Acute Renal Failure (ARF): BUN and Creatinine Interpretation

SimpleNursing Editorial Team Apr 4, 2018
kidney illustration for acute renal failure bun and creatinine interpretation

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  1. How to Diagnose ARF
  2. Two Conditions of ARF
  3. Interpreting BUN and Creatinine

In our previous articles, we already discussed the phases of acute renal failure, namely:

  • Phase 1 – Oliguric
  • Phase 2 – Diuretic
  • Phase 3 – Recovery

We also talked about the three main causes of acute renal failure which are:

  • Low blood pressure
  • Low volume
  • Low oxygen

The mentioned causes are mainly due to low perfusion and low mean arterial pressure (MAP) that will cause the body to break down. So now, we’re going to assess how severe the acute renal failure is by determining your laboratory values.

Think of it this way: You’re driving your friend’s car, and you got into a car accident. The first thing that your friend is going to ask you once you break the news is, “How bad is it?” or “What’s the damage?” With acute renal failure, there are levels to determine the progression of the condition.                      

How to Diagnose ARF

To determine the severity of acute renal failure, doctors will request for specific tests for verification. Some of these tests may include:

  • Urine specific gravity
  • Blood urea nitrogen (BUN) and creatinine
  • Glomerular filtration rate – function test

Technically, this is breaking down the pathophysiology of kidney function, and we want to have a more vivid picture of the status of the condition, or how much of the kidney is no longer working or how much of it is still working.

Urine specific gravity

Urine specific gravity is going to show the concentration of the urine. If the test result indicates 1.030 or higher, you can expect very concentrated, dark, and stinky urine due to lack of water inside the body.

You will notice this with clients who are fond of drinking alcohol and soda but drink minimal water. Blood becomes concentrated, and it is turned into very dark urine output. In the phases of ARF, this is called as the oliguric phase.

BUN and Creatinine

As what was mentioned before, if BUN increases but the creatinine level is normal, the main reason is dehydration and NOT acute renal failure. Now, if the BUN and creatinine are both elevated, this means that you have a kidney problem.

Glomerular Filtration Rate

Glomerular filtration rate measures the creatinine in the blood. Having a GFR test result of below 65 is a sign of ARF. So remember the magic number – 65. Anything below 65 is a serious kidney issue, and anything above 65 is passing. The number that has to show on the GFR result should be above 85 to become normal.

Two Conditions of ARF

Now, we will determine how severe the condition is and in doing so, you have to know the two conditions of acute renal failure which are:

  • Extrarenal failure (prerenal) – 20:1 ratio for BUN/creatinine
  • Intrarenal failure – 10:1 ratio for BUN/creatinine

The difference between the two is their location. Extrarenal or prerenal failure means that the problem is outside the kidneys while intrarenal means the problem is inside the kidneys. To give you a more straightforward example, if there’s a fire (damage) and it happened outside the house (kidneys), it’s called prerenal. On the other hand, if the fire happened inside the house, it’s called intrarenal. Between the two, prerenal is more preferred than intrarenal to avoid serious damage.

Interpreting BUN and Creatinine

If your BUN is at 40, the expected creatinine will be at two; there’s enough space separating the BUN and creatinine. Since creatinine is the main factor that determines the severity of ARF, the moment it starts rising, and the numbers become closer, that’s a sign that therapeutic modality should be provided immediately.

Remember, if the “fire” is inside the kidneys, there will be depleted oxygen content and will start to suffocate the kidneys. Therefore, your kidneys need to be rebooted with treatment modalities that Mike likes to call “HDTV.”

To know more about these HDTV treatment modalities, access our other nursing videos here.

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