Glomerular Filtration Rate (GFR)

In this lesson we're going to take a look at glomerular filtration rate.

The first thing I'm sure you're asking is what the heck is glomerular filtration rate? Well it's a lab test, and we use it to look at kidney dysfunction. We're going to get into the pathophysiology of it in just a minute, but the thing you have to understand is that it is a rate and it's based on time. So we basically want to make sure that the glomerulus, an important part of the kidney, is filtering out in 90 to 120 ml per minute based on a body mass of 1.73 meters squared. I know this is a fancy rate, but the important part is that you actually look at the number part first. So 90 to 120. Again it's an indication of kidney dysfunction, it can also be a predictor of future kidney dysfunction.

Things that influences our age, gender and even race.

So let's get into the patho of why we look at this. Well as you can see here you have blood supply to the kidneys, and the blood filtration happens in this area called the glomerulus. the glomerulus is responsible for the first stage of filtration of blood and getting waste products out. If there's damage to the glomeruli, you have this decrease in glomerular filtration. This means that there's actually an increase in a waste product in blood. The reason we look at the glomerular filtration rate, or GFR, is because the glomeruli are the first stage in filtration and getting out waste products. If we can see that there's damage or injury to this area of the kidney we can actually make future assessments about kidney injury or disease.

So how are we going to send this lab off?

Well it's done by serum, so we can either send a green, a red top tube or lavender top tube to the lab.

Commonly it's going to be sent with other labs, I'm so things like your renal labs or other chemistries.

The other thing that you may want to consider doing especially if your patient is suspicious of kidney injury, is to talk about doing a creatinine clearance at the same time. We can use both GFR and creatinine clearance to determine how well the kidneys are working. Just remember that would creatinine clearance you need a 24-hour urine, so you want to disregard that first sample of urine and then start collecting every sample thereafter for the next 24 hours.

So what is a normal GFR look like?

So like we talked about, it is based on time. Anything between 90 and 120 seconds is considered a normal GFR. The only real time that you're going to have an increased GFR, and this is really rare, is it you have some sort of a long term kidney damage, and basically your kidneys are filtering out a ton of urine, but the problem is is that you're spilling out albumin, which is a really important protein. Again that's rare, but as long as your patients albumin and protein levels are normal, and your GFR is maybe slightly elevated, then that's going to be considered a normal value. The thing you want to look at with increased GFR is if your patient has any other kidney biomarkers, like BUN or creatinine, that go up or if your patient is losing protein.

We pay more attention to a decreased GFR. Another important thing about GFR is that depending on what rate the glomerulus is filtering, it helps us to tell what percentage of the kidneys are actually working. So anything 90 mL per minute is pretty much normal function. A good rule of thumb is that the number in mils per minute, is equivalent to the predicted or estimated amount of functioning kidney. So use this as an example. So let's say you have a patient whose GFR 60, well you can predict about 60% of the kidney function is normal. So patient with a GFR of 15, you can estimate that about 15% of the kidney function is working. Now I know that that's a really broad rule of thumb, but make sure you're talking to your providers about what their goals are and what they consider as normal GFR rates for patients depending on how chronic their diseases.

Causes for decreased GFR or going to be injury to the glomerulus, you can actually have damage to the nephrons and the glomeruli, so something like an acute kidney injury, or you can have long-term kidney damage like chronic kidney disease. Also toxicity is going to decrease your GFR as well as diabetes and potential hypertension, because of the amount of injury that's occurring specifically to the kidneys.

We were talking about GFR, we're looking at our lab values in our eliminations for our nursing concept.
So let's recap.

Normal values for your GFR are 90 to 120 mL per minute. Again that's based on factors like age, body size, gender, and even race.

GFR is a specific value that we look at what we're looking at the kidneys. Nothing else is going to influence it.

Increased GFR is going to be normal in most circumstances, but just take a look at your albumin.

A decreased GFR is going to help you figure out how much damage is happening in the kidneys. The lower the rate, the worse the kidney damage.

And lastly, GFR is an essential renal value. Just like with BUN and creatinine, this is going to take a look at the kidneys.

That's it for our lesson on GFR.Make sure you check out all the resources attached to this lesson. Now, go out and be your best selves today. And, as always, happy nursing!!