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!)

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