The two core blood tests for kidney function are serum creatinine and blood urea nitrogen (BUN). These are included in routine blood panels and give a snapshot of how well your kidneys are filtering waste. But several other blood markers round out the picture, from estimated glomerular filtration rate (eGFR) to electrolytes like potassium and phosphorus. Here’s what each test measures and what your results actually tell you.
Serum Creatinine
Creatinine is a waste product left over from normal energy production in your muscles. It enters your bloodstream at a fairly constant rate, and healthy kidneys filter it out and send it into your urine. Because the supply is so steady, a rising creatinine level is one of the earliest signals that your kidneys aren’t keeping up with their filtering job.
Your doctor may order a creatinine test to investigate symptoms that could point to kidney trouble, to screen for kidney disease if you have diabetes or high blood pressure, or to track whether existing kidney disease is stable or getting worse. Normal creatinine ranges vary by age, sex, and muscle mass, so a single number doesn’t tell the whole story on its own. That’s where eGFR comes in.
Estimated Glomerular Filtration Rate (eGFR)
Your eGFR isn’t a separate blood draw. It’s a calculation that uses your creatinine result along with your age and sex to estimate how many milliliters of blood your kidneys filter per minute. The current standard is the CKD-EPI creatinine equation from 2021, which the National Kidney Foundation and American Society of Nephrology recommend as a race-free formula.
eGFR is the number doctors use to stage chronic kidney disease:
- Stage 1 (G1): eGFR above 90 ml/min, but other signs of kidney damage are present
- Stage 2 (G2): eGFR of 60 to 89, with other signs of damage
- Stage 3a (G3a): eGFR of 45 to 59
- Stage 3b (G3b): eGFR of 30 to 44
- Stage 4 (G4): eGFR of 15 to 29
- Stage 5 (G5): eGFR below 15, meaning the kidneys have lost almost all function
An eGFR above 90 with no other markers of damage is generally considered normal. Notice that stages 1 and 2 both require additional evidence of kidney problems (like protein in the urine) because eGFR alone can be normal or near-normal even when early damage is present.
Blood Urea Nitrogen (BUN)
BUN measures the amount of urea nitrogen in your blood. Urea is another waste product your kidneys are responsible for clearing. Like creatinine, it rises when your kidneys aren’t filtering efficiently. BUN is included alongside creatinine in both the basic metabolic panel (BMP) and the comprehensive metabolic panel (CMP), so you’ve likely had it checked even if you didn’t realize it.
One useful detail is the BUN-to-creatinine ratio. The normal range is roughly 10 to 20. A ratio above 20 can suggest that something other than kidney disease itself is driving the problem, often dehydration or reduced blood flow to the kidneys. This helps doctors distinguish between kidneys that are structurally damaged and kidneys that are temporarily underperforming because the body is low on fluid or blood pressure has dropped.
Cystatin C
Cystatin C is a protein produced by cells throughout your body at a relatively steady rate, and like creatinine, healthy kidneys filter it out. The advantage of cystatin C is that it isn’t affected by muscle mass, age, or sex the way creatinine is. That makes it especially useful for people whose creatinine levels might be misleading: older adults who have lost muscle, very muscular individuals, people with amputations, or anyone whose body composition falls outside average ranges.
Cystatin C is typically used as a confirmatory test. If your creatinine-based eGFR puts you near a decision point, like the boundary between stage 2 and stage 3, your doctor may order a cystatin C test to get a more precise estimate. An eGFR calculated from both creatinine and cystatin C together is generally accepted as more accurate than either marker alone. The 2021 CKD-EPI guidelines include a combined creatinine-cystatin equation for exactly this purpose.
Electrolytes: Potassium, Phosphorus, and Calcium
Your kidneys do far more than remove waste. They also regulate the balance of key minerals in your blood. When kidney function declines, these minerals can drift out of their normal ranges, sometimes with serious consequences.
Potassium is one of the most closely watched. Healthy kidneys keep potassium in a tight range, but damaged kidneys may let it build up. High potassium can affect your heart rhythm, which is why it’s monitored carefully in people with kidney disease.
Phosphorus follows a similar pattern. The kidneys filter excess phosphate from the blood and excrete it in urine. In the later stages of chronic kidney disease and kidney failure, damaged kidneys can’t clear phosphate fast enough, and levels rise. High phosphate pulls calcium out of bones over time, weakening them. Phosphate levels are tightly linked to calcium, vitamin D, and parathyroid hormone, so doctors often check all four together to understand what’s driving an abnormal result.
Calcium itself can shift when kidneys struggle to activate vitamin D, which your body needs to absorb calcium from food. Low calcium triggers the parathyroid glands to release more hormone, creating a cascade that eventually harms bone density. These mineral imbalances rarely cause obvious symptoms until they’re fairly advanced, which is why routine blood work catches them before you’d notice anything wrong.
Albumin
Albumin is the most abundant protein in your blood. It keeps fluid inside your blood vessels and carries hormones and nutrients throughout your body. Healthy kidneys don’t let albumin pass through their filters, but damaged kidneys can leak it into your urine, gradually lowering the amount in your blood. A lower-than-normal blood albumin level can be a sign of kidney disease, though it can also drop with liver disease, malnutrition, or chronic inflammation.
When albumin drops low enough, fluid can seep out of blood vessels and accumulate in the lungs, abdomen, or legs. This is one reason doctors check albumin both in the blood (to see how much you have) and in the urine (to see how much you’re losing). The blood test is part of a comprehensive metabolic panel; the urine test is often ordered separately.
BMP vs. CMP: Which Panel Checks Kidneys
If your lab order says “basic metabolic panel,” that includes eight substances: creatinine, BUN, glucose, calcium, and four electrolytes (sodium, potassium, chloride, and bicarbonate). That covers the essential kidney markers. A comprehensive metabolic panel includes all eight BMP tests plus six more that measure liver enzymes and proteins like albumin. Your doctor may choose a CMP to get a broader view of organ health in a single draw.
Neither panel includes cystatin C or phosphorus. Those are ordered separately when there’s a specific reason to check them, like monitoring someone with established kidney disease or investigating an unexpected creatinine result.
What Can Affect Your Results
Creatinine levels naturally vary with muscle mass. A bodybuilder and a small-framed older adult could have very different “normal” creatinine values, even with equally healthy kidneys. Dehydration temporarily raises both BUN and creatinine because there’s less fluid diluting those waste products in your blood, though BUN tends to spike more, which is why the BUN-to-creatinine ratio helps flag dehydration.
Certain medications can also interfere with creatinine test results. Some drugs cause falsely low readings, which could mask a real kidney problem, while others push readings artificially high. If your results seem inconsistent with how you feel or with other test findings, your doctor may retest or switch to cystatin C for a cleaner picture. A high-protein meal the night before a blood draw can temporarily nudge BUN upward, though most kidney panels don’t require fasting unless other tests on the same order do.