Most people don’t think about their kidneys until something goes wrong. But by then, it’s often too late. Your kidneys filter about 120 to 150 quarts of blood every day, removing waste and extra fluid. If they start to fail, you might not feel it-until you’ve lost half their function. That’s why kidney function tests matter. They’re simple, quick, and can catch problems before you even notice symptoms.

What Do Kidney Function Tests Actually Measure?

There are three main tests doctors use to check how well your kidneys are working: creatinine in your blood, your estimated GFR, and urinalysis. Each one tells a different part of the story.

Serum creatinine is a waste product from your muscles. When your kidneys are healthy, they flush it out through urine. When they’re struggling, creatinine builds up in your blood. But here’s the catch: creatinine doesn’t rise until you’ve already lost about half your kidney function. That’s why it’s not a good early warning sign on its own.

Estimated Glomerular Filtration Rate (eGFR) is the number doctors rely on most. It’s not directly measured-it’s calculated using your creatinine level, age, sex, and sometimes race. The result tells you how well your kidneys are filtering blood. A normal eGFR is 90 or higher. If it drops below 60 for three months or more, it’s a sign of chronic kidney disease. The UK Kidney Association uses the CKD-EPI formula, which is more accurate than older methods, especially for people with higher kidney function.

Urinalysis looks at what’s in your urine. The most important part is checking for albumin-a type of protein that shouldn’t be there. Healthy kidneys keep protein in your blood. If it’s leaking into your urine, it’s one of the earliest signs of damage. The test used today isn’t a dipstick (those are outdated). Instead, labs measure the Albumin-to-Creatinine Ratio (ACR). A result over 70 mg/mmol is considered high. If it’s between 3 and 70, your doctor will ask you to repeat the test. One high result could be from dehydration or infection. Two or three high results over weeks confirm kidney damage.

Why You Can’t Rely on Just One Test

Many people think if their creatinine is normal, their kidneys are fine. That’s dangerous thinking. I’ve seen patients with normal creatinine but high ACR-clear signs of early kidney damage. Why? Because creatinine is a lagging indicator. It only rises after significant damage has happened. Albumin in the urine, on the other hand, shows up early.

Think of it like a car’s check engine light. Creatinine is the light that comes on after the engine is already overheating. ACR is the warning beep you hear before it gets that bad. You need both.

That’s why the UK Kidney Association and the American Kidney Fund both say: Always test both eGFR and ACR together. One without the other is like checking only the oil level in your car and ignoring the brake fluid. You might miss something critical.

Who Should Be Tested and How Often?

You don’t need to get tested unless you’re at risk. But if you fall into any of these groups, you should be checked at least once a year:

• People with diabetes (type 1 or type 2)
• People with high blood pressure
• Those with heart disease or a history of heart attack
• People who’ve had acute kidney injury in the past
• Anyone with a family history of kidney failure
• People over 60

The CDC recommends a basic metabolic panel-including creatinine and eGFR-as part of your yearly checkup. If you’re in a high-risk group, your doctor should also order a urine ACR test every year. Some people with early kidney disease may need testing every 3 to 6 months to track changes.

And don’t assume you’re safe just because you feel fine. Kidney disease doesn’t cause pain. It doesn’t make you tired until it’s advanced. That’s why testing is so important.

What If Your Results Are Abnormal?

If your eGFR is below 60 or your ACR is high, don’t panic. A single abnormal result doesn’t mean you have kidney disease. Doctors look for patterns. If your numbers stay abnormal over three months, that’s when a diagnosis of chronic kidney disease is made.

What happens next depends on how bad it is:

• eGFR 60-89 with high ACR: Early kidney damage. Focus on controlling blood pressure, blood sugar, and diet.
• eGFR 30-59: Moderate decline. You’ll likely be referred to a kidney specialist (nephrologist). Medications like ACE inhibitors or SGLT2 inhibitors may be started.
• eGFR 15-29: Severe decline. Planning for dialysis or transplant may begin.
• eGFR below 15: Kidney failure. Dialysis or transplant is needed.

For urine results:

• ACR 3-70 mg/mmol: Borderline. Repeat test in 3 months.
• ACR above 70 mg/mmol: Confirmed proteinuria. Needs medical evaluation and treatment.

Many people with early kidney disease can slow or even stop the damage with lifestyle changes and medication. The key is catching it early.

When Creatinine Isn’t Enough: Cystatin C

There are cases where creatinine-based eGFR doesn’t give the full picture. For example:

• If you’re very muscular-more muscle means more creatinine, which can make your eGFR look worse than it is.
• If you’re very underweight or elderly-less muscle means less creatinine, which can hide kidney problems.
• If you’re pregnant-hormones change how creatinine is processed.
• If you’re under 18-standard equations aren’t designed for children.

In these cases, doctors may order a cystatin C test. It’s a protein made by all your cells, and your kidneys filter it out. It’s not affected by muscle mass, diet, or age the way creatinine is. Some hospitals now use cystatin C to calculate a second eGFR to cross-check the creatinine-based one. If both match, you can trust the result. If they’re very different, your doctor knows to dig deeper.

What About 24-Hour Urine Tests?

You might hear about collecting urine for 24 hours. That test measures total protein, sodium, potassium, and other substances. It’s accurate-but it’s a hassle. You have to collect every drop of urine for a full day. It’s messy, easy to mess up, and rarely needed anymore.

Today, ACR is the gold standard for checking protein in urine. It’s just as accurate, much easier, and done with a single morning sample. Unless your doctor suspects a rare kidney disorder, you probably won’t need a 24-hour collection.

What You Can Do Now

You don’t have to wait for symptoms. If you’re over 40, or have diabetes, high blood pressure, or a family history of kidney disease, ask your GP for a simple blood and urine test. It’s covered by the NHS. No appointment needed in many cases-you can just walk into your local clinic.

And if your results are normal? Keep getting checked. Kidney disease creeps up slowly. One test every year is enough for most people. But if you’re high risk, don’t skip it.

Don’t ignore your kidneys. They’re silent workers. But when they fail, they take your whole body down with them. Catching problems early isn’t just smart-it’s life-saving.