From the Guidelines
Low blood urea nitrogen (BUN) levels can result from several causes including overhydration, malnutrition, severe liver disease, and pregnancy, as well as errors in laboratory or blood sampling procedures, such as dilution of predialysis BUN blood sample with saline or drawing predialysis BUN blood sample after the start of dialysis 1.
Causes of Low BUN
Some of the key causes of low BUN levels include:
- Overhydration or excessive fluid intake, which dilutes blood components, including BUN
- Malnutrition or low protein intake, which reduces the amount of protein being metabolized into urea
- Severe liver disease, which impairs the liver's ability to produce urea during protein metabolism
- Pregnancy, which involves normal physiological changes including increased glomerular filtration rate and plasma volume, leading to lower BUN levels
- Certain medications, such as anabolic steroids and growth hormones, that promote protein synthesis rather than breakdown
- Genetic disorders affecting the urea cycle, which may result in decreased BUN production
- Errors in laboratory or blood sampling procedures, such as dilution of predialysis BUN blood sample with saline or drawing predialysis BUN blood sample after the start of dialysis 1
Investigation and Treatment
When investigating low BUN, healthcare providers typically evaluate:
- Hydration status
- Nutritional intake
- Liver function
- Medication use Additional tests may be ordered, such as:
- Liver function tests
- Nutritional assessments
- Tests for rare genetic disorders Treatment focuses on addressing the underlying cause rather than the BUN level itself, as recommended by recent clinical practice guidelines for hemodialysis adequacy 1.
From the Research
Causes of Low BUN Lab Work Up
- Malnutrition is a common cause of low BUN (Blood Urea Nitrogen) levels, as it can lead to decreased protein intake and subsequent decreased urea production 2, 3, 4, 5.
- Liver disease, particularly cirrhosis, can also cause low BUN levels due to impaired liver function and decreased ability to detoxify ammonia 2, 3, 4, 5.
- Urea cycle disorders, such as ornithine transcarbamylase deficiency and argininosuccinic aciduria, can also lead to low BUN levels due to impaired urea production 6.
- Other factors that can contribute to low BUN levels include:
- It is essential to note that low BUN levels can have various causes, and a comprehensive evaluation, including medical history, physical examination, and laboratory tests, is necessary to determine the underlying cause 2, 3, 4, 5, 6.