Chronic kidney disease (CKD) is diagnosed with two simple tests: a blood test that estimates how well your kidneys filter waste, and a urine test that checks for protein leaking into your urine. For the diagnosis to count as “chronic,” at least one of these results must remain abnormal for more than three months. That three-month rule is what separates CKD from a temporary dip in kidney function caused by dehydration, infection, or medication side effects.
The Two Core Tests
The blood test measures creatinine, a waste product your muscles produce at a fairly steady rate. Healthy kidneys clear creatinine efficiently, so when levels rise in your blood, it signals that your kidneys are falling behind. Your doctor plugs that creatinine value into a formula along with your age and sex to produce a number called eGFR (estimated glomerular filtration rate). The current standard formula, adopted in 2021, no longer includes a race variable, which had been controversial for years. An eGFR of 90 or above is normal. Below 60 for three months or longer means CKD.
The urine test looks for albumin, a protein that healthy kidneys keep in the bloodstream. When the filtering units in your kidneys are damaged, albumin slips through into your urine. The result is reported as a urine albumin-to-creatinine ratio (ACR). A normal ACR is less than 30 mg/g. A result between 30 and 299 mg/g signals early kidney damage and raises your risk of kidney failure, heart failure, and stroke. An ACR of 300 mg/g or higher, confirmed on a repeat test, points to more significant kidney disease.
You can have CKD with a normal eGFR. In stage 1, kidney function looks fine on the blood test, but protein in the urine reveals underlying damage. That’s why both tests matter and why relying on only one can miss early disease.
What Can Throw Off Your Results
Because eGFR is based on creatinine, anything that temporarily raises creatinine in your blood can make your kidney function look worse than it actually is. Eating a large amount of cooked meat before the test, taking creatine supplements, being very muscular, doing intense exercise right before your blood draw, or being dehydrated can all push creatinine up and eGFR down. Certain medications have the same effect. If your result comes back unexpectedly low, your doctor will likely retest before drawing conclusions.
The Three-Month Confirmation
A single abnormal result doesn’t equal CKD. Acute kidney injury from dehydration, surgery, or a drug reaction can temporarily lower eGFR or raise urine protein. CKD is defined as abnormal kidney structure or function lasting more than three months. So your doctor will repeat tests after that window to see whether the problem has resolved or persisted. Once results are confirmed as chronic and stable, a more thorough workup begins to figure out what’s causing the damage.
CKD Stages by eGFR
Once CKD is confirmed, your eGFR determines the stage:
- Stage 1: eGFR 90 or above with evidence of kidney damage (such as protein in urine)
- Stage 2: eGFR 60 to 89, mild loss of function
- Stage 3a: eGFR 45 to 59, mild to moderate loss
- Stage 3b: eGFR 30 to 44, moderate to severe loss
- Stage 4: eGFR 15 to 29, severe loss
- Stage 5: eGFR below 15, kidney failure
Stage 3 is where most people are first diagnosed, often because routine bloodwork catches a low eGFR before any symptoms appear. Stages 1 and 2 rarely cause noticeable symptoms and are usually found only when someone is being monitored for diabetes, high blood pressure, or another condition that puts kidneys at risk.
When Imaging Is Used
Blood and urine tests tell your doctor how well your kidneys work, but not what they look like. An ultrasound fills that gap. It’s painless, uses no radiation, and can reveal structural clues that support or refine a CKD diagnosis.
Doctors look at kidney size, symmetry, and how bright the tissue appears on the screen compared to the adjacent liver. In CKD, kidneys often appear smaller than normal, and the outer layer of kidney tissue (the cortex) thins out. The tissue also tends to look brighter than it should, a sign of scarring. An ultrasound can pick up cysts too. Four or more cysts in both kidneys, in someone without a family history of polycystic kidney disease, suggest acquired cystic kidney disease, a condition linked to CKD. It also helps rule out blockages like kidney stones or an enlarged prostate pressing on the urinary tract, which can mimic or worsen CKD.
When a Kidney Biopsy Is Needed
Most people with CKD never need a biopsy. The combination of blood tests, urine tests, and imaging is enough to confirm the diagnosis and guide treatment, especially when the cause is clearly diabetes or long-standing high blood pressure.
A biopsy becomes necessary when the cause of kidney damage is unclear or the disease is behaving unexpectedly. Common triggers include unexplained kidney failure, large amounts of protein in the urine (nephrotic syndrome), blood in the urine coming from the kidney’s filtering units, or signs of an autoimmune condition like lupus. If your kidney function drops rapidly and nobody can explain why, a biopsy lets a pathologist examine a tiny sample of kidney tissue under a microscope to identify the specific type of damage. For people with a kidney transplant, a biopsy is standard when there are signs of rejection, such as a rising creatinine level, fever, swelling, or decreased urine output.
Who Should Be Screened
CKD is often silent until it’s advanced, which makes screening essential for people at higher risk. Diabetes and high blood pressure are the two biggest risk factors, accounting for the majority of CKD cases. If you have either condition, guidelines recommend checking eGFR and urine albumin at least once a year. Your doctor may test more frequently if you’re at higher risk of progression or if the results would change your treatment plan.
Other risk factors that warrant regular screening include a family history of kidney disease, heart disease, obesity, recurrent kidney infections, and a history of acute kidney injury. Age also plays a role, since kidney function naturally declines over time. If you fall into any of these categories and haven’t had your kidney function checked recently, it’s a straightforward blood draw and urine sample that can catch problems years before symptoms start.