Blood Work: Understanding BUN and Creatinine

In order to determine kidney function, a blood test called the BUN Creatinine is performed. BUN stands for blood urine nitrogen. It measures how much urea is present in the bloodstream. High levels of urea indicate that the kidneys are having trouble filtering it out. The most noticeable effect of impaired kidney function is a marked decrease in the output of urine. This is a sign of dehydration.

Cystic fibrosis (CF) patients are prone to dehydration due to the abnormal function of the cystic fibrosis trans-membrane regulator (CFTR) protein. This is primarily what causes the characteristically salty sweat in people with CF. Dehydration and the loss of electrolytes during sweating can wreak havoc on the kidneys. When this happens, the imbalance of electrolytes can result in a buildup of nitrogen in the blood that is caused by a buildup of urea in the body.

Urea is a molecule comprised of carbon, oxygen, nitrogen and hydrogen in the formula CH₄N₂O. This chemical is released into the bloodstream by the liver as the body breaks down proteins. In healthy kidneys, urea is removed from the blood, and passes easily into the urine. When kidney function is impaired, a buildup of urea occurs.

A normal BUN level is usually in the range of 10 to 20 milligrams per deciliter (mg/dL). It is just as important to prevent the BUN from being too high as it is from it falling below the normal range.

Possible Causes of high BUN

1. High protein diets

2. Certain medications such as the antibiotic, tobramycin

3. Dehydration

4. Congestive Heart failure

5. Presence of Addison's disease

6. Blockage of urinary tract due to kidney stones

7. Intestinal bleeding

Possible Causes of low BUN

1. Liver disease

2. Malnutrition

3. Low protein diet (common in vegetarians and vegans)

4. Severe muscle injury

5. Pregnancy

Typically a BUN level is measured at the same time as a creatinine measurement. Creatine (note spelling) is a protein that generally remains at a consistent level in the bloodstream. It's made in the liver, then serves as the energy source when muscles are used. As the breakdown of muscles takes place, the waste product creatinine is formed and is then filtered through the kidneys. By comparing the amount of BUN and creatinine, the doctor can determine whether kidney function is normal or impaired. A normal ratio of BUN:creatinine is anywhere in the range of 10:1 to 20:1.

Patients with CF, particularly those who have pancreatic insufficiency, tend to be malnourished. This can result in lower BUN/Creatinine ratios. As mentioned before about the levels of urea, the BUN/Creatinine ratio can be elevated due to dehydration. In general men have higher BUN than women. The BUN level also increases naturally with age.

Blood work for the purpose of monitoring BUN/Creatinine is done every other day throughout the course of intravenous antibiotics. This is to ensure that the concentration of the prescribed medication is not so high that it compromises the patient's kidney function. It is important for patients undergoing IV therapy to remain adequately hydrated in order to maintain normal and healthy kidney function.