How a nephrologist assesses your kidney function

Doctors use laboratory values to interpret your medical condition.  With respect to kidney disease, the BUN and the creatinine help your nephrologist (as well as your internist and family physician) determine if your kidneys are working correctly. These two tests are commonly ordered for many reasons and are invaluable tools to help your doctor assess your condition. Let’s define what BUN (pronounced by spelling out the letters “B”, “U”, and “N”) and creatinine mean from a simple country nephrologist’s perspective.

Creatinine is a molecule made by the muscles in your body. Creatinine is produced at a constant rate. Creatinine, for the purposes of this discussion, is cleared (or removed) from the kidney filters unchanged by the rest of the body’s metabolism. If the kidneys are having trouble getting rid of the creatinine, the molecules accumulate in the body and the laboratory number increases.

Let’s say a normal creatinine is 1. Patients require kidney dialysis when the value becomes 4-5 and they complain of symptoms consistent with needing dialysis. In other words, the kidney filters are messed up and you now need an alternative method to remove toxins and water from your body — this method of removal we call kidney dialysis.

To be more specific with respect to the function of the kidneys and take into account the differences among human beings, we use an equation called creatinine clearance to calculate how well a person’s kidneys are working. Creatinine is a component of the calculation. Why do we go through all this trouble to serve you?

Creatinine can be an inaccurate marker of kidney function in certain circumstances. For example, men have more muscle mass than women on average and therefore produce more creatinine. This fact must be taken into account when determining if the level of creatinine is high or low. If men have more muscle mass, their creatinine should be higher at baseline because more muscle means more creatinine around. In addition, that’s why the lab differentiates whether a person is African American or not when giving results for the creatinine clearance or GFR (glomerular filtration rate) to determine a patient’s kidney renal function.

BUN or blood urea nitrogen is a term to describe the breakdown products of protein in your body. This value is affected by many things including:

  • blood loss through the gastrointestinal tract
  • the use of steroids for patients with COPD and emphysema
  • the level of hydration in the body
  • in patients receiving intravenous nutrition in the hospital who are prescribed too much protein. Kidney doctors via consultation help other physicians including critical care doctors and hospitalists prescribe TPN (total parenteral nutrition) for the patients they serve if the patient is suffering from complex electrolyte disorders.

A high BUN suggests toxins are not removed from the body. A low BUN suggests malnutrition. Because BUN is given to us in terms of concentration, we also use this laboratory value to determine a patient’s volume status. If the value is high we consider a patient might be dehydrated, for example.

Therefore, we look at BUN with respect to the other laboratory values to make meaning of the term. For example, the BUN can be low when a patient is not eating. In other words, when a patient does not eat enough, there may not be enough protein in the body to convert to BUN, resulting in a low value. In this setting, there is a risk for malnutrition. That’s why the renal panel or kidney panel includes a patient’s albumin, which is a better marker of nutritional status.

In conclusion, we nephrologists look at the removal of creatinine from the body when making decisions regarding kidney dialysis instead of the clearance of BUN from the body. BUN is used to help us make an assessment of the condition of the kidneys but the creatinine is better because the BUN is affected by so many things. So if the creatinine clearance is low (suggesting the need for dialysis) and the BUN is low (at first glance suggesting the patient is not in need of dialysis because there too few toxins in the body to make dialysis worth it), patients may still need kidney dialysis to not only remove fluid from the body but also clear toxins not represented by the BUN.

Michael Aaronson is a nephrologist who blogs at his self-titled blog, Michael L. Aaronson M.D.

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