GFR

Now that I am in the radiology department I spend a lot of time focusing on GFR and kidney function. Why? Good question!

In MRI and CT we give contrast to a lot of patients. In CT the contrast is iodine based. In MRI the contrast is gadolinium (metal) based. Both types of contrasts are filtered out through the kidneys and thus the reason kidney function is so important in this department. The way we assess kidney function is by checking a patient’s creatinine level in their blood. Luckily for us we have machine called the i-Stat that can test the blood and give a result in two minutes. The result transfers into Cerner (our EMR) and the computer then uses that result to calculate the GFR. Great… except I didn’t really have an understanding of why we were checking the creatinine, what GFR really was, or why there is a GFR result for African Americans and non-African Americans. I decided to do a little reasearch and I figured, since this is a nursing blog and all, why don’t I share what I have learned?

What is “GFR”?

GFR stands for glomerular filtration rate. Basically, the GFR tells you the flow rate of fluids through the kidney. Your glomeruli are the capillaries in your nephrons inside the kidney. Blood is filtered across the capillary membranes helping to remove waste that can ultimately be excreted through the urine. Taking you back to anatomy and physiology in nursing school aren’t I? *shudders*

A simple google search will bring up lots of GFR calculators. Typically the GFR calculator takes into account serum creatinine, age, gender, and race (African American versus not) and then it will give you the estimated GFR. A GFR >60 indicates a generally healthy kidney. Less than 60 can indicate potential kidney disease. Less than 15 can indicate full on failure. Here is a little infographic that is patient centered.

Why creatinine?

Why does the GFR equation use creatinine? In the most basic terms, creatinine is a waste product of creatine. Creatine is used by the muscle cells for energy. Your kidneys help filter the creatinine out of the blood to be excreted in the urine. Low creatinine typically indicates good kidney function (which makes sense, healthy kidneys will filter out creatinine effectively). High creatinine indicates the opposite, kidney function is probably on the lower end because the kidneys are unable to filter out the waste product. Creatinine is primarily filtered out through the kidneys which is why it is a pretty good indicator of kidney function.

Why is the result different based on race?

Many, many times I have looked at my labs and wondered why the GFR had a result for African Americans and then essentially everyone else. It wasn’t until I started working here and paying attention to the GFR that I decided to look it up. Turns out studies show we have “higher than average” muscle mass so we generate higher levels of creatinine. Higher creatinine levels lead to higher filtration rates. The difference in results account for this.

Now I can actually explain to my patients why I am taking blood after I start an IV. I like to be able to asnwer my patient’s questions so of course I had to do a little learning on my end. Hopefully some of you will also find this information useful! (Also here is a great reference for frequently asked questions from the National Kidney Foundation because, why not!)

Resident-splaining

One thing that absolutely drives me nuts is having a new resident come to the trauma unit, that I have worked on for almost three years, and “resident-splain” something obvious to me!

What is resident-splaining? It’s when a resident condescendingly “explains” something to you that they assume you know nothing about because you’re just a nurse…

I’ve had a resident (not a very good one at that) start to “explain” calcium in the blood to me. Why? Well, we had given quite a lot of blood products and I asked about giving some calcium as the ABG showed the ionized calcium was low. This is common. Massive infusions almost always drop the serum calcium due to the citrate used in the unit of blood (if this is new to you, here is an article that explains it rather well). Like I said, I know this. Trauma nurses are typically very aware of this because, you know, we give a lot of blood. Trauma… Bleeding… But hey, I’m just a nurse.

Now, she’s not giving me the calcium I need. She starts explaining calcium in the blood and why I should go by the ionized calcium instead of the calcium level on his BMP. Remember, I told her the ionized calcium on the ABG was low… Ionized. Calcium. The level she is currently explaining to me. That level. That’s not enough, she’s not even looking at me while she is talking and it’s in a very condescending tone.

Bruh.

I finally stop her with this statement: “I’m well aware of the purpose of an ionized calcium which is why I told you what it was on the ABG that I just ran (can you hear the attitude in my voice?). I don’t need an explanation, I need calcium. Can you order that or did you need me to throw that order in real quick?” Her:

*blank stare* “Oh, yeah I can put that in for you…” *quickly and quietly begins ordering what I need*

I had no more issues with her for the duration of her rotation on our unit.

It’s irritating. So so irritating. I’m far too outspoken to have someone resident-splain things to me. Don’t try me buddy…

Look at your labs

You have drawn blood, or phlebotomy has come and drawn blood for you. Great. Labs are sent and you go on about your shift. The lab doesn’t call you with any critical values so you figure things must be ok.

And then your patient starts to have more ectopy. His rhythm has definitely changed. Or maybe her blood pressure seems to be lower than normal. Maybe he is more altered than he was.

What’s going on?! This doesn’t make sense. Then you look at your labs… Four hours later…

The values weren’t critical but they are abnormal for your patient. His potassium is 2.9. Her H/H is 7/21, a full three points lower than it was on her last set. His glucose is 61 and he normally runs in the 200’s.

Oh. Ohhhhhh…. Well, shit.

That is four hours that your patient has had abnormal but totally treatable lab values. That is four hours of treatment that your patient did not receive. That is four hours too long. When you send labs or have labs sent for you, you need to remember to check the results. Do not assume that the lab will call you if something is wrong. The lab is responsible for calling when the values are critical or dramatically different than the previous set. However, for some patients, it does not take a critical value for them to exhibit changes. Each person is different, while a hemoglobin and hematocrit of 7/21 may be totally fine for one patient it may be too low for another. One patient may function totally fine with a glucose of 61 while someone that lives higher may exhibit altered mental status. This patient may show no signs at all that their potassium is 2.9 while that patient begins to throw all the ectopy EVER whenever their potassium is less than 3.3. Each patient is an individual and should be treated as such.

Your patient and their labs are your responsibility. Take the minute to give them a check, that minute could save a life.