Renal Labs: Uncomplicated BUN and Creatinine Interpretation

Today, Mike Linares will teach you about one of the favorite topics in nursing exams – BUN and creatinine.

Let’s begin.

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

  1. Hydrogen ions
  2. Urea
  3. 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, with clients who have renal failure, they will be experiencing 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 in the form of urine. BUN (blood urea nitrogen) technically means the amount of urea concentration in the blood.

Creatinine is the by-product of muscle breakdown. Creatinine is filtered by the kidneys and is also passed out from the body as urine.

Normal Levels

Kidneys filter out creatinine and BUN; this is evident in a metabolic panel. What we want to see in our metabolic panel for creatinine and BUN is a good ratio. 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

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

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

To explain further, the body filters typically creatinine and BUN. However, the most significant indicator for kidneys failing in its 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 just mean that you are just dehydrated. To remember this, keep in mind that your “BUNs get burned” when you are dehydrated.

Now, if your BUN and creatinine are both increased, you have 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,” then you’re probably just 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)

If you want to check out other topics that commonly show up in nursing exams, visit