrointestinal bleeding: BUN can increase due to gastrointestinal bleeding, while creatinine may not be affected as much 1. In monitoring renal function, a patient-based monitoring regimen should be developed, considering both medication and individual risk factors, and suggesting a monitoring interval based on a patient’s combined risk 1.
From the Research
Definition of BUN and Creatinine
- Blood Urea Nitrogen (BUN) and creatinine are two commonly used tests to estimate kidney function, particularly glomerular filtration rate (GFR) 2.
- BUN measures the amount of urea in the blood, while creatinine measures the amount of creatinine, a waste product from muscle metabolism, in the blood.
- These tests are useful for clinicians to assess kidney function, but they have limitations and should be interpreted in conjunction with other tests and clinical indicators 2, 3.
Indications of Kidney Function
- Elevated BUN and creatinine levels can indicate impaired kidney function, but the ratio of BUN to creatinine (BUN/Cr) can provide additional information about kidney function 4.
- A higher BUN/Cr ratio has been associated with worsening kidney function in patients with heart failure 4.
- Creatinine is the most widely used functional biomarker of the kidney, but its use has substantial shortcomings, and understanding these limitations is critical for accurate interpretation of creatinine values 3.
Assessment of Kidney Function
- Glomerular filtration rate (GFR) is the best indicator of kidney function, and can be estimated using various equations, such as the creatinine-based Cockcroft-Gault and the modification of diet in renal disease 5.
- Cystatin C is advocated as a functional marker of kidney function, and its clinical significance is under study 5.
- Proteinuria, or the presence of protein in the urine, adds value to GFR estimation and can indicate kidney damage 5, 6.
- Assessment of kidney function is important for the detection and management of chronic kidney disease, and various methods, including GFR and albuminuria, are used to assess kidney function 6.