From the Guidelines
To calculate BUN, a blood sample is drawn and analyzed in a laboratory, with the most recent and highest quality study suggesting that the blood specimen should be obtained from the arterial needle prior to connecting the arterial blood tubing or flushing the needle, to prevent dilution of the blood sample 1. The normal process involves collecting blood from a vein, typically in the arm, using standard venipuncture techniques. No special preparation is usually required before the test, though fasting may be recommended in some cases. The blood sample is then analyzed using automated chemistry analyzers that measure the nitrogen content in urea. Normal BUN values typically range from 7 to 20 mg/dL (2.5 to 7.1 mmol/L), though reference ranges may vary slightly between laboratories. BUN is an important indicator of kidney function, as urea is a waste product filtered by the kidneys. Elevated levels may suggest kidney dysfunction, dehydration, high protein intake, or certain medications affecting kidney function. BUN is often interpreted alongside creatinine levels in what's called the BUN-to-creatinine ratio, providing more comprehensive information about kidney function than either test alone. Some key considerations when calculating BUN include:
- Obtaining the blood specimen from the arterial needle prior to connecting the arterial blood tubing or flushing the needle, to prevent dilution of the blood sample 1
- Avoiding the risk of underestimating predialysis BUN level because of saline dilution or by sampling the blood after treatment has begun 1
- Using the most accurate determination of PNA in patients undergoing PD, which uses Equation 2, but this requires measurement of UNA and dialysate protein losses 1 It's also important to note that the calculation of PNA can be affected by various factors, such as protein losses in dialysate, urinary protein losses, and nitrogen balance studies. Therefore, the most accurate calculation of BUN should take into account these factors and use the most recent and highest quality study available, such as the one published in the American Journal of Kidney Diseases 1.
From the Research
Calculation of BUN
To calculate the Blood Urea Nitrogen (BUN) level, it is essential to understand the normal range and the factors that affect it.
- The normal BUN range is 10 to 20 mg/100 mL 2.
- BUN levels can be affected by various factors, including renal function, dehydration, and extrarenal problems such as heart failure or liver disease.
Interpretation of BUN Results
When interpreting BUN results, it is crucial to consider the serum creatinine level as well.
- A BUN to serum creatinine ratio of greater than 25:1 may indicate an extrarenal problem 2.
- A ratio of 10:1 may indicate renal failure, either acute or chronic 2.
- High BUN concentrations, such as above 28 mg/dL, have been associated with adverse outcomes in critically ill patients, even after correction for renal failure and other confounding factors 3.
Factors Affecting BUN Levels
Several factors can affect BUN levels, including:
- Renal function: impaired renal function can lead to elevated BUN levels 2, 3.
- Dehydration: dehydration can cause a increase in BUN levels 2.
- Nonsteroidal anti-inflammatory drugs (NSAIDs): NSAID use has been associated with increased BUN levels in older adults 4.
- Hemodialysis: BUN measurements can be used to determine blood recirculation in hemodialysis patients, but the accuracy and reproducibility of these calculations may be limited 5.