Kidney Function Tests Explained
Last updated 2026-10-01
What the kidneys do
Your kidneys filter waste and extra fluid from the blood and help control blood pressure, electrolytes, and red blood cell production. Kidney testing checks both how well they filter and whether anything is leaking that should stay in the blood.
Creatinine and eGFR
Creatinine is a waste product from normal muscle activity. Healthy kidneys remove it steadily, so a higher blood level can signal reduced filtering. Labs use creatinine along with age and sex to calculate an estimated glomerular filtration rate, or eGFR. The eGFR is a better summary of kidney function than creatinine alone, and it is grouped into stages to describe how much filtering remains.
Creatinine also depends on muscle mass. A muscular person may run a bit higher, while a frail person may run lower even with reduced function. The eGFR accounts for some of this.
BUN
Blood urea nitrogen is another waste product the kidneys clear. It is less specific than creatinine and is affected by hydration, diet, and other conditions. BUN is most useful when read next to creatinine and the eGFR.
Urine protein and albumin
Filters should hold protein in the blood. When protein or its main component, albumin, appears in urine, it can be an early sign of kidney stress. A urine albumin-to-creatinine ratio is often used because it corrects for how dilute the sample is. For some conditions, this ratio rises before the eGFR falls.
Urinalysis
A simple urine test can add useful signals, including blood, protein, white cells, and other findings. It is often paired with the blood tests to complete the picture.
Putting it together
- eGFR tells you roughly how much filtering remains.
- Urine protein tells you whether the filters are leaking.
- BUN and electrolytes add context about fluid and balance.
Many findings are mild and stable, while others need attention. Diabetes and high blood pressure are the most common long-term threats to kidney function, which is why both are managed carefully. Clinicians may repeat testing to confirm a trend before acting.
How kidney tests are ordered
Kidney function is usually checked with a blood draw, sometimes paired with a urine sample. No fasting is typically required. The tests are ordered for routine screening, to monitor a condition such as diabetes or high blood pressure, or to look into swelling, fatigue, or changes in urination.
Why the estimate, not just creatinine
Creatinine alone rises and falls with muscle mass, so labs combine it with age and sex to estimate filtration. That estimate is the more useful summary. Because it is an estimate, a value near a stage boundary is often rechecked before it is treated as a firm change.
Following a trend over time
A single result is a snapshot. Doctors watch the direction across months or years, alongside urine protein, to see whether function is stable or shifting. Mild findings that hold steady are common and need less concern than a steady decline. Interpretation belongs with a clinician.
This article is general information, not medical advice; talk with a clinician about your results.