Kidney Function Tests: Creatinine, GFR, and Urinalysis Explained

Why Your Kidneys Need Regular Checkups

Your kidneys work nonstop-filtering about 120 to 150 quarts of blood each day to remove waste and extra fluid. But here’s the problem: you won’t feel anything wrong until they’re already badly damaged. About half your kidney function can be gone before your creatinine levels even start to rise. That’s why simple blood and urine tests are your best early warning system.

Most doctors don’t wait for symptoms. They check kidney health during routine blood work. If you’re over 40, have high blood pressure, diabetes, or a family history of kidney disease, getting these tests yearly isn’t optional-it’s essential. The three key tests? Serum creatinine, eGFR, and urinalysis. Together, they tell you if your kidneys are filtering right-or starting to fail.

What Creatinine Really Tells You

Creatinine is a waste product from your muscles. Every day, your body makes a steady amount. Healthy kidneys filter it out and flush it through urine. When kidney function drops, creatinine builds up in your blood.

But here’s the catch: creatinine alone doesn’t give the full picture. It’s a late signal. You can lose up to 50% of kidney function before your creatinine level climbs above normal. That’s why it’s never used alone. A creatinine level of 1.2 mg/dL might be normal for a muscular man but high for a small woman. Age, sex, and muscle mass all affect it.

For example, a 70-year-old woman with a creatinine of 1.1 mg/dL might have worse kidney function than a 30-year-old man with the same number. That’s why doctors don’t just look at the number-they plug it into a formula to calculate eGFR.

Understanding eGFR: The Real Measure of Kidney Function

eGFR stands for estimated Glomerular Filtration Rate. It’s not measured directly. It’s calculated using your creatinine level, age, sex, and sometimes race. This number tells you how well your kidneys are filtering blood. Think of it as the kidney’s miles per gallon.

The most accurate formula used today is the CKD-EPI equation, adopted by labs worldwide since 2009. It replaced the older MDRD formula because it’s more precise, especially for people with near-normal kidney function.

Here’s what eGFR numbers mean:

• 90 or above: Normal kidney function (even if there’s minor damage)
• 60-89: Mildly reduced function-monitor closely
• 45-59: Moderately reduced-likely early kidney disease
• 30-44: Severely reduced-needs specialist care
• 15-29: Very severe decline-preparation for dialysis may begin
• Below 15: Kidney failure-dialysis or transplant needed

But eGFR isn’t perfect. If you’re very muscular, very overweight, pregnant, or under 18, the calculation can be off. In those cases, doctors may use cystatin C, another blood marker that’s less affected by muscle mass. It’s not routine yet, but it’s becoming more common for tricky cases.

Urinalysis: The Silent Sign of Kidney Trouble

One of the earliest signs of kidney damage? Protein leaking into your urine. Healthy kidneys keep protein in your blood. When the filters break down, albumin-a type of protein-starts slipping through.

That’s why urinalysis matters more than you think. A simple dipstick test used to be common, but it’s unreliable. Today, the gold standard is the Albumin-to-Creatinine Ratio (ACR). It measures how much albumin is in your urine compared to creatinine, giving a precise number.

Here’s what ACR levels mean:

• Below 3 mg/mmol: Normal
• 3 to 70 mg/mmol: Microalbuminuria-early kidney damage
• Over 70 mg/mmol: Overt proteinuria-significant kidney injury

If your ACR is borderline, your doctor will ask for a second test, usually with your first morning urine. One high result isn’t enough. Kidney damage shows up over time. Two out of three elevated tests over three months confirm the problem.

Even if your eGFR is normal, an abnormal ACR means your kidneys are already stressed. This is why both tests are needed. One checks filtering speed. The other checks filter quality.

When and How Often Should You Get Tested?

You don’t need to wait for symptoms. The CDC and UK Kidney Association both recommend testing for anyone with risk factors:

• Diabetes (type 1 or 2)
• High blood pressure
• Heart disease
• Obesity
• Family history of kidney failure
• Previous acute kidney injury

If you have one or more of these, get an eGFR and ACR test at least once a year. If you’ve already been diagnosed with early kidney disease, your doctor may want tests every 3 to 6 months.

Even if you’re healthy and under 40, consider asking for these tests during your next physical. Many people don’t realize they’re at risk until it’s too late. A 2023 study found that nearly 1 in 3 adults with early kidney damage had no symptoms-and weren’t being monitored.

What About 24-Hour Urine Tests?

You might hear about 24-hour urine collections. These are used in specific cases-not for routine screening. Your doctor might order one if:

• ACR results are unclear
• You’re losing large amounts of protein
• They need to measure other substances like sodium, potassium, or oxalate

The process is simple but annoying: you collect every drop of urine for 24 hours in a special container. You start by emptying your bladder in the morning and saving that sample. Then you collect everything for the next 24 hours, including the first urine the next morning.

It’s not fun, but it gives a complete picture of what’s being flushed out over a full day. Most people only need this if their doctor suspects a rare kidney disorder or needs to track treatment response.

What If My Results Are Abnormal?

Don’t panic. An abnormal result doesn’t mean you have kidney failure. It means your kidneys are sending a warning sign.

Most early kidney damage can be slowed or even reversed. The key is catching it early and treating the cause:

• If you have diabetes, tighter blood sugar control can reduce protein leakage.
• If you have high blood pressure, ACE inhibitors or ARBs (common blood pressure meds) protect your kidneys and reduce protein in urine.
• Reducing salt, quitting smoking, and losing weight all help.
• Some medications, like NSAIDs (ibuprofen, naproxen), can hurt kidneys-especially if you already have damage. Talk to your doctor before taking them regularly.

Your doctor may refer you to a nephrologist (kidney specialist) if your eGFR drops below 45 or if you have persistent proteinuria. Early intervention can delay dialysis by years-or even prevent it.

What You Can Do Today

You don’t need a medical degree to protect your kidneys. Here’s what actually works:

• Ask your doctor for your latest eGFR and ACR numbers-and what they mean for you.
• Keep your blood pressure under 130/80 if you have kidney issues.
• Control your blood sugar if you’re diabetic.
• Drink water regularly, but don’t overdo it-your kidneys handle fluid best with steady intake.
• Avoid long-term use of NSAIDs unless prescribed.
• Don’t ignore swelling in your ankles, foamy urine, or fatigue. These can be signs, even if your tests are normal.

Kidney disease doesn’t come with a siren. It creeps in quietly. But with just two simple tests-eGFR and ACR-you can catch it before it’s too late. Don’t wait until you feel sick. Your kidneys are working right now. Make sure they’re still working well.