The single most important lab value for identifying kidney failure is your estimated glomerular filtration rate, or eGFR. An eGFR below 15 is the marker for kidney failure, while an eGFR below 60 signals that your kidneys aren’t working properly. But eGFR doesn’t work alone. Doctors look at a combination of values, including creatinine, urine albumin, and sometimes additional markers, to build a complete picture of how well your kidneys are filtering waste.
eGFR: The Core Number
Your eGFR estimates how many milliliters of blood your kidneys filter per minute. It’s calculated from a blood test, most often using your creatinine level along with your age and sex. Here’s how the stages break down:
- Stage 1 (eGFR 90 or above): Normal filtering, though other signs of kidney damage like protein in the urine may be present
- Stage 2 (eGFR 60–89): Mild loss of function
- Stage 3a (eGFR 45–59): Mild to moderate loss
- Stage 3b (eGFR 30–44): Moderate to severe loss
- Stage 4 (eGFR 15–29): Severe loss
- Stage 5 (eGFR below 15): Kidney failure
These thresholds were reaffirmed by the 2024 international clinical guidelines (KDIGO), which also explicitly rejected adjusting the cutoffs by age. Whether you’re 35 or 75, the same numbers apply.
Serum Creatinine
Creatinine is a waste product your muscles produce at a fairly steady rate. Healthy kidneys filter it out efficiently, so when your blood creatinine starts climbing, it usually means your kidneys are falling behind. There’s no single “normal” creatinine number because the baseline varies with age, sex, and body size. What matters more is the trend: a creatinine level that’s rising over time, or one that spikes suddenly, points to declining kidney function.
The catch is that creatinine reflects muscle mass almost as much as it reflects kidney function. Research on kidney transplant recipients found that eGFR was falsely reduced by about 6 points for every 10 kilograms of lean body mass. In highly muscular people, nearly half of positive CKD screening results were false alarms. On the flip side, someone with very low muscle mass (from prolonged illness, amputation, or aging) can have a creatinine level that looks reassuringly normal even though their kidneys are struggling. Certain medications, including an antibiotic called trimethoprim, can also push creatinine readings higher without any actual change in kidney function.
Cystatin C: A More Reliable Alternative
When creatinine is unreliable, doctors can order a blood test for cystatin C. This protein is produced by nearly all cells in the body at a constant rate, so it isn’t thrown off by muscle mass the way creatinine is. A normal cystatin C level in adults falls roughly between 0.6 and 1.0 mg/L, with higher values suggesting reduced kidney function.
Cystatin C is especially useful for people whose body composition makes creatinine misleading: athletes, people with obesity, those with amputations or spinal cord injuries, and patients who have lost significant muscle during hospitalization. The 2024 clinical guidelines now recommend combining creatinine and cystatin C for a more accurate eGFR whenever cystatin C testing is available.
Urine Albumin-to-Creatinine Ratio
Healthy kidneys keep proteins in your blood and out of your urine. When the kidney’s filtering units are damaged, albumin (a common blood protein) leaks through. The urine albumin-to-creatinine ratio, or uACR, measures this leakage from a single urine sample.
- Normal: Less than 30 mg/g
- Moderately increased (30–299 mg/g): Higher risk of kidney failure, heart failure, and stroke
- Severely increased (300 mg/g or higher): Strong indicator of kidney disease and severe risk of kidney failure
A uACR result above 300 mg/g, confirmed on a repeat test, is considered diagnostic of kidney disease even if your eGFR hasn’t dropped dramatically yet. This makes it one of the earliest warning signs available. It’s also why routine kidney screening includes both a blood test (for eGFR) and a urine test (for albumin), not just one or the other.
Blood Urea Nitrogen (BUN)
BUN measures the amount of urea, another waste product, in your blood. Like creatinine, it rises when the kidneys can’t keep up. But BUN is a less specific marker because it’s also influenced by how much protein you eat, whether you’re dehydrated, and whether your body is breaking down tissue from illness or surgery. Your doctor will typically look at BUN alongside creatinine and eGFR rather than relying on it alone.
Lab Values in Acute Kidney Injury
Kidney failure isn’t always a slow process. Acute kidney injury, or AKI, is a sudden drop in kidney function that develops over hours or days, often during a hospitalization or severe illness. The diagnostic criteria are more time-sensitive than those for chronic kidney disease:
- A creatinine increase of 0.3 mg/dL or more within 48 hours
- A creatinine level rising to at least 1.5 times baseline within 7 days
- Urine output dropping below 0.5 mL per kilogram of body weight per hour for 6 hours
Meeting any one of these criteria is enough for a diagnosis. The speed of the creatinine change is what separates AKI from chronic kidney disease, which develops over months or years.
Hemoglobin and Anemia
Failing kidneys produce less of a hormone that tells your bone marrow to make red blood cells, so anemia often accompanies kidney disease. Almost half of people with stage 3a CKD already have anemia, and by stage 5 (kidney failure) more than 9 out of 10 do. Anemia is generally defined as hemoglobin below 12 g/dL in women or below 13 g/dL in men.
Hemoglobin isn’t used to diagnose kidney failure directly, but persistently low levels that don’t respond to iron supplements can be an early clue that kidney function has declined further than other labs suggest. If you’ve been told your hemoglobin is low without an obvious explanation, a kidney workup is a reasonable next step.
How These Values Work Together
No single lab result confirms kidney failure on its own. A high creatinine in a muscular person might be completely normal. A normal creatinine in someone with very little muscle mass might be hiding real damage. An elevated BUN could mean dehydration rather than kidney disease. That’s why doctors interpret these values as a panel: eGFR and creatinine from a blood draw, uACR from a urine sample, and sometimes cystatin C when the standard tests are hard to interpret. Together, they tell a much more reliable story than any one number alone.