Renal functions refer to BUN and creatinine. let’s start there. I know it’s a basic start. Now, do not make the mistake of calling the BUN a “bun.” That is a rookie mistake. It is referred to a B.U.N. meaning blood urea nitrogen.
If elevated, we referred to that state as azotemia (Azotemia (azot, “nitrogen” + -emia, “blood condition”) is a medical condition characterized by abnormally high levels of nitrogen-containing compounds. Another renal function is known as the creatinine. Now, creatinine is a breakdown product of muscle and stays relatively constant. A normal reference value for creatinine is less than 1.5, but that is quite variable based on the muscle mass of the individual. If you have a frail 90-year-old lady who weighs 80 lbs and her creatinine is 1.5, this demonstrates significant renal insufficiency. However, when a body builder, with a large muscles, has a creatinine of 1.7 that may be a completely normal value for this larger gentleman.
If renal functions are elevated, we need to figure out if it is renal, prerenal or postrenal. This must identify this. Prerenal problems are a problem with a lack of blood flow going into the kidneys. This is (almost always) either dehydration or congestive heart failure. (I will estimate 24 out of 25 times they are dry.) Both of these states mean the body is not getting adequate blood flow to the kidneys to keep the renal functions adequate.
The initial way to tell if this is a prerenal problem is to look at the BUN to creatinine ratio. If the ratio is greater than 20, it suggests a pre-renal problem. A renal cause of azotemia means that the kidneys are actually diseased and sick. A way to help with this is your BUN and creatinine ratio. Again, if less than 20, that suggests a sick kidney. This is also a time when that fractional excretion of sodium or FeNa is helpful because a FeNa greater than 2 says that the body is not filtering sodium the way it should and again implies a sick kidney.
The most common postrenal problem, by far, is the prostate. Do a rectal exam and see if the prostate is enlarged. Talk to the patient about the history of urinary flow. Is it easy to initiate flow? Do they feel like they empty their bladder completely? The prostate is kind of like a doughnut that the urethra goes through, and when the prostate becomes enlarged, it pinches down on the urethra. A patient who has an enlarged prostate, will void, yet not completely empty their bladder because of the pressure on their urethra. So, you would ask them, “Do you feel like you are completely emptying your bladder after you pee?” If a patient has elevated BUN or creatinine, you need to look at the BUN to creatinine ratio and apply that to patient care. Students can often get confused here. If you have a ratio that is greater than 20 yet the BUN to creatinine values are in normal range, we do not care about that. We do not look at ratio if the BUN to creatinine are normal. So, once again we only look at the BUN to creatinine ratio if the functions are elevated.