The two most important blood tests for kidney function are serum creatinine and estimated glomerular filtration rate (eGFR). These are typically ordered together as part of a basic or comprehensive metabolic panel, and they tell you how well your kidneys are filtering waste from your blood. Several other blood tests, including blood urea nitrogen (BUN), cystatin C, and electrolyte levels, fill in the picture when more detail is needed.
Serum Creatinine: The Core Test
Creatinine is a waste product your muscles generate constantly as they use energy. Healthy kidneys filter it out of the blood and send it into your urine. When kidney function declines, creatinine builds up in the bloodstream, so a higher-than-normal level is one of the earliest signs something is off.
Normal serum creatinine ranges are 0.74 to 1.35 mg/dL for adult men and 0.59 to 1.04 mg/dL for adult women. These ranges differ because men generally carry more muscle mass, which produces more creatinine. That’s also why this test has limitations: a very muscular person may have slightly elevated creatinine without any kidney problem, while someone with low muscle mass (older adults, people with certain chronic illnesses) can have “normal” creatinine even when their kidneys are struggling.
Vigorous exercise and high-protein meals can temporarily raise creatinine levels. Your doctor may ask you to avoid strenuous activity for 24 hours before the blood draw, and some labs recommend fasting for 8 to 12 hours, though requirements vary by test panel.
eGFR: What Your Creatinine Number Actually Means
A creatinine level on its own doesn’t tell you how much filtering power your kidneys have. That’s where eGFR comes in. It takes your creatinine result and runs it through a formula that factors in your age and sex to estimate how many milliliters of blood your kidneys clean per minute. You don’t need a separate blood draw for this. The lab calculates it automatically from the same creatinine sample.
The current standard is the 2021 CKD-EPI equation, which was a significant update because it removed race as a variable. Previously, the formula adjusted results based on whether a patient was identified as Black, a practice that the National Kidney Foundation and the American Society of Nephrology determined was scientifically unjustified since race is a social construct, not a biological one. All U.S. labs now use the race-free version.
eGFR is measured in mL/min/1.73 m² and maps directly to stages of chronic kidney disease (CKD):
- 90 or above: Normal kidney function (Stage 1 if other signs of damage exist)
- 60 to 89: Mildly reduced function (Stage 2)
- 30 to 59: Moderately reduced function (Stage 3)
- 15 to 29: Severely reduced function (Stage 4)
- Below 15: Kidney failure (Stage 5)
A single low reading doesn’t necessarily mean you have kidney disease. Doctors look for a pattern over at least three months before making a diagnosis, because temporary factors like dehydration, medications, or illness can push eGFR down temporarily.
Blood Urea Nitrogen (BUN)
BUN measures how much urea, another waste product, is circulating in your blood. Your liver produces urea when it breaks down protein, and your kidneys are responsible for clearing it. When they can’t keep up, BUN rises.
The relationship between BUN and creatinine is often more informative than either number alone. In typical kidney disease, BUN and creatinine rise together in roughly a 10-to-1 ratio. If BUN shoots up far beyond that ratio while creatinine stays relatively stable, the problem is more likely dehydration, a high-protein diet, or gastrointestinal bleeding rather than kidney damage itself. A ratio lower than expected can point to liver disease, since the liver is producing less urea, or to very low protein intake.
BUN is almost always included in a standard metabolic panel, so you’ll usually get this result alongside creatinine without requesting it separately.
Cystatin C: A More Precise Alternative
Cystatin C is a small protein produced by virtually every cell in your body at a steady rate. Unlike creatinine, its levels aren’t significantly affected by muscle mass, age, sex, body size, or nutritional status. That makes it especially useful for people whose creatinine results may be misleading.
Your doctor is most likely to order cystatin C in a few specific situations. If your creatinine-based eGFR falls between 45 and 60 and you have no other signs of kidney damage, a cystatin C test can confirm whether you truly have early kidney disease or whether your creatinine is simply running high for non-kidney reasons. It’s also the preferred marker for people with conditions that alter creatinine production, including sickle cell disease, muscular dystrophy, spina bifida, and patients undergoing chemotherapy who may have reduced muscle mass.
When cystatin C is available, labs can calculate eGFR using a combined creatinine-cystatin C equation, which is considered the most accurate non-invasive estimate of kidney function currently available.
Electrolytes: Potassium and Phosphorus
Healthy kidneys regulate the balance of minerals in your blood. As kidney function declines, certain electrolytes start drifting out of their normal range, and tracking them becomes part of routine monitoring.
Potassium is one of the first to shift. Damaged kidneys struggle to excrete excess potassium, leading to elevated blood levels that can cause dangerous heart rhythm problems. Phosphorus follows a similar pattern. Normal blood phosphorus sits between 2.5 and 4.5 mg/dL, and levels above that range may prompt testing for kidney disease if it hasn’t already been diagnosed. In people with known CKD, high phosphorus accelerates bone loss and contributes to hardening of the arteries, so it’s checked regularly.
These electrolyte tests don’t diagnose kidney disease on their own, but abnormal results on a routine panel sometimes serve as the first clue that something is wrong.
Hemoglobin and Anemia Screening
Your kidneys produce a hormone that signals your bone marrow to make red blood cells. As kidney function drops, production of this hormone falls, and anemia develops. This is one of the reasons people with advancing kidney disease feel persistently fatigued, short of breath, or dizzy.
A complete blood count (CBC) measures hemoglobin, the oxygen-carrying protein in red blood cells. Hemoglobin below about 12.0 g/dL in the context of kidney disease points to this specific type of anemia. It’s not typically an early marker, since it tends to appear as CKD reaches moderate to advanced stages, but it’s part of the broader picture doctors monitor once kidney problems are established.
What a Standard Panel Includes
If your doctor orders kidney function tests, you’ll most likely get a basic metabolic panel (BMP) or comprehensive metabolic panel (CMP). Both include serum creatinine, eGFR (calculated automatically), and BUN. A CMP adds liver enzymes and a few extra proteins. If your results raise questions, follow-up tests like cystatin C, a full electrolyte panel, or a CBC may be added.
Urine tests, particularly the urine albumin-to-creatinine ratio, are often ordered alongside blood work because they detect protein leaking into the urine, a sign of kidney damage that blood tests alone can miss. Together, blood and urine results give the most complete snapshot of how your kidneys are performing.