Yes, blood work is one of the most reliable ways to detect kidney problems, and it’s often the first place they show up. A standard blood panel can measure how well your kidneys filter waste by tracking two key markers: creatinine and blood urea nitrogen (BUN). These tests are routine, require no fasting, and can flag kidney disease well before you notice symptoms.
That said, blood tests have real limitations. They work best for measuring filtration, not for spotting structural damage like cysts, blockages, or scarring. And creatinine, the most common blood marker, has a significant blind spot: roughly 50% of your kidney function can be lost before creatinine levels rise noticeably in the blood. Understanding what blood work catches, what it misses, and what additional tests fill the gaps gives you a much clearer picture.
The Two Main Blood Markers
When your doctor orders a kidney panel, two numbers do most of the heavy lifting: serum creatinine and blood urea nitrogen.
Creatinine is a waste product your muscles produce at a fairly steady rate. Healthy kidneys filter it out of your blood and into your urine. When your kidneys aren’t filtering efficiently, creatinine accumulates in the bloodstream. The catch is that “normal” creatinine varies a lot depending on your age, sex, and muscle mass. A muscular 25-year-old and a sedentary 75-year-old can have very different creatinine levels that are both technically normal for them. This makes a single creatinine reading surprisingly tricky to interpret on its own.
Blood urea nitrogen (BUN) measures another waste product, urea, which comes from the breakdown of protein in your diet. High BUN can signal that your kidneys aren’t clearing waste effectively. But BUN is even less specific than creatinine. Dehydration, a high-protein diet, burns, certain medications, and even a recent heart attack can all push BUN levels up without any kidney damage present. Low BUN, on the other hand, sometimes points to liver disease or malnutrition rather than a kidney issue. Doctors typically look at BUN alongside creatinine rather than relying on it alone.
How eGFR Turns Creatinine Into a Clearer Answer
Because raw creatinine numbers are so variable, labs now automatically calculate a value called estimated glomerular filtration rate, or eGFR. This formula takes your creatinine level and adjusts it for age and sex to estimate how many milliliters of blood your kidneys actually filter per minute. The result is a much more useful number than creatinine alone.
An eGFR of 90 or above is considered normal. Between 60 and 89, kidney function is mildly decreased, though many people in this range have no symptoms and no progression. Below 60 is where doctors pay close attention, because it typically means moderate kidney disease. Below 15 signals kidney failure.
For over 20 years, eGFR formulas included a race variable that adjusted results for Black patients. In 2021, the National Kidney Foundation and the American Society of Nephrology recommended dropping that variable, concluding that race is a social construct and its inclusion in clinical formulas contributed to health disparities. Labs across the U.S. now use the updated race-free equation.
Kidney Disease Stages by the Numbers
Chronic kidney disease is classified into stages based on eGFR, which helps you and your doctor understand where things stand:
- Stage 1 (eGFR 90+): Kidney damage is present (detected through urine tests or imaging) but filtration is still normal
- Stage 2 (eGFR 60–89): Mildly decreased function, often with no noticeable symptoms
- Stage 3a (eGFR 45–59): Mild to moderate decrease in function
- Stage 3b (eGFR 30–44): Moderate to severe decrease
- Stage 4 (eGFR 15–29): Severe decrease, with symptoms becoming more apparent
- Stage 5 (eGFR below 15): Kidney failure
Notice that Stage 1 has a normal eGFR. This is the critical limitation of blood work: in the earliest stage of kidney disease, your blood tests can look completely fine. Damage is already happening, but your kidneys are still compensating well enough that filtration rates haven’t dropped. This is why doctors sometimes pair blood work with urine tests to catch problems sooner.
What Blood Work Can Miss
Blood tests measure filtration. They tell you how well your kidneys are cleaning your blood right now. They don’t directly reveal structural problems like kidney stones, cysts, tumors, or blockages in the urinary tract. For those, imaging tests like ultrasound or CT scans are needed.
There’s also the sensitivity problem. Because creatinine doesn’t rise meaningfully until about half of kidney function is already gone, a “normal” creatinine result doesn’t guarantee healthy kidneys. You could lose a substantial amount of filtering capacity and still have blood work that looks unremarkable. This is one reason eGFR was developed, and why trends over time matter more than any single test result. A creatinine level that’s creeping upward over several years tells a different story than one isolated reading.
In some cases, doctors order a kidney biopsy to identify a specific disease, assess how well it might respond to treatment, or determine how far damage has progressed. This goes far beyond what any blood draw can reveal.
Cystatin C: A More Precise Alternative
If your doctor suspects that creatinine-based results might be unreliable for you, there’s a second blood marker called cystatin C. Unlike creatinine, which is influenced by muscle mass, cystatin C is produced at a constant rate by virtually all cells in the body. It isn’t affected by how muscular you are, making it a better option for people who are very athletic, very thin, elderly, or anyone whose muscle mass might skew creatinine readings.
Research shows cystatin C outperforms creatinine as a marker of true kidney function, particularly at higher filtration rates where creatinine can appear normal despite early damage. It’s also a stronger predictor of cardiovascular disease risk and progression to kidney failure. For elderly patients and people from non-white ethnic groups, cystatin C has been shown to be more reliable at identifying those at increased risk. Current guidelines recommend expanded use of cystatin C testing, though it’s not yet as routine as creatinine in most labs.
The Urine Test That Fills the Gap
One of the most important complements to blood work is a urine test called the albumin-to-creatinine ratio, or UACR. Albumin is a protein that healthy kidneys keep in the blood. When it starts leaking into urine, even in tiny amounts, it’s an early sign that the kidney’s filtering units are damaged.
A normal UACR is below 30 mg/g, though normal values tend to be slightly lower in men (around 17 mg/g) than in women (around 25 mg/g). A result between 30 and 300 mg/g indicates microalbuminuria, a level of protein leakage that’s too small to cause symptoms but significant enough to flag early kidney damage. This test can detect problems at Stage 1, when eGFR is still normal, making it essential for catching kidney disease as early as possible.
Microalbuminuria is also an independent predictor of heart disease and cardiovascular death in the general population. Even UACR levels within the technically “normal” range, if on the higher end, are associated with increased risk of developing high blood pressure. This makes the test valuable beyond kidney health alone. It’s a simple spot urine sample, not a 24-hour collection, so it’s convenient and widely available.
Preparing for a Kidney Blood Test
Kidney function blood tests do not require fasting. Unlike cholesterol or blood sugar panels, measurements of creatinine and BUN aren’t influenced by what you ate that morning. You can eat and drink normally before the draw.
That said, a few things can temporarily affect your results. Intense exercise in the 24 hours before the test can raise creatinine because of increased muscle breakdown. Eating an unusually large amount of cooked meat (a significant source of creatinine) could nudge levels up slightly. And dehydration can elevate both creatinine and BUN, making kidney function look worse than it actually is. Staying normally hydrated and avoiding extreme exertion the day before gives you the most accurate baseline.
If your results come back borderline, your doctor will typically retest before drawing conclusions. A single elevated creatinine reading could reflect something as simple as a hard workout or a day of not drinking enough water. The pattern over time is what matters most.