The main blood test for kidney function is a creatinine test, which measures a waste product your muscles naturally produce. Your kidneys filter creatinine out of your blood, so when levels rise, it signals that your kidneys aren’t cleaning your blood as well as they should. Doctors use your creatinine result to calculate a second, more useful number called eGFR, which estimates how efficiently your kidneys are filtering. These two values, often run as part of a routine blood panel, are the cornerstone of kidney health screening.
Creatinine: The Core Kidney Marker
Creatinine is a waste product that forms when your muscles break down during normal, everyday use. It enters your bloodstream at a fairly constant rate, gets filtered out by your kidneys, and leaves your body in urine. That predictable cycle is what makes it so useful as a kidney marker: if your kidneys are struggling, creatinine builds up in the blood instead of being cleared out.
A standard blood draw is all that’s needed. Your doctor may ask you to fast for 8 to 12 hours beforehand if creatinine is being measured alongside blood sugar or other metabolic markers. Avoiding strenuous exercise right before the test is also a good idea, since intense physical activity can temporarily raise creatinine levels and skew results.
One important caveat: creatinine isn’t a perfect measure on its own. Because it comes from muscle tissue, your baseline level depends heavily on how much muscle you carry. A young, muscular person will naturally have higher creatinine than an older, smaller-framed person, even if both have perfectly healthy kidneys. Diet plays a role too. Eating a large amount of meat shortly before a test can bump your creatinine up, since meat contains creatine that your body converts to creatinine. Creatine supplements have the same effect. For these reasons, doctors rarely interpret a creatinine number in isolation.
eGFR: What Your Creatinine Result Actually Means
Your lab report will typically include a value called eGFR, or estimated glomerular filtration rate. This number takes your creatinine level and adjusts it for your age and sex to estimate how many milliliters of blood your kidneys clean per minute. It’s a much more practical measure than raw creatinine because it accounts for some of the body-size and age differences that make creatinine tricky to interpret on its own.
The current standard formula, adopted in 2021 by the National Kidney Foundation, no longer includes a race-based adjustment that older equations used. Labs now calculate eGFR using only creatinine, age, and sex. A normal eGFR is 90 or above (measured in mL/min/1.73 m²). That number represents strong filtration. The lower it drops, the more kidney function has been lost.
Here’s how the stages of chronic kidney disease (CKD) map to eGFR ranges:
- Stage 1 (eGFR 90+): Normal filtration, but other signs of kidney damage may be present (like protein in urine)
- Stage 2 (eGFR 60–89): Mildly decreased function
- Stage 3a (eGFR 45–59): Mild to moderate decrease
- Stage 3b (eGFR 30–44): Moderate to severe decrease
- Stage 4 (eGFR 15–29): Severe decrease
- Stage 5 (eGFR below 15): Kidney failure
An important detail: an eGFR of 60 to 89 by itself doesn’t automatically mean you have kidney disease. Without other evidence of kidney damage, stages 1 and 2 don’t meet the full criteria for a CKD diagnosis. Your doctor would look for additional markers like protein in your urine or abnormal imaging before making that call.
BUN: The Other Kidney Number on Your Lab Report
Blood urea nitrogen, or BUN, is the second kidney-related value you’ll commonly see on a metabolic panel. When your body breaks down protein from food or from your own tissues, it produces urea as a byproduct. Like creatinine, urea travels through your blood and gets filtered out by your kidneys. A rising BUN level can indicate that your kidneys aren’t removing waste effectively.
BUN is less specific to the kidneys than creatinine, though. Dehydration, a high-protein diet, certain medications, and even gastrointestinal bleeding can all push BUN higher without any real change in kidney function. That’s why doctors look at BUN alongside creatinine rather than relying on it alone. The ratio between the two values can help distinguish a kidney problem from other causes.
Cystatin C: A More Precise Alternative
For some people, creatinine-based estimates don’t tell the full story. That’s where cystatin C comes in. This is a small protein produced at a steady rate by virtually all cells in the body, not just muscle cells. Because its production doesn’t depend on muscle mass, physical activity, or diet, it gives a cleaner signal of how well the kidneys are filtering.
Research published in the American Journal of Kidney Diseases has shown that cystatin C correlates more closely with direct measurements of kidney filtration than creatinine does, particularly in populations where creatinine can be misleading. People with very high or very low muscle mass, older adults losing muscle with age, and those with unusual diets all tend to get more accurate eGFR results when cystatin C is factored in. The National Kidney Foundation now recommends that labs use equations incorporating both creatinine and cystatin C when a more precise estimate is needed.
Cystatin C isn’t ordered as routinely as creatinine. You’re most likely to see it used when a creatinine-based eGFR falls in a borderline range and your doctor wants to confirm whether kidney function is truly reduced before starting treatment or making a diagnosis.
The Full Renal Function Panel
When your doctor orders a comprehensive look at kidney health, you’ll get more than just creatinine and BUN. A renal function panel includes a set of electrolytes that your kidneys are responsible for balancing: sodium, potassium, chloride, bicarbonate, calcium, and phosphorus. The panel also calculates something called the anion gap, which reflects the balance between positively and negatively charged particles in your blood.
These electrolytes matter because your kidneys do far more than just filter waste. They regulate the concentration of minerals in your blood, control acid-base balance, and manage fluid levels. When kidney function declines, potassium can climb to dangerous levels, phosphorus can rise while calcium drops, and the acid-base balance can shift. Tracking these values helps doctors catch complications early, especially in people already diagnosed with CKD.
What Can Affect Your Results
Several non-kidney factors can influence the numbers on your lab report, and understanding them helps you avoid unnecessary worry. High muscle mass raises creatinine. So does a meal heavy in red meat eaten within hours of the test. Creatine supplements will do the same. On the other hand, people with very low muscle mass, including older adults and those with chronic illness, may show deceptively normal creatinine levels even when kidney function is declining. Their muscles simply aren’t producing enough creatinine to trigger an abnormal reading.
Dehydration is one of the most common causes of temporarily elevated creatinine and BUN. If you haven’t been drinking enough water before the test, your results may look worse than your actual kidney function warrants. Intense exercise in the hours before a blood draw can also spike creatinine briefly. For the most accurate results, follow any fasting instructions from your doctor, stay reasonably hydrated, skip heavy workouts the morning of the test, and mention any supplements you take.
A single abnormal result doesn’t necessarily mean kidney disease. Doctors typically repeat the test after a few weeks to see whether the values persist. Kidney disease is defined by sustained abnormalities, not a one-time reading.