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The Bun Creatinine Ratio. The Biomarkers of Hydration and Renal Function.

Chris Park , On November 18, 2022

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What is the BUN Creatinine ratio used for?

The BUN: Creatinine ratio is a measurement of either renal failure or a cause of dehydration in the body. Without this objective criteria dehydration is a clinical diagnosis, formed from the signs and symptoms of dehydration on the body.

What is the BUN creatinine ratio?

BUN is the measurement of urea in the blood performed via a blood urea nitrogen test. Urea is a chemical waste product produced when your body breaks down protein used by your cells . During this process, your liver produces ammonia — which contains nitrogen — and combines with other elements, such as carbon, hydrogen and oxygen, to form urea. The urea travels from your liver to your kidneys through your bloodstream.

Creatinine is a molecular protein produced by the muscle and is relatively freely filtered by the kidneys. Hence, it is commonly used as a kidney function test as most adults make the same amount of creatinine every day. There are relatively few things that can affect the creatinine level hence the majority of fluctuations can be attributed to a change in kidney function.

BUN and creatinine together are used to estimate the kidney function. But since kidney function is determined to some extent by a person’s fluid intake, a dehydrated body can lead to a high BUN: creatinine ratio. This is also called pre-renal azotemia.

A BUN/Creatinine ratio of more than 20 is considered high and dehydrated.

What causes a high BUN creatinine ratio?

1. Dehydration is one of the factors that causes a high BUN creatinine ratio. Since you are affecting one side of the equation. The good news is that this is reversible by increasing the fluids consumed (consult your nephrologist if you are on dialysis) to get a lower BUN:creatinine ratio.

  • An example of an individual with a high BUN:creatinine Ratio due to dehydration:

    A person of 40 years of age and 150lb ran a couple of miles in the scorching heat without a drink. If we sampled this person’s lab, his chemistry profile may look something like: BUN level of 28 mg/dl and creatinine of 1.1 mg/dl. Notice, that the BUN/Creatinine ratio in this case is higher than 20. Therefore, we can conclude that this individual is dehydrated. If this same person had a bottle of water (1liter or 32oz water bottle) the lab profile would change to:
    BUN of 16 mg/dl and creatinine of 0.95 mg/dl. In this example with a bottle of water the BUN/Creatinine ratio is less than 20 and BUN and creatinine have normalized.
    In summary, a high BUN-Creatinine ratio – more than 20 and the person has a normal creatinine, usually indicates dehydration. However, if BUN and creatinine are both elevated significantly and the ratio is < 20, it may indicate an underlying kidney problem.

2. A kidney problem can be the cause of a high BUN:creatinine ratio. In this regard, both BUN and creatinine are elevated proportionately, and the ratio hence is not necessarily higher than 20.

  • An example of an individual with a high BUN:creatinine Ratio due to kidney problems:

    A 50 year old person who weighs roughly 150 lbs, who is experiencing high blood pressure and has the pre-diabetes glucose level will show different laboratories. His lab reveal a BUN level of 28 mg/dl and a creatinine level of 1.6 mg/dl. He is hydrated since the BUN-Creatinine ratio is less than 20 and his BUN and creatinine levels are elevated in a “hydrated” state. This reveals he has an kidney problems.

Renal failure patients are normally diagnosed with a BUN and creatinine test when undergoing urine sampling. When a patient is diagnosed with CKD a special renal diet is needed.

What Causes a low BUN creatinine Ratio?

There are some circumstances that cause a low BUN:creatinine ratio. In patients experiencing a low BUN/creatinine ratio can be caused by very low protein intake as seen in cachexia or liver cirrhosis. Some cases of malnutrition will also have very low BUN: Creatinine ratio. People with an eating disorder or those who are undergoing very extreme dietary modifications to drop weight can have very low BUN levels in the blood. Drinking way too much water can also give very low BUN and creatinine ratio in the blood tests.

Can a High Protein Diet Raise Bun and creatinine Levels in the Blood?

Yes, in fact, when we eat protein, the peptide bond gets broken to yield amino acids. These are then absorbed through the gut and are the building blocks for protein. As the peptide bond breakage creates nitrogenous waste products, ammonia is produced which is converted into a nontoxic waste called urea by the liver. The measurement of urea in the blood is done via Blood urea nitrogen test or simply referred to as BUN. Therefore, the very definition of BUN can be explained as the breakdown of protein. One of the commonest causes of high BUN in the blood is a very high protein diet. If someone is working out extensively to gain weight, he/she is likely ingesting a very high protein amount (almost 2-3 g/kg/day). In these cases, a lot of nitrogenous waste is produced due to the protein breakdown, leading to ammonia generation that is eventually converted into urea. This could also happen if you are inadvertently eating surprising protein foods. Since urea is non toxic, it is eliminated via urine. Now, generally speaking, a high protein diet will also cause muscle gain, and hence the blood creatinine level will eventually rise and catch up, normalizing the BUN: creatinine ratio. Thus, high protein intake can cause a high BUN/Creatinine ratio but only initially. Again, if the same person is working out a lot and does not hydrate quickly, the BUN to creatinine ratio will be higher.

Can Medicine cause a high BUN creatinine ratio?

Yes, diuretics typically do. Commonly used for water retention, this class of medications can elevate the BUN and creatinine levels. Hence, patients using diuretics should get laboratories done to assess electrolyte and renal function levels periodically.

How can you improve your BUN creatinine ratio?

As mentioned above, in healthy adults, a high BUN: Creatinine ratio, particularly if it is > 20 is a sign of inadequate hydration. If you are exercising regularly and sweating a lot, you will likely get dehydrated fast. One of the best ways to stay rehydrated during exercise is by using electrolyte rich hydration. This electrolyte powder delivers magnesium, sodium and potassium, all electrolytes depleted with exercise. Magnesium potentiates sodium delivery to exercising muscles and helps retain potassium in the body, leading to better muscle function and lesser cramping. As magnesium dilates the blood vessels, it can improve perfusion (blood supply) of the kidneys while exercising, and this will eventually translate to an improvement of the BUN: creatinine ratio. Thus, in simple terms, an easy way to improve your BUN/Creatinine levels is by improving your hydration.

Chris Park

Chris Park is a board-certified sports nutritionist, registered dietitian and former college rugby player. Chris loves strength training and pushing his body to the limit, which comes in handy when working with professional athletes and designing their nutritional needs. Chris also enjoys helping others by writing about nutrition.