Kidney Function Tests Explained: Creatinine, GFR, and Urinalysis

Most people don’t think about their kidneys until something goes wrong. But your kidneys are working 24/7-filtering waste, balancing fluids, and keeping your blood pressure in check. The problem? Kidney disease often has no symptoms until it’s advanced. That’s why simple blood and urine tests can be lifesavers. Three key tests-creatinine, GFR, and urinalysis-are the foundation of kidney health checks. They’re not complicated, but understanding them helps you catch problems early.

What Creatinine Tells You About Your Kidneys

Creatinine is a waste product your muscles make as they break down energy. Healthy kidneys filter it out of your blood and send it out in your urine. When kidney function drops, creatinine builds up in your bloodstream. That’s why doctors measure it in blood tests.

But here’s the catch: creatinine doesn’t rise until you’ve lost about half your kidney function. That makes it a late warning sign. A person with 40% kidney function might still have a "normal" creatinine level. That’s why creatinine alone isn’t enough-it’s just the first clue.

Normal creatinine levels vary by age, sex, and muscle mass. For adult men, it’s usually 0.7 to 1.3 mg/dL. For women, it’s 0.6 to 1.1 mg/dL. But if you’re very muscular or very elderly, your numbers might look off even if your kidneys are fine. That’s why doctors don’t rely on creatinine by itself.

What GFR Really Means (And Why It Matters More Than Creatinine)

Glomerular Filtration Rate, or GFR, is the gold standard for measuring how well your kidneys are filtering blood. It’s not directly measured-it’s calculated using your creatinine level, age, sex, and sometimes race. The most accurate formula used today is the CKD-EPI equation, which replaced the older MDRD method in 2012.

GFR is measured in milliliters per minute per 1.73 square meters of body surface area. Think of it as your kidney’s filtration speed. A GFR of 90 or above is normal. But here’s how the stages break down:

• Stage 1: GFR 90+ - Kidneys are working normally, but there may be early damage (like protein in urine)
• Stage 2: GFR 60-89 - Mild decrease in function
• Stage 3a: GFR 45-59 - Mild to moderate loss
• Stage 3b: GFR 30-44 - Moderate to severe loss
• Stage 4: GFR 15-29 - Severe loss
• Stage 5: GFR under 15 - Kidney failure

Doctors use GFR to track progression over time. If your GFR drops by more than 5 points in a year, it’s a red flag. But GFR isn’t perfect. It can be misleading if you’re under 18, pregnant, extremely muscular, or very overweight. In those cases, a cystatin C test might be used instead. Cystatin C is another blood marker that’s less affected by muscle mass and gives a clearer picture of kidney function.

Urinalysis: The Silent Early Warning System

While creatinine and GFR tell you how well your kidneys are filtering, urinalysis tells you what’s leaking out. The most important part of this test is checking for albumin-a type of protein that shouldn’t be in your urine.

When your kidneys are damaged, tiny holes form in their filters. Albumin slips through, showing up in your urine long before creatinine rises. That’s why protein in urine is one of the earliest signs of kidney disease, even if your blood tests look normal.

Doctors don’t use old-fashioned dipstick tests anymore. Instead, they use the Albumin-to-Creatinine Ratio (ACR). This test measures how much albumin is in your urine compared to creatinine. A normal ACR is under 3 mg/mmol. If it’s between 3 and 70, it’s considered early kidney damage. Above 70 means more serious damage.

One abnormal ACR result isn’t enough. You need two out of three positive tests over 3-6 months to confirm kidney disease. This avoids false positives from things like dehydration, intense exercise, or a fever.

Why You Need Both Blood and Urine Tests

Some people think if their creatinine and GFR are normal, they’re fine. But that’s not true. You can have normal kidney function and still have early damage-shown only by protein in your urine.

Conversely, someone with low GFR might not have proteinuria if their damage is from high blood pressure alone, not from kidney disease itself. That’s why both tests are required. The UK Kidney Association and the National Kidney Foundation both say: always check both eGFR and ACR together.

For example, a 60-year-old with type 2 diabetes might have a GFR of 70 (mildly reduced) but an ACR of 50. That combination means they have diabetic kidney disease-even though their GFR hasn’t dropped into the danger zone yet. Catching it here lets them change their diet, start a new medication, or control blood sugar better to slow the damage.

Who Should Get Tested and How Often

You don’t need to be sick to get tested. If you fall into any of these groups, you should get checked at least once a year:

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

The CDC recommends everyone get a basic metabolic panel (which includes creatinine) once a year as part of a routine checkup. If you’re in a high-risk group, you also need the ACR test. Some clinics now do both tests automatically for patients with diabetes or hypertension.

If you’ve been diagnosed with early kidney disease (Stage 1 or 2), you’ll likely be tested every 6 to 12 months. If you’re in Stage 3 or higher, your doctor may want to see you every 3 to 6 months.

What Happens If Your Results Are Abnormal

If your creatinine is high or your GFR is low, your doctor won’t panic right away. They’ll check for other causes-dehydration, certain medications (like NSAIDs), or a recent infection. They might repeat the test in a few weeks.

If your ACR is high, they’ll likely ask for a morning urine sample to confirm. They’ll also check your blood pressure and maybe order an ultrasound to see if your kidneys look normal on imaging.

If kidney disease is confirmed, treatment focuses on slowing progression. That means:

• Controlling blood pressure (target is usually under 130/80)
• Managing blood sugar if you have diabetes
• Switching from NSAIDs to safer pain relievers
• Reducing salt and protein intake
• Starting an SGLT2 inhibitor or ACE inhibitor, which protect kidneys even in people without diabetes

Many people with early kidney disease never progress to dialysis. With the right care, they live full lives.

Common Myths About Kidney Tests

Myth: "I feel fine, so my kidneys must be okay." Reality: Kidney disease is silent until it’s advanced. Feeling fine doesn’t mean your kidneys are healthy.

Myth: "Only people with diabetes need kidney tests." Reality: High blood pressure is actually the second leading cause of kidney failure. Anyone with hypertension should be tested.

Myth: "A normal creatinine means I’m fine." Reality: Creatinine misses early damage. You need the ACR test to catch it.

Myth: "I have to collect urine for 24 hours." Reality: For most people, a single morning urine sample is enough for ACR. The 24-hour collection is only used in special cases.