Understanding Renal Function (BUN and creatinine)

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Now, let's talk about renal functions and the ancillary testing required when evaluating BUN & creatinine. Initially if you are doing just a basic CBC and chemistry panel,  we get our BUN and creatinine. It gives me a really good glimpse of how well the kidneys are functioning.

The BUN, let's talk about the blood, urea, nitrogen.

BUN is a product of the breakdown of your blood and it's very volatile. Creatinine is a product of muscle mass. So a creatinine of 1.2 could be very normal if you have a muscular guy (like me... OK ok ok...JK.) A creatinine of 1.2 could be borderline renal failure if you have a 90 pound female who's got very little muscle mass.

BUN is volatile.

Creatinine is very steady state.

So if I said to you, "Let's just say my BUN is 20 and my creatinine is 1. If I go to sit in the sauna for an hour and I get really dehydrated, my BUN will go to 40 and my creatinine will minimally elevate to 1.2. The BUN is very volatile. And one of the things that we have to look at when we look at BUN and creatinine is where's the problem coming from?

So if the BUN and creatinine are within a normal range ... and remember, the normal range for creatinine is variable based on the patient. So a normal range is based on muscle mass. It's not really based on a patient's age. You can assume that as people get older their muscles atrophy but when you're looking at creatinine and patient's muscle mass, that's when you really need to look at a little better numbers than just the creatinine, like the GFR with a creatinine clearance, and you look at some formulas.

So GFR is really what we use to put them in a category of a stage of renal failure or renal insufficiency, chronic renal insufficiency. So when it comes to someone who's got hepatic functions that are up, you always have to ask one of two questions. Is it hepato-necrotic or cholestatic? You've got to put them in one of those two categories. And we'll cover liver functions on one of the upcoming podcasts here at Talk EM.

But you to put it in one of two categories. It's kind of like if someone's got a white count, you have to put them in a category. Is it infectious response or stress response? Like if you have someone anemic, on our past podcast. You have to ask how fast did they get there. So you have to ask preliminary questions. So when someone's BUN or creatinine is elevated you have to ask this question. "Is it a problem that's occurring before the kidney? Is it a problem with the kidney or is it a problem after the kidney?

So as a PA or nurse practitioner ... it doesn't make a difference if you're a doc ...it doesn't make a difference. When someone's BUN and creatinine are up you have to ask "Where's the problem." Is it pre-renal, renal or post-renal. You always have to ask that question.

So as a good general rule, the BUN normal value is about the same as bicarb, it's about 24. Normal creatinine is about the same value as the total bilirubin or a normal INR, not on Warfarin, so it's about 1. So it's important that you always have a good conceptual idea of what a normal value is for a normal person.

So a normal BUN is about 24, normal creatinine is about 1. So if you have a patient whose BUN and creatinine is elevated, your first thought has got to be, "Where's the problem?" Is it pre-renal, renal, or post-renal? This needs to be cognitively addressed and it's got to be put in your note. Okay?

Now my advice on the approach to the workup. Let's say someone comes in, their BUN is 36 and their creatinine is 1.4. Well what do you do with this? Well, first thing you've got to do is look at their baselines, look at their past history. What's been their BUN and creatinine? Where's their baseline? If you don't do that I think you're negligent. If you don't even ask the question, "What's the role of renal functions," you're completely missing the ball. And if you say that to a physician, "Oh, BUN and creatinine is 36 and 1.6." And they're like, "What's their baseline?" Like, "Oh, I don't know. I didn't even look for it." You forever are going to be labeled in the doc's mind as someone who just doesn't get it. Either you just don't get it or you're really young.

It's a capital offense. It's a capital cognitive mistake. So if BUN and creatinine are up, you have to know what's their baseline. And even if you can't find it ... let's say they've never been to your facility before and you have no access to old records ... well, can you contact their primary doc? Can you contact a hospital that they were at and get a discharge summary? Can you take proactive action to find it?

Too many of us say, "Well, I don't have the information." Did you even look for it or did you take the path of least resistance? But let's just say there's no way to access files. Well you need to lead with that. "Listen, I've got a guy whose BUN is 36, creatinine 1.6, I don't have old labs on him. I tried to access old hospital records. I can't find them so I don't know what his baseline is and he said he's never had a problem with his kidneys before. So, first thing you always got to do is to ask, what's their baseline.

And then the second question's going to be, are they on any nephrotoxic agents. So if I ask you this question right now, "What are the three most common nephrotoxic agents?" The most common iatrogenic nephrotoxic agents, and by far it is ACE inhibitors, NSAIDS and contrast studies.

So when you're looking at someone whose BUN and creatinine are up, your first step has got to be, "What's your baseline?" Second is, are they on a nephrotoxic agent? If they had a contrast study three days ago, okay, that's going to tell you a lot. If they've been taking a lot of Motrin because their rheumatoid arthritis is acting up, well that's going to tell you a lot. If they're on Indomethacin because of gout, tells you a lot. If they're on an ACE or was recently started on an ACE, all of that's going to help you a lot.

Now again, as I said earlier, when you look at someone's BUN and creatinine, if you don't know their baseline, I think you're really naïve and young in medicine, but also, you've got to immediately know those nephrotoxic agents. If you go to a supervising doc and say, let's say you get part one right, "Oh, I got a BUN of 36, creatinine 1.6, and the renal functions were normal six months ago. I checked it." And you're like, "Whoo! I remembered that. Whoo! I got it."

And the doc's like, "Well, are they on Motrin? Have they been taking NSAIDS or are they on the ACE inhibitor?" And you're like, "Uh." If you go back to the chart you look like an idiot again because everybody knows those are so nephrotoxic, you've got to start there. So the two primary questions when there's an increased BUN and creatinine is, What's their baseline and are they on one of the preliminary nephrotoxic agents? Cool.

So once you've eliminated them or weighed them in, at least, then you have is a pre-renal, renal or post-renal. My advice is always look at post-renal obstructive uropathy. Always go there. Okay? When you go there it's pretty easy to rule it out. Because what is the most common post-renal obstructive uropathy? By far it's the prostate. By far it's the prostate.

A big prostate is going to put him into urinary retention and that can put him into renal failure. So you have to eliminate the prostate right away. I've got to say it's got to be 49 out of 50 cases.

Now, what about a kidney stone? Could a kidney stone give you a post-obstructive uropathy? And the answer is, not if you've got two good kidneys. Okay, if you have a fully obstructed kidney stone on the right, 100% obstructive, there is a hydroureter, hydronephrosis. If you have 100% blockage of the right kidney, how much is that going to affect renal function? The answer is it shouldn't. It shouldn't. The other kidney should compensate.

And just so you guys know, as a side note, I was always really bent out of shape when a CT scan came back and said hydroureter, hydronephrosis. Because I'm like, "Oh my gosh, we've got to get that stone out of there because that's going to damage the kidney." Like, this becomes an emergency to me, an urgency to me. And the bottom line is, that stone, based on K-9 studies, could stay there two weeks. And as long as the other side is good, they're fine. Don't worry about it. So there's not an emergency if there's hydroureter, hydronephrosis. You've got a lot of time there. That was a real misunderstanding that I had.

So, that bags the question then, how do you take that off the table? How do you rule out post-obstructive uropathy? Well, you've got to think. Do they have renal obstruction? So you do a rectal exam and make sure they don't have a big prostate. You ask them, "Are you peeing well? Do you feel like you're emptying your bladder? You could always do a bladder scan or when in doubt, put a Foley catheter in. If the Foley catheter gets 300 cc's of urine out, it ain't the prostate. If you get 1600 cc's out you know, okay, you've got a urological problem that needs to be addressed.

So, the first step has always got to be rule out post-obstructive uropathy. Now once you've ruled that out and taken that off the table then you've got to go, "Is it a pre-renal problem or a renal problem? The next thing you do, is you've got to look at the ratio of BUN to creatinine, BUN to creatinine. Remember, normal BUN is about 24, normal creatinine is about 1. Now if those values are normal, I don't have to look at the ratio. The ratio's irrelevant. But if either one of those are high, I want to look at the ratio.

So let me go back to that scenario of my BUN of being 24, my creatinine of 1, and let's say I go in a sauna. So the ratio, is it greater than 20:1 or less than 20:1? That's the magic number. That's what everybody's going to talk about. If the ratio is greater than 20 that suggests blood is not appropriately getting to the kidneys. Well what's the most common cause of that? They have a pre-renal elevation of the BUN, also known as a pre-renal azotemia.

So if someone says they've got an azotemia, well what does that mean? It means their BUN is high. That's all. It's not a magic thing. If they have a pre-renal azotemia it means they're not getting enough blood to their kidneys. Well why wouldn't that happen? By far, the most common is, they're dehydrated.

What's the most common cause of a pre-renal azotemia? They're dry. They're dehydrated. Super common! What's the second most common? Causes CHF. If their heart ain't beating right, if their heart is decreased in the rejection fraction or the cardiac output, then guess what? They're going to have an elevated BUN.

Now, question for you. Question for you. If I went into a sauna and my BUN was 24 and my creatinine was 1 ... so I go into the sauna for an hour and I take 80 of Lasix. I'm going to pause for a second. I'm going to ask you this question. What would you expect my BUN and creatinine to do? So I go in 24 and 1, I sit in there an hour and I'm a sweaty dude, and I take 80 Lasix, and I pee out a liter. What do you expect my BUN and creatinine to do? How would those numbers change? Think about it and let me know.

Well answer is, BUN is going to go to 40, creatinine is going to go to 1.4. The ratio is much higher than 20. That's what dehydration will do. It'll give you that BUN and creatinine of much greater than 20. Super [inaudible 00:14:32]. How do you do it? Well, replace fluid.

Now let's change gears for a second. Now let's say I'm going into that sauna and yeah, I took my 80 of Lasix but I also took 800 of Ibuprofen, I started myself on some Lisinopril, an ACE inhibitor, and I just had a contrast study. So now I go in at 24 and 1, now I come out and my BUN is 40, my creatinine is 4. My ratio is much less than 20 so I have a kidney hit. Okay? So when you a renal problem that elevates your BUN and creatinine, you're going to clearly look at your nephrotoxic agents, you're going to give them a lot of IV fluids and you've got to talk to Nephrology at this point because it's kind of a big deal.

And I will tell you this, that an ultrasound is to the kidney what an EKG is to a cardiologist. So if you're going to workup someone for a renal problem, you really need to do an ultrasound. Order that right away, get that in. And ultrasounds are kind of like that with the reproductive organs. If anybody's got a testicle problem you ultrasound them. Don't pass go, don't collect $200.00. If anybody's got what you think is a gynecological ovarian problem, ultrasound them.

And I'll take it one step further. If you have any question about someone's leg, ultrasound it. If someone comes in with a cellulitis of one leg, ultrasound it. Make sure there's not a DVT under there because it's a lethal problem. Colleagues, I just want to urge you to have a low threshold to pull the trigger on ultrasounds. That's my advice. So with kidney problems, reproductive organ problems and legs, just have a very low threshold to order it. That's a good standard of care.

So BUN and creatinine, step one, look at old values. Step two, look at nephrotoxic agents. Step three is, rule out post-obstructive uropathy first. When in doubt put a Foley catheter in. I remember being a hospitalist and they called me and said, "John, come admit this guy with acute renal failure. Nephrology has already been consulted." So I go down and see the guy and as a hospitalist I had a Foley catheter put in. He had the 1600 cc's out, and guess what, we didn't need Nephrology. We needed Urology. It was a Urological consult not a Nephrology consult. Okay?

So if someone's got increased VUN and creatinine, we want to try to perfuse the kidney. And that's going to be our preliminary work, so we want to give them IV fluids. Now what's the limiting factor of IV fluids? Can their heart take it? That's all. So when we want to hydrate somebody we just got to make sure that their heart can handle it and it doesn't put them into failure. So you need to weigh that in clinically. Do you know what their ejection fraction is? Are they on medicines for CHF?

Guys, a 250 cc bolus could put someone into failure. And is that the end of the world? No. You'd rather not but as long as you're monitoring the patient, are they sounding wet? Are they tachypneic? Are they hypoxic? You've got to monitor that. And last thing I'll say is, remember Dopamine. Remember Dopamine has three different dosing structures with three different physiologic phenomenon. And low dose Dopamine is considered renal dose Dopamine, that if you put someone on low dose Dopamine it's supposed to dilate their renal arteries, perfuse the kidneys a little bit better and if someone's got an elevated BUN-creatinine and it's a problem with the kidneys, a good question is to say, "Hey, do you like renal dose Dopamine here, which is just a really low dose of Dopamine?"

And you'll get some conflicting answers. So I don't ever pull the trigger on renal dose Dopamine by myself without talking to somebody else. It's kind of like adding bicarb to an IV solution in someone who you're concerned about rhabdomyolysis. There's a little bit of controversy there, so I don't pull the trigger on that without talking to someone smarter than me or a higher authority than me. But it shows an insightful question. Okay?

So, in conclusion with the renal functions. You've got to know how to work with these. And my hope is that this podcast gave you real measurable tools to assess BUN and creatinine. Now apply it. Apply it clinically.

and the good thing about medicine is, guys, see one, do one, teach one. If you really want to learn about BUN and creatinine, teach this to somebody else if you can. Find a way to, "Hey, can I teach you something? Can I show you something?" My experience is when I first started working as a hospitalist and I started thinking there are super sick people and running codes and having near arrests ... well the bottom line is, I'm like, "I want to get good at this stuff."

So the best way to get really good at something is to change your paradigm into becoming a teacher. So I'm like, "Well, I'm going to become an ACLS instructor." So I became an ACLS instructor and the better I became as an instructor, the better I was able to manage critically ill patients.

So when you can turn your mind into that of a teacher, let me show you how to do this, you learn at a much higher level. So I'm going to challenge you to do that with the BUN and creatinine. So good luck, God bless and I will see you in the next podcast. And once again, let me encourage you just to check out the website, CME4Life.com. Look at their podcasts. If you're looking for educational programs, there's tremendous amount of educational programs there. The podcasts hopefully will be helpful to you. There are blogs on that website. They have two programs that are a secret weapon.

If you're a PA, you need category one CME, they have two performance improvement programs. One is on medical malpractice defense and one is on patient satisfaction. To do the programs will take you a couple hours over a three month period and it's good for 40 category one CME hours. It is ridiculous how many category one CME hours you can get. For nurse practitioner's it's 20, still a ridiculous amount of hours. And CME [inaudible 00:20:13] self-assessment programs. Very, very little input of your effort to get tremendous credits.

So you do these two home tests. It's good for, if I remember correctly, 30 category one CME hours, because the NCCPA gives you 1.5 times the hours with self-assessment and 2 times the hours with performance improvement.

Good luck. God bless. I'll see you on the next episode, where next episode we are going to talk about liver functions.

If you need to learn EKGs... we have a secret weapon for you right here.

Or other secrets of EM education, urgent care or critical care visit www.CME4Life.com