Kidney function tests are a group of blood and urine tests that measure how well your kidneys are filtering waste and maintaining the balance of key substances in your body. The most common version is a renal function panel, which uses a single blood draw to check up to nine markers at once. Some situations also call for urine tests that detect early signs of kidney damage before symptoms ever appear.
What a Renal Function Panel Includes
A standard renal function panel measures several substances in your blood, each revealing something different about how your kidneys are performing. The core markers are creatinine, blood urea nitrogen (BUN), and glucose. The panel also checks electrolytes: sodium, potassium, chloride, and bicarbonate. Albumin (a protein made by your liver) and calcium round out the list.
Your doctor may order this panel as part of a routine checkup, to monitor an existing condition like diabetes, or to investigate symptoms such as swelling, fatigue, or changes in urination. The test requires a standard blood draw from your arm, and results are typically available within a day or two.
Creatinine and eGFR: The Core Kidney Markers
Creatinine is a waste product your muscles produce at a fairly steady rate. Healthy kidneys filter it out continuously, so when creatinine builds up in your blood, it signals that filtration is slowing down. Normal serum creatinine falls between 0.74 and 1.35 mg/dL for adult men and 0.59 to 1.04 mg/dL for adult women.
Rather than interpreting creatinine alone, labs use it to calculate your estimated glomerular filtration rate, or eGFR. This number estimates how many milliliters of blood your kidneys clean per minute, adjusted for your age, sex, and body size. An eGFR of 90 or above is normal. Below that, the number maps to specific stages of chronic kidney disease:
- Stage 1: eGFR 90 or higher, but with other evidence of kidney damage (such as protein in the urine) persisting for three months or more
- Stage 2: eGFR 60 to 89 with kidney damage present for three months or more
- Stage 3a: eGFR 45 to 59 for three months or more
- Stage 3b: eGFR 30 to 44 for three months or more
- Stage 4: eGFR 15 to 29 for three months or more
- Stage 5: eGFR below 15, or on dialysis
One important caveat: creatinine levels are influenced by muscle mass, diet, and certain medications. A very muscular person can have higher creatinine without any kidney problem, while someone with low muscle mass may show deceptively normal levels even when kidney function has declined. Because of these limitations, UK and international guidelines recommend confirming borderline results (eGFR 45 to 59 with no protein in the urine) using a different blood marker called cystatin C, which is not affected by muscle mass the same way creatinine is.
BUN and the BUN-to-Creatinine Ratio
Blood urea nitrogen measures the amount of urea, another waste product, circulating in your blood. Your liver produces urea when it breaks down protein, and your kidneys excrete it. A rising BUN can point to kidney trouble, but it can also spike from dehydration, a high-protein diet, or gastrointestinal bleeding. BUN alone is not a reliable indicator of kidney health.
The BUN-to-creatinine ratio adds context. A normal ratio falls between 10 and 20. When the ratio climbs above 20, it often suggests a “prerenal” cause, meaning something is reducing blood flow to the kidneys (like dehydration or heart failure) rather than damage to the kidneys themselves. This distinction helps your doctor decide what to investigate next.
What Electrolytes Reveal About Your Kidneys
Your kidneys regulate electrolytes by adjusting how much gets reabsorbed or excreted. When kidney function declines, these levels drift out of their normal ranges, which can cause symptoms ranging from muscle cramps to dangerous heart rhythm changes.
The key electrolytes on the panel and their normal ranges are sodium (135 to 145 mmol/L), potassium (3.6 to 5.5 mmol/L), and bicarbonate (22 to 29 mmol/L). High potassium is one of the more concerning findings in kidney disease because it can affect heart rhythm. Low bicarbonate signals that the kidneys are struggling to neutralize acid in the blood, a condition called metabolic acidosis that becomes more common in later stages of kidney disease.
Urine Tests: Catching Damage Early
Blood tests tell you how well your kidneys are filtering right now. Urine tests can catch damage that’s already started but hasn’t yet lowered your eGFR. The most important urine test is the urine albumin-to-creatinine ratio, or uACR, which measures tiny amounts of a protein called albumin leaking into your urine.
Healthy kidneys keep albumin in the bloodstream. When the kidney’s filtering units are injured, albumin slips through. A uACR below 30 mg/g is normal. A result between 30 and 299 mg/g indicates moderately increased albumin loss and raises your risk of kidney failure, heart failure, and stroke. A uACR of 300 mg/g or higher, confirmed on a repeat test, points to significant kidney disease and severe cardiovascular risk.
This test is particularly valuable for people with diabetes or high blood pressure, where kidney damage can progress silently for years. A simple spot urine sample is all that’s needed.
The 24-Hour Urine Collection
In some cases, your doctor may ask for a 24-hour urine collection instead of a spot sample. This involves collecting every drop of urine you produce over a full 24-hour period in a special container that must be kept cold. The results give a more precise measurement of how much creatinine (and sometimes protein or other substances) your kidneys are clearing.
If you’re asked to do one, try to choose a day when you’ll be home. Accuracy depends on capturing every sample during the window and keeping the container refrigerated. Forgetting even one bathroom trip, going past the 24-hour mark, or spilling part of the collection can throw off results. Vigorous exercise and acute stress during the collection period can also skew numbers, as can certain foods like coffee, bananas, and citrus fruits. Your provider will let you know if any dietary restrictions apply.
Preparing for Your Test
Whether you need to fast before a renal panel depends on what else your doctor is testing alongside it. If the panel includes a glucose measurement or is bundled with a basic metabolic panel, fasting for 8 to 12 hours is common. Your provider will give specific instructions.
Let your provider know about all medications, supplements, and vitamins you take before the test. Some can alter creatinine, BUN, or electrolyte levels and lead to misleading results. Don’t stop taking prescription medications on your own, but do mention everything so your doctor can interpret the numbers in context.
What Abnormal Results Mean in Practice
A single abnormal result rarely means you have kidney disease. Creatinine can spike temporarily from intense exercise, a protein-heavy meal, or dehydration. Electrolytes fluctuate with what you’ve eaten and how hydrated you are. Doctors look at the pattern across multiple markers and, when something looks off, repeat the test after a few weeks or months before drawing conclusions.
If your eGFR is persistently below 60 or your uACR stays at 30 mg/g or higher on repeat testing, that meets the clinical definition of chronic kidney disease. At that point, monitoring becomes more frequent, and management focuses on protecting the kidney function you still have, typically by controlling blood pressure, managing blood sugar if you have diabetes, and adjusting your diet.
For people with no known risk factors, kidney function tests are often part of a standard annual blood panel. For those with diabetes, high blood pressure, a family history of kidney disease, or existing heart disease, testing at least once a year with both a blood panel and a uACR is standard practice.