Kidney disease is diagnosed primarily through two simple tests: a blood test that estimates how well your kidneys filter waste, and a urine test that checks for protein leaking from damaged kidneys. These two measurements, often done together, can catch kidney disease years before symptoms appear. Most people discover it through routine screening or bloodwork ordered for another reason entirely.
The Two Core 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, they tell your doctor both how well your kidneys are working and whether they’re sustaining damage.
Your eGFR is calculated from a standard blood draw that measures creatinine, a waste product your muscles produce at a steady rate. Healthy kidneys clear creatinine efficiently. When they can’t keep up, creatinine builds in your blood, and your eGFR drops. The calculation factors in your age and sex to produce a score. An eGFR above 90 is normal. Below 60 signals meaningful kidney impairment, and below 15 indicates kidney failure. A single low reading doesn’t automatically mean chronic disease. Your eGFR has to stay below 60 for at least three months before chronic kidney disease (CKD) is confirmed.
The UACR comes from a urine sample, no 24-hour collection required. It measures how much albumin, a protein that should stay in your blood, is spilling into your urine. A result above 30 mg/g is abnormal and counts as a marker for kidney disease. Levels between 30 and 300 mg/g indicate moderate albumin leakage, while anything above 300 mg/g suggests more significant kidney damage. This test can pick up problems even when your eGFR still looks normal, making it especially valuable for early detection.
Who Should Get Screened and How Often
If you have diabetes or high blood pressure, you’re in the highest-risk group for kidney disease, and both your eGFR and UACR should be checked regularly. How often depends on your results. With normal values, once a year is typically enough. If your numbers fall into a mildly abnormal range, testing twice a year makes sense. People with more advanced disease may need bloodwork three or four times a year to track how quickly things are changing.
Even without diabetes or hypertension, screening becomes reasonable after age 50. There’s no firm consensus on exact testing intervals for otherwise healthy older adults, but getting a baseline and rechecking periodically gives you and your doctor something to compare against over time.
What Other Blood Tests Reveal
Beyond creatinine, a basic metabolic panel measures blood urea nitrogen (BUN), another waste product the kidneys filter. BUN alone doesn’t diagnose kidney disease because it fluctuates with hydration and protein intake. But when both BUN and creatinine are elevated together, kidney impairment is likely. An isolated rise in BUN with normal creatinine often just means you’re dehydrated.
As kidney function declines, your body loses the ability to manage certain minerals. Phosphorus levels are a key example. In people with CKD, a serum phosphorus above 3.5 mg/dL has been independently linked to worse outcomes, with risk climbing for every additional 0.5 mg/dL increase. At levels of 5 mg/dL or higher, the risk of death rises substantially in dialysis patients. Potassium can also creep up as kidneys lose their ability to excrete it, which is why electrolyte panels become routine monitoring tools once kidney disease is identified.
Some labs also measure cystatin C, an alternative marker for kidney filtration that isn’t affected by muscle mass the way creatinine is. It’s not available everywhere, and most people can be monitored with creatinine alone, but it can help clarify borderline cases.
What a Urine Sample Shows Under a Microscope
A urinalysis goes beyond protein measurement. When a lab examines urine sediment under a microscope, they look for structures called casts, which are tiny tube-shaped formations that take the shape of the kidney’s filtering tubes. Different types of casts point to different problems. Red blood cell casts indicate microscopic bleeding from the kidney itself and appear across many kidney diseases. Waxy casts are found in people with advanced or long-standing kidney failure. The type and quantity of these casts help narrow down what’s going on inside the kidneys without needing to look directly.
What Ultrasound Can Show
A kidney ultrasound is painless, requires no radiation, and gives your doctor a structural picture of your kidneys. Normal adult kidneys measure roughly 10 to 12 cm long, with a cortex (the outer filtering layer) about 7 to 10 mm thick. When kidney disease progresses, the kidneys often shrink and the cortex thins, visible signs that filtering tissue has been lost over time.
Ultrasound also detects structural problems that blood and urine tests can’t. Simple cysts are extremely common and usually harmless, appearing as smooth, round, fluid-filled pockets. Complex cysts with irregular walls, internal dividers, or calcifications need closer evaluation. In polycystic kidney disease, the ultrasound is often diagnostic on its own: both kidneys are enlarged and packed with cysts of varying sizes, eventually losing the normal distinction between their inner and outer layers.
Hydronephrosis, where urine backs up and swells the kidney’s drainage system, is graded on ultrasound from mild pelvis expansion to severe dilation with cortical thinning. This finding can explain a sudden drop in kidney function if a stone or obstruction is blocking urine flow.
When a Biopsy Is Needed
Most kidney disease is diagnosed and managed without ever taking a tissue sample. But in certain situations, a kidney biopsy becomes the only way to get a definitive answer. The main reasons doctors recommend one include unexplained rapid loss of kidney function, heavy protein in the urine (nephrotic syndrome), persistent blood in the urine that appears to come from the kidneys, and kidney involvement in systemic diseases like lupus. If you’ve had a kidney transplant and the new organ starts underperforming, a biopsy can reveal whether rejection is the cause.
The procedure uses a needle guided by ultrasound to extract a small core of kidney tissue. A pathologist then examines it under multiple types of microscopy to identify the specific disease process. That said, if your clinical picture and lab work clearly point to a known pattern, your doctor may skip the biopsy entirely. It’s reserved for cases where the diagnosis genuinely changes the treatment plan.
Acute vs. Chronic: How Timing Matters
Not all kidney problems are chronic. Acute kidney injury (AKI) can happen over days, from severe dehydration, a medication reaction, or a urinary blockage. The key distinction is duration. Kidney disease is classified as chronic only when reduced function (eGFR below 60) or markers of damage (like albuminuria) persist for three months or longer. This is the single most reliable criterion.
If you get an abnormal kidney result for the first time, your doctor will typically retest in a few weeks or months before making a chronic diagnosis. Previous lab work showing normal kidney function helps establish that a new problem is acute. Ultrasound findings like small, thin-cortex kidneys suggest the damage has been building for a while, pointing toward chronic disease even without months of prior lab data.