Kidney disease is diagnosed primarily through two simple tests: a blood test that measures how well your kidneys filter waste, and a urine test that checks for protein leaking into your urine. These tests can detect kidney damage years before you feel any symptoms, which is why they’re routinely ordered for people with diabetes, high blood pressure, or a family history of kidney problems. Depending on the results, your doctor may order imaging or additional bloodwork to pinpoint the cause and severity.
The Two Core Screening Tests
The foundation of kidney disease diagnosis comes down to two numbers: your estimated glomerular filtration rate (eGFR) and your urine albumin-to-creatinine ratio (UACR). Together, these tell your doctor how effectively your kidneys are working and whether they’re letting things through that they shouldn’t be.
For the eGFR, a standard blood draw measures creatinine, a waste product created by normal muscle breakdown. Your kidneys constantly remove creatinine from your blood, so when they’re not filtering well, creatinine levels rise. A lab then plugs your creatinine level, age, and sex into a standardized formula to estimate your filtration rate. The current standard is the 2021 CKD-EPI equation, which no longer uses race as a variable. A normal eGFR is above 90 mL/min. Below 60 for three months or longer points to chronic kidney disease.
The urine test looks for albumin, a protein that healthy kidneys keep in the blood. When the kidneys’ tiny filters are damaged, albumin slips through into urine. A quick dipstick test can detect larger amounts, but the more precise measurement is the UACR, which compares albumin to creatinine in a single urine sample. A UACR below 30 mg/g is normal. Between 30 and 300 mg/g indicates moderate protein leakage, sometimes called microalbuminuria. Above 300 mg/g signals more serious damage.
What Symptoms Trigger Testing
Most early kidney disease causes no symptoms at all, which is why screening matters so much for people at risk. But certain signs prompt doctors to order kidney tests even in people who haven’t been screened before.
Foamy or sudsy urine is one of the more recognizable clues. High concentrations of protein in urine change its appearance noticeably. Swelling around the eyes or in the legs, ankles, and feet can develop when damaged kidneys let too much protein escape or fail to remove excess fluid. Blood in the urine, whether visible or detected on a routine urinalysis, is relatively specific to a kidney or urinary tract problem and always warrants further investigation.
Other signs are less obvious. Persistent fatigue, unexplained nausea, loss of appetite, or a metallic taste in your mouth can all develop as waste products build up in the blood. Urinating more frequently at night, flank pain, or consistently high blood pressure that’s hard to control can also point toward kidney trouble. Dizziness and low blood pressure sometimes signal dehydration-related kidney injury. In more advanced cases, confusion, drowsiness, or slurred speech may appear.
How eGFR Stages Work
Once blood and urine tests confirm kidney disease, doctors classify it into stages based on your eGFR. This staging system helps guide treatment decisions and gives you a clearer picture of where things stand.
- Stage 1 (G1): eGFR above 90. Kidney function is normal, but other signs of damage exist, such as protein in urine or abnormal imaging.
- Stage 2 (G2): eGFR of 60 to 89 with evidence of kidney damage. A mildly reduced rate that often has no symptoms.
- Stage 3a (G3a): eGFR of 45 to 59. Mild to moderate loss of function.
- Stage 3b (G3b): eGFR of 30 to 44. Moderate to severe loss of function.
- Stage 4 (G4): eGFR of 15 to 29. Severe loss of function, and planning for potential dialysis or transplant typically begins.
- Stage 5 (G5): eGFR below 15. The kidneys have lost almost all function.
Staging always considers both eGFR and albuminuria together. Someone with a normal eGFR but persistently high albumin in their urine still has kidney disease and may face a higher risk of progression than their filtration number alone would suggest.
When Creatinine Alone Isn’t Enough
Creatinine is the standard marker for estimating kidney function, but it has a significant limitation: it’s influenced by muscle mass, age, and sex. A very muscular person may have higher creatinine that doesn’t reflect kidney problems, while someone with low muscle mass (due to aging, malnutrition, or a chronic illness) may have misleadingly normal creatinine levels despite real kidney damage.
In these situations, doctors can add a second blood marker called cystatin C. This protein is produced by nearly all cells in the body at a relatively steady rate, making it less dependent on body composition. An eGFR calculated from both creatinine and cystatin C together is generally more accurate than either marker alone. The NIDDK now considers this combined approach the preferred method. Cystatin C can be falsely elevated by chronic inflammation, obesity, smoking, and thyroid problems, so your doctor interprets it alongside your full clinical picture.
Additional Blood Markers
Beyond eGFR, a broader blood panel reveals how kidney disease is affecting the rest of your body. As kidney function declines, several chemical imbalances develop that help confirm the diagnosis and guide treatment.
Potassium levels often rise because damaged kidneys can’t clear it efficiently. High potassium is concerning because it can cause dangerous heart rhythm problems. Phosphorus tends to build up as well, which over time pulls calcium from bones and deposits it in blood vessels. Low calcium and high phosphorus together are a hallmark of advancing kidney disease and a reason doctors monitor bone health closely. A basic or renal metabolic panel captures most of these markers in a single blood draw.
Imaging and Biopsy
An ultrasound is often the first imaging test ordered. It’s painless, uses no radiation, and shows the size and shape of your kidneys, whether there are cysts or masses, and whether urine flow is blocked. Small, shrunken kidneys on ultrasound suggest long-standing chronic disease, while normal-sized or enlarged kidneys may point to a more recent or reversible problem.
A kidney biopsy, where a small tissue sample is taken with a needle, is reserved for specific situations. Doctors typically recommend it when there’s unexplained sudden kidney failure, large amounts of protein in the urine (nephrotic syndrome), or signs of an inflammatory kidney condition. The biopsy reveals the exact type and severity of damage at the tissue level, helps determine whether the damage is potentially reversible or permanent, and directly shapes treatment decisions. It’s not part of routine screening but becomes important when blood and urine tests alone can’t explain what’s happening.
Preparing for Kidney Function Tests
A standard creatinine blood test doesn’t always require fasting, but if your doctor orders a renal function panel or basic metabolic panel, you may be asked to fast for 8 to 12 hours beforehand. Fasting means nothing except plain water: no coffee, juice, flavored water, or soda. You should also avoid chewing gum, smoking, and exercise during the fasting window, as these can shift your blood chemistry enough to affect results.
Ask your doctor whether to take your usual medications and supplements before the test. Some can influence creatinine or other markers. If you accidentally eat or drink something during a required fast, mention it before the blood draw rather than after. You may need to reschedule to get accurate results. The urine test for albumin requires no special preparation, just a sample collected at the office or lab.