Risk Factors and Signs of Kidney Disease

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Session 37

What do this patient’s decreased creatinine clearance and oliguria tell you? What risk factors are behind his condition? Would you give him a contrast dye?

As always, we’re joined by Dr. Mike Natter from BoardVitals, as we cover the renal system. The kidney physiology is probably the most complicated of all the physiologies that you’ve got to be pretty nerdy to understand what’s going on.

If you need some help with your Level 1 or Step 1 exams, check out BoardVitals. Use the promo code BOARDROUNDS to save 15% off your purchase.

Listen to this podcast episode with the player above, or keep reading for the highlights and takeaway points.

[03:08] Question of the Week

A 65-year-old gentleman who is a smoker with a past medical history of hypertension, diabetes mellitus, and he presents with chest pain. After an initial evaluation, he’s taken for an emergency coronary angiogram.

Three days later, he developed an elevated serum creatinine, polyuria, and hyperkalemia. His lab findings are as follows:


Plasma concentration: 2.0

Urine concentration: 200


Serum or plasma concentration: 80

Urine concentration: 0


Plasma concentration: 5

Urine concentration: 50

Uric acid

Plasma concentration: 3

Urine concentration: 15


Plasma concentration: 0.2

Urine concentration: 100

The question is as follows:

Assuming a urine flow rate of 1mL/min, what is this patient’s filtration fraction?

(A) 10%

(B) 20%

(C) 30%

(D) 40%

(E) 50%

[05:10] The Big Picture

From a clinical perspective, this is not particularly one-to-one relevant to what we do in the hospital or the clinic. That being said, there is a significance to understanding how we get to some things. Oftentimes, this is already calculated in the hospital. But this question is for you to understand what it all means.

At the end of the day, the question is about math. You have to understand what formula needs to be applied. Once you know it, the question becomes quite simple.

When you talk about a 65-year-old guy who has renal dysfunction and goes for a coronary angiogram, we shoot some contrast dye into the coronary arteries to see if there is stenosis for instance. The problem with running a contrast is getting that contrast out of the body because of allergic reactions and how kidneys filter it. So it’s renal toxic.

'When we have a high contrast load, we can do some harm to the kidneys.'Click To Tweet

Hence, you want to reserve those coronary angiograms and similarly any kind of contrast-loading dye studies for people that really need them and those that have any renal dysfunction. So this guy might have taken an additional hit to his kidneys if he already started out with some kidney problems, to begin with.

The two leading causes of end-stage renal disease or any kind of CKD (chronic kidney disease) are hypertension and diabetes. And this guy is set up for some kidney problems potentially.

[07:40] Clinical Relevance

Additionally, the patient developed oliguria, which is characterized by decreased urine output. Hyperkalemia means high in potassium. A potassium level of 5 in the lab result is really not that elevated so maybe it’s elevated from his baseline of maybe 3.5 or 4.

'We do worry about hyperkalemia especially in folks who have kidney dysfunction because hyperkalemia can lead to things like dysrhythmias and cardiac issues.'Click To Tweet

All this stuff is making it more clinically relevant to us. Then we need to dive into some definitions.

[08:24] What is PAH?

PAH stands for para-aminohippuric acid. This boils down to understanding physiology. As you recall, the nephron is super complex with a lot of secretion and absorption. There are only a few ways to measure renal plasma flow, what’s being seen by the nephron, and what’s being put out in the urine.

One way to figure that out is by using PAH because it’s a nice correlate for that renal plasma flow. It’s neither absorbed nor secreted by the nephron. Whatever you put into the plasma, the PAH is just going to be standing on the other end and it’s going to be dumped into the urine. So this is helpful for this math problem.

[09:53] What is a Filtration Fraction?

The question is asking for the filtration fraction. The kidney is generally built for filtering the blood but it’s really the glomerulus and what the nephron is seeing that’s being filtered. What fraction of what the plasma is showing is actually going to be filtered? So it’s asking the fraction of what we’re filtering at the kidney level.

First, think about the glomerular filtration rate (GFR). It’s the rate at which the glomerulus is filtering. Usually, we’re given the measurement, but if not, we can calculate that.

The filtration fraction is essentially equal to the GFR divided by that renal plasma flow. The renal plasma flow is the amount of blood flowing through to the kidneys, or in this case, the nephron. From here, you can do a bit of plugging and chugging.

The GFR wasn’t given in the passage so we have to figure it out on our own. The GFR can also be calculated by the creatinine clearance.

The creatinine clearance is obtained by getting the urine creatinine multiplied by the urine flow rate then divide by the plasma creatinine concentration. 200 x 1 / 2 = 100. So the GFR is equated to the creatinine clearance of 100 mL/min.

The renal plasma flow is equal to the clearance of PAH because PAH is neither secreted nor absorbed. So you get the urine PAH multiplied by the urine flow of 1 then divide that by the plasma PAH concentration. This is what is seen and what’s excreted by the kidney. 100 x 1 / 0.2 = 500 mL/min.

Now, we have the PAH clearance equivalent to the renal plasma flow and now we can plug and chug. Again, the filtration fraction is the GFR divided by the renal plasma flow. 100 / 500 = 20%.

So this all leads us to answer choice B.

[13:50] What If You Don’t Remember the Formulas?

If you don’t know the formulas and what those terms are, it’s impossible to answer this question. Ultimately, all the information is going to be provided by the computer. But you shouldn’t have that mindset of not liking these kinds of questions. You should be looking to learn something even if it’s not fun.

'It's nice to know where we derive what we do on a daily basis to understand.'Click To Tweet

And this is what separates us from other providers. We have to know the deeper definitions and understanding of where things are coming from.

[15:50] BoardVitals

If you’re looking for some more help with your Step 1 or Level 1 prep, go to BoardVitals and see how they can help you prepare for your exams. They have a huge QBank of questions with amazing explanations and answers. Get access to Ask Questions depending on the level of package you get. Use the promo code BOARDROUNDS to save 15% off your purchase.


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