How to Interpret BUN and Creatinine Levels

Blood urea nitrogen (BUN) and creatinine levels are frequently prescribed by health care providers (HCP). And as a nursing student, it’s essential to understand the importance of interpreting these BUN and creatinine lab results accurately.

BUN measures the amount of nitrogen in the blood that comes from urea, a waste product of protein metabolism. On the other hand, creatinine is a waste product of muscle metabolism excreted by the kidneys. These two tests are often used together to evaluate kidney function, with abnormal levels indicating possible kidney disease or dysfunction.

Interpreting BUN and creatinine levels can be challenging, especially for nursing students new to this area of practice.

The kidneys’ primary function is mainly to filter the following:

• Hydrogen ions
• Urea
• Creatinine

You can easily remember this because your kidneys look like a hook; so that’s HUC (pronounced as “hook”).

Hydrogen ions are acidic. Therefore, patients with renal failure will experience metabolic acidosis because the kidneys have increased hydrogen ions.

Urea is the by-product or waste product of ammonia that is detoxified into the liver, then goes into a portion of the portal vein, and is finally sent to the kidneys for filtering. Once filtered, urea is excreted out of the body as urine. BUN (blood urea nitrogen) technically means the urea concentration in the blood.

Creatinine is the by-product of muscle breakdown. The kidneys filter creatinine, and is also passed out from the body as urine.

Normal Bun and Creatinine Levels

Kidneys filter out creatinine and BUN; this is evident in a metabolic panel. We want to see a good ratio in our metabolic panel for creatinine and BUN. This means that normal laboratory values should show as:

• Creatinine – 0.7 to 1.2 mg
• BUN – less than 20 mg/dL

Acute Renal Failure Labs

In acute renal failure, you have to identify the following indicators:

1. Urine output – how much urine (pee) the patient has excreted
2. Glomerular filtration rate (GFR) – how fast glomeruli (the little washer machines) wash blood milliliters per minute in the kidneys. Normal GFR should be between 85 to 110 mL/min. Acute renal failure patients will show a GFR of less than 60.
3. BUN and creatinine ratio – if creatinine and BUN have an increased ratio. A creatinine that shows more than 1.2 mg will equate to a BUN that’s thrice as much. For example, if you have a creatinine level of 3 mg, your BUN will be 60 mg/dL.

To explain further, the body typically filters creatinine and BUN. However, the most significant indicator for kidneys failing in their primary function (filtering), is the presence of high creatinine. This is because BUN can be high in the body but not in the potty.

This could mean that the patient is dehydrated. To remember this, keep in mind that your “BUNs get burned” when you are dehydrated.

Now, if BUN and creatinine increase, there is kidney involvement. If in case you are still confused with kidney involvement, just remember this by recalling that you have two kidneys, therefore, two lab values should both be increased. If you only have “burned BUNs,” you’re probably dehydrated.

In the other lectures, we will have a comprehensive discussion on the topics of:

• Acute renal failure
• Chronic renal failure
• Creatinine ratios
• Oliguric phase
• Diuresis phase
• Recovery phase from acute renal failure
• Recovery phase from intrarenal and extrarenal
• ABGs with renal failure (sodium, calcium, potassium phosphate)

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