Being Kind to Your Kidneys: Kidney Function Tests

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A big part of “being kind” to your kidneys is taking charge early on. In discussing diabetes, we often talk about “knowing your numbers,” such as your A1C and blood pressure. These are important numbers, of course, but what’s often overlooked is your kidney numbers. These numbers may or may not be something that your doctor talks to you about — if you don’t know about your kidney test results, ask! In the meantime, here’s a rundown of some common tests, often called kidney function tests, that you should be familiar with.

Microalbumin test
The microalbumin test is a urine test that checks for very small (micro) amounts of protein called albumin in your urine. The microalbumin test may also be called a:

• Urine albumin test
• Albumin-to-creatinine ratio
• Microalbumin-to-creatinine ratio

You may be asked to give a random urine sample or a timed urine sample (such as overnight), or you may be asked to collect your urine over a 24-hour period. (A microalbumin test is not the same as a routine urine dipstick test, by the way). It’s recommended that everyone with diabetes have a microalbumin test done at least once a year. Have you had yours yet?

Protein in the urine can signify several things, including kidney damage from uncontrolled diabetes, high blood pressure, liver disease, heart failure, and lupus. It can also occur due to strenuous exercise, blood in the urine, urinary tract infections, and certain medications, like aspirin and some antibiotics.

The goal is a microalbumin level of less than 30 milligrams (mg). If the result is 30 or higher, you will need to have the test repeated 2–3 more times over the next 3–6 months. A reading of 30 to 299 mg (called microalbuminuria) may indicate early kidney disease. A reading of 300 mg or more (called proteinuria) signifies more advanced kidney disease. The presence of protein in the urine can also indicate an increased risk for heart disease.

Estimated glomerular filtration rate (eGFR)
While the microalbumin test can indicate the presence of kidney damage, another test, called the eGFR, tells you how well your kidneys are working. Your doctor actually calculates your eGFR by measuring the level of a substance known as creatinine in your blood and then plugging that result into an equation. Basically, the eGFR measures how well your kidneys are filtering creatinine out of your blood.

The eGFR should be checked at least once a year. You may need it checked more often if you have signs of early kidney damage or if you have kidney disease. Healthy kidneys usually filter 100 milliliters per minute (ml/min). An eGFR lower than 60 ml/min suggests some kidney damage. Less than 15 ml/min indicates kidney failure.

The eGFR is just an estimate, however, and it can be affected by things like age (it’s not valid for those under the age of 18 or over the age of 75), muscle mass, malnutrition, obesity, and certain medications. But, it’s a helpful tool to give your doctor a better sense of what’s going on with your kidneys. Chances are, your doctor (or the lab) calculates your eGFR, but you may not have been told your result. So, the next time you see your doctor, ask and make sure it’s been done.

Creatinine is a waste product that forms when muscle tissue breaks down. The kidneys filter out creatinine and excrete it in the urine. When kidneys aren’t working as well as they should, creatinine can build up in the blood. A blood or serum creatinine test can tell you how well your kidneys are working: To do this, your doctor will plug your creatinine result into an equation to determine the eGFR, as I mentioned above.

Your doctor will likely check your blood creatinine level at least once a year, and more often if you have kidney disease or high blood pressure. Sometimes, your doctor will check your urine creatinine level, as well to determine the extent of kidney damage.

Normal creatinine results range from 0.6 to 1.3 milligrams per deciliter (mg/dl), but the reference range can vary from lab to lab, between men and women, and by age. A creatinine above the reference range can indicate kidney damage, but the level may be high due to a high meat intake, dehydration, or creatine supplements.

Blood urea nitrogen (BUN)
The blood urea nitrogen (BUN) test measures the amount of nitrogen in the blood that’s formed from urea, a by-product of protein breakdown made in the liver. The kidneys excrete BUN in the urine. The BUN is yet another test that tells how your kidneys are working. A high BUN can mean that the kidneys aren’t getting rid of urea and therefore aren’t doing their job. But high levels may also be due to heart failure, dehydration, fever, and a high-protein diet. Low BUN levels can occur during pregnancy or with liver disease or damage.

It’s common for BUN to be measured along with creatinine to determine the BUN-to-creatinine ratio. Normal BUN ranges for men are usually 8–24 mg/dl and 6–21 mg/dl for women. A normal BUN-to-creatinine ratio is usually 10:1 to 20:1. High BUN-to-creatinine ratios may result from sudden kidney failure, shock, severe dehydration, kidney stones, or bleeding in the digestive tract.

The two key kidney tests that you should make sure you get done every year are the microalbumin test and the eGFR. Find out if they’ve been done, what the results are, and what it means if your results aren’t within the target range.

More next week!

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