Kidney Function Tests Explained: Creatinine, GFR, and Urinalysis

Most people don’t think about their kidneys until something goes wrong. But by the time you feel tired, swollen, or notice changes in your pee, up to half your kidney function might already be gone. That’s why kidney function tests-like creatinine, GFR, and urinalysis-are some of the most important blood and urine checks you can get. They don’t need to be scary. In fact, they’re simple, quick, and can catch problems long before you feel sick.

What Your Kidneys Actually Do

Your kidneys aren’t just filters. They’re busy, 24/7 maintenance crews. They clean about 120 to 150 quarts of blood every day, turning waste into about 1 to 2 quarts of urine. They also balance your salt and water, control blood pressure, make red blood cells, and keep your bones strong by activating vitamin D. When they start to slow down, it’s not always obvious. That’s why tests matter.

Creatinine: The Waste Product That Tells a Story

Creatinine is a chemical your muscles make as they work. It’s harmless-until your kidneys can’t flush it out. When kidney function drops, creatinine builds up in your blood. That’s the basic idea behind the creatinine blood test.

But here’s the catch: creatinine doesn’t rise until you’ve lost about 50% of your kidney function. That’s why it’s called a late marker. Two people can have the same creatinine level but very different kidney health. A muscular 30-year-old might naturally have higher creatinine than a frail 70-year-old. That’s why doctors don’t look at creatinine alone. They use it to calculate something more useful: eGFR.

eGFR: The Real Measure of Kidney Performance

eGFR stands for estimated Glomerular Filtration Rate. It’s not measured directly-it’s calculated using your creatinine level, age, sex, and sometimes race. It tells you how well your kidneys are filtering blood, in milliliters per minute per 1.73 square meters of body surface area.

Here’s what the numbers mean, based on current guidelines:

• 90 or higher: Normal kidney function
• 60 to 89: Mildly reduced function
• 45 to 59: Moderately reduced
• 30 to 44: Severely reduced
• 15 to 29: Very severely reduced
• Below 15: Kidney failure (dialysis or transplant likely needed)

These stages are used worldwide. In Australia, the UK, and the US, labs now report eGFR automatically when you get a creatinine test. You don’t have to ask for it. If your eGFR is below 60 for three months or more, it’s considered chronic kidney disease-even if you feel fine.

Why eGFR Isn’t Perfect

eGFR is great-but it’s not foolproof. It can be inaccurate if you’re:

• Under 18
• Pregnant
• Very muscular (bodybuilders, athletes)
• Very overweight
• On a very low-protein diet

In those cases, doctors might order a cystatin C test. It’s another blood marker, less affected by muscle mass or diet. Some hospitals now use cystatin C to cross-check eGFR, especially for high-risk patients.

Urinalysis: The Hidden Clue in Your Pee

One of the earliest signs of kidney damage? Protein leaking into your urine. Healthy kidneys don’t let protein pass through. But if the filters are damaged-like a torn sieve-albumin (a type of protein) slips out.

That’s why urinalysis is just as important as blood tests. The most accurate test isn’t the old dipstick you might remember from school. It’s the Albumin-to-Creatinine Ratio, or ACR. This test measures how much albumin is in your urine compared to creatinine. It’s done on a single morning sample-no 24-hour collection needed.

Here’s what ACR levels mean:

• Less than 3 mg/mmol: Normal
• 3 to 70 mg/mmol: Mild to moderate proteinuria (needs repeat test)
• Above 70 mg/mmol: Significant kidney damage

Doctors don’t diagnose based on one test. If your ACR is high, they’ll ask you to repeat it in a few weeks. One high result could be from dehydration, infection, or even intense exercise. Two or three high results over time? That’s a red flag.

What About 24-Hour Urine Tests?

You might hear about collecting all your urine for 24 hours. It sounds extreme, and it is. But it’s still used sometimes-for example, if you have heavy protein loss or need to measure other substances like sodium, potassium, or oxalate. Most people don’t need it. ACR gives the same info for kidney damage with way less hassle.

Who Needs These Tests?

You don’t need to wait for symptoms. If you have any of these, you should be tested at least once a year:

• Diabetes (type 1 or 2)
• High blood pressure
• Heart disease
• Obesity
• Family history of kidney failure
• Age over 60
• Previous acute kidney injury

Even if you’re healthy, getting a basic metabolic panel (which includes creatinine and eGFR) once a year during your check-up is smart. The CDC recommends it for everyone over 50. In Australia, Medicare covers these tests for people with chronic conditions.

What Happens If the Tests Show a Problem?

Early kidney disease doesn’t always need drugs. Often, it just needs lifestyle changes:

• Control your blood sugar if you have diabetes
• Keep your blood pressure under 130/80
• Reduce salt and processed foods
• Stay hydrated-but don’t overdo water
• Quit smoking
• Avoid NSAIDs like ibuprofen or naproxen unless prescribed

If your eGFR drops below 45 or your ACR stays high, your doctor might refer you to a nephrologist. They’ll look for the cause-maybe it’s glomerulonephritis, polycystic kidney disease, or something else. Treatment depends on the root problem, but the goal is always the same: slow the damage.

What Doesn’t Work

There’s a lot of misinformation out there. Don’t believe these myths:

• “Drinking lots of water cures kidney disease.” Water helps, but it won’t fix damaged filters.
• “Herbs or detox teas clean your kidneys.” Some herbs can actually harm your kidneys. Avoid unregulated supplements.
• “If my creatinine is normal, I’m fine.” Not true. eGFR and ACR tell the full story.
• “I don’t need tests if I feel fine.” That’s exactly when you need them most.

What to Ask Your Doctor

If you’ve had a kidney test, here’s what to ask next:

• What’s my eGFR? Is it trending up or down?
• What’s my ACR? Was it checked?
• Do I need a repeat test? When?
• Should I be tested for cystatin C?
• What lifestyle changes should I make right now?

Don’t leave the appointment without knowing your numbers. Write them down. Keep track. Small changes now can keep you off dialysis later.

Final Thought: Early Detection Saves Kidneys

Kidney disease doesn’t come with a warning siren. It sneaks in quietly. But with just two simple tests-eGFR from blood and ACR from urine-you can see what’s happening before it’s too late. If you’re at risk, get tested. If you’re not, ask your doctor if you should start. Your kidneys won’t tell you they’re in trouble. But these tests will.