What does a low glomerular filtration rate mean
May 31, 2012 12:16 AM   Subscribe

Medical professionals, please tell me the possible significance of my glomerular filtration rate

Just had a physical and my glomerular filtration rate (GFR) was 71, but the reference range says the normal value is greater than 90. What does this result mean? How worried should I be and what should I do to follow up? I'm only 35, normal weight, and relatively healthy.

My BUN is 16.0 and my creatinine is 0.9. All other metabolic tests are within the normal range as well. Why would my GFR be off when the rest of my tests are normal?

I have noticed some possible swelling and tenderness in my legs (VERY MILD) as well as a sense that my skin can dent easily, which seemed a little strange to me. But it is so mild that if I pointed it out to someone they would probably not notice. Also, very dry eyes. I'm not sure if this could be related.

Of course I plan to follow up with my GP. But can you help me interpret these results? Thanks in advance.
posted by davisnot to Health & Fitness (7 answers total) 2 users marked this as a favorite
The blood test GFR number is really eGFR - estimated glomerular filtration rate. It's calculated from your creatinine. Creatinine can be raised by heavy exercise. BUN can be raised by high protein consumption. A BUN of 16 mg/dL and creatinine of 0.9 mg/dL is still within the "normal" range, though with the symptoms you mention, it could potentially indicate the beginnings of kidney disease.

When you follow up with your GP and you show concern about your lab results and mention your symptoms, he'll probably want to do a 24-hour urine collection. That will determine your true creatinine clearance (a better measure of GFR) and other signs of kidney problems like protein or minute traces of blood.

IANAD, but have you noticed any change in how much you urinate, either more or less? Or has your urine been more foamy in the toilet? Those are things to mention to your doctor.

Good luck. Kidney failure is a terrible thing to be facing, though you may be a long way off yet if you're headed down that road.
posted by WasabiFlux at 1:12 AM on May 31, 2012

First, this is not medical advice. I am just a lady on the Internet who knows a little bit about this, based on both personal experience and on reading a bunch of articles, so I am not the best source of advice. Your doctor will definitely have better information for you than I.

Glomerular filtration rate is difficult to measure, so the number you have is an estimate of your GFR. Getting an accurate measurement is difficult, costly, and time consuming. Because of this, doctors use estimated GFR. Sometimes these estimates can underestimate numbers for people whose GFR is above 60. The formula that your lab used to estimate your GFR does not take your body mass index into account, which is an important factor. This means that your GFR could be under or overestimated. Just something to keep in mind.

Basically, GFR measures how much fluid flows through your kidneys. Lower numbers mean that your kidney function may be impaired. That's why, if you have googled GFR, you are probably getting a lot of information about kidney disease.

From what I understand, a GFR above 60 is usually considered okay for most adults, if there are no other signs of kidney damage. However, if you have other signs of kidney damage/disease, it is really an important thing to catch early. A lot of kidney care can be preventive, causing you to not need to do more intensive and difficult treatments in the future. Interestingly, the BUN and creatinine don't actually start elevating until you've lost more than something like 50% of your kidney function, so GFR is a more commonly used measurement for catching kidney disease early.

Please bring your other symptoms up to your doctor when you talk. I am really glad to hear that you plan to follow up with your GP. If you don't get a satisfactory answer from them, please get a second opinion. If you have copies of old tests, I would also bring those in when you talk to your doctor.

To give you an idea of what to expect, your doctor will probably want to do a simple urine test. They may also want to do a more complicated urine test that involves collecting all of your urine for 24 hours. I'm not sure how common this is anymore. Basically, they want to see how much protein is in your urine. The longer test assesses how much creatinine your body is actually passing, giving them a better idea of your GFR.
posted by k8lin at 1:14 AM on May 31, 2012 [2 favorites]

I just want to add that if you catch kidney disease early, it can really change the course of the disease and treatment. Catching it early may allow you to entirely prevent having to do things like dialysis. Kidney disease doesn't have to be terrible; in fact, a lot of people have mildly impaired kidney function that never progresses to complete kidney failure.

13% of American adults have elevated GFRs [1]. Not all of these individuals progress to Stage V kidney disease, which is when replacement therapy options (dialysis or transplant) need to happen. I'm not trying to say that end-stage renal disease or chronic kidney disease aren't terrible, by any means; I just want to be clear that an elevated GFR on a blood test does not automatically mean that you are going to need to go on dialysis.

I say this because going to the doctor early is really important. Don't avoid it because it seems scary. I know, it's not the best. Pat yourself on the back afterwards for doing something un-fun and stressful but that is really good for you, and then take yourself out for a treat. Ice cream or something.

[1] Coresh, J., Selvin, E., Stevens, L. A., Manzi, J., Kusek, J. W., Eggers, P., Van Lente, F., et al. (2007). Prevalence of chronic kidney disease in the United States. JAMA: the journal of the American Medical Association, 298(17), 2038. (I'm really sorry, but this is likely behind a paywall.)
posted by k8lin at 1:37 AM on May 31, 2012 [1 favorite]

Hey, it's also entirely possible that the test is inaccurate. Talk to your GP, wait for followup tests if that's what they suggest. All tests have a false positive rate, and if you're otherwise healthy and have no symptoms a bad test result is in many cases more likely to be a false positive than an accurate reading (I don't know about GFR and kidney disease specifically).

I'm not saying "don't follow up on this" but I am saying "wait until you know more before you start freaking out" :)

And, as k8lin says, even if there is actually something wrong, it may be very manageable. (And that article is actually free!).
posted by mskyle at 6:35 AM on May 31, 2012 [1 favorite]

I just recently, last month in fact, dealt with a very similar situation, and I think sharing my own experience might help. Possible TMI follows!

I had a serious reaction after my doctor tried me on Crestor for my cholesterol. After only a week, I ached so badly I could hardly walk. Frighteningly, my urine also turned dark. I stopped taking the Crestor immediately and called my doctor, at which point I was sent to the hospital for a battery of tests to check for kidney and muscle damage.

The most important lesson I have learned is to get copies of all my own medical records to track my own test results because communication between doctors and patients is, surprisingly often, sadly lacking.***

Anyway, during the testing, My eGFt came up as 58, which as you know from your Googling is REALLY low. If it is under 60, the National Kidney Foundation suggests hydrating the patient immediately, follow-up monitoring, etc.

My old doctor did none of this, and shared no information from the tests with me. I was put on Cipro for a kidney infection. When it didn't help, I called the office, and they told me all my test results were "fine". So I asked why they put me on Cipro in the first place, and they stonewalled me.

It was incredibly frustrating, and eventually resulted in my changing doctors.

But my NEW doctor is great! And he ordered the wonderful (that is very much a sarcastic hamburger wonderful) 24-hour Urine test.

I went to the lab, and they gave me a big orange container and a plastic tray. I had to collect my urine in the plastic tray every time I went to the bathroom, pour it into the big orange container, and then bring the urine-filled container back to the lab 24 hours later. Yay!

Oh, and did I mention this all happened on my 46th birthday? Yeah.

It wasn't really that bad, honestly. I was embarrassed, but the lab does this stuff all the time, and the Pee Jug, as I called it, was in a big brown shopping bag they thoughtfully provided, so no one else knew what I was up to. Once I got past the, "I can't believe I have to carry my urine around!" moment, I was good to go.

And the test motivated me to give my bathroom a really thorough cleaning, because ick. Oh, and I washed my hands like whoa. And got all OCD about the Pee Jug, too, because GOD FORBID that lab thought I was careless with my urine!

So, 24 hours later, I return my big orange Pee Jug in its shopping bag to the lab, feeling like I'd just made a trip to the Bodily Waste Shopping Mall ("Yes, I'll take a couple cups of urine, please. And make that to go!").

They took a little blood from my arm, and that was it.

[The end result for me showed my "creatinine clearance" rate was much higher than the eGFt, thankfully, so I just need to go back in 3 months for follow-up testing.]

I went through all this with you because I DO think you should bring up your eGFT with your doctor, as the swelling you are experiencing sounds like non-pitting edema. The "non-pitting" is like you described, where dents you make don't disappear right away, and it could be an indication there's something wonky going on.

So you might be dealing with a minor kidney issue, and it is best to be sure about these things, for your own peace of mind.

And if you do have to go through that 24-hour urine test, now you know what to expect!

***Worst Case medical Communication Scenario Example (hypochondriacs, stop reading NOW):

A local woman had some blood tests done, and sadly the results were Very Bad. The nurse called to tell her, got the answering machine and chose not to leave a message because of the sensitive nature of the diagnosis. But then, the nurse didn't indicate on the chart that she never reached the patient.

The blood test results are forwarded to a specialist, at which point the specialist's office dutifully places a call to the woman's home.

So the first indication this woman had that something was wrong was when she answered the phone that morning and a voice on the other end said, "Hi, I'm calling to set up your oncology appointment!"

Hell of a way to learn you have cancer.
posted by misha at 9:48 AM on May 31, 2012 [1 favorite]

k8lin gives very good advice. Also, perhaps you were a bit dehydrated when you took the test. Things like hydration and protein intake can also affect this value.
posted by mochapickle at 12:05 PM on May 31, 2012

I went through all this with you because I DO think you should bring up your eGFT with your doctor, as the swelling you are experiencing sounds like non-pitting edema. The "non-pitting" is like you described, where dents you make don't disappear right away, and it could be an indication there's something wonky going on.

Other way round - what he's describing (making dents in the skin that remain after you move your finger away) sounds like pitting oedema, which to me is more concerning than an isolated eGFR drop (which is not hugely abnormal either).

Q: If you just had an annual physical, i assume they did your cholesterol and sugars. Did you fast before the blood test? If you were a little dehydrated that could account for it. When blood volume drops, the kidneys cut back a little bit so you don't waste so much water.
posted by chiquitita at 6:05 AM on June 1, 2012

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