Kidney Function Test Guide: BUN, Creatinine, and eGFR

What Kidney Function Tests Measure

The kidneys filter waste from the blood, balance fluids and electrolytes, and help control blood pressure. Kidney function testing does not measure one single thing. It uses several markers to estimate how well the kidneys are working and whether they are leaking anything they should keep in the blood. Because the kidneys are quiet organs, early changes often show up on lab tests before any symptoms appear.

The Core Blood Markers

Creatinine

Creatinine is a waste product from normal muscle activity. Healthy kidneys remove it from the blood, so a rising level can suggest reduced filtering. The result depends partly on muscle mass, age, and sex, which is why it is always compared with a personal reference range.

Blood urea nitrogen

Blood urea nitrogen, or BUN, is another waste product that the kidneys clear. It can rise with dehydration, a high protein load, or reduced kidney function. BUN is most useful when read next to creatinine.

Estimated glomerular filtration rate

The estimated glomerular filtration rate, or eGFR, is calculated from creatinine along with age and sex. It estimates how much blood the kidneys filter each minute. A lower eGFR generally suggests reduced filtering, and clinicians sometimes use it to describe stages of kidney function.

Urine-Based Kidney Tests

Blood tests are only part of the picture. Urine tests can detect albumin, a protein that should mostly stay in the blood. When small amounts of albumin appear in urine, it can be an early sign that the kidneys are under strain, especially in people with diabetes or high blood pressure. A urine albumin to creatinine ratio is a common way to measure this. A urinalysis may also look for blood, cells, or casts that hint at kidney or urinary problems.

Why a Clinician Orders Kidney Testing

Testing may be ordered to monitor a long term condition such as diabetes or hypertension, to investigate swelling, fatigue, or changes in urination, or to check for the effects of medications that can stress the kidneys. It is also part of many routine health panels. People with a family history of kidney disease may be tested periodically as well.

How Samples Are Collected

Blood markers come from a routine blood draw, usually from a vein in the arm. Urine tests may use a random sample collected at the lab or a timed collection over several hours, depending on what is being measured. For some tests, a clinician may ask you to drink water beforehand so a sample is available. Follow the lab's instructions for the specific order.

How Results Are Reported

Results are reported as values with reference ranges, and the ranges vary by laboratory, age, and sex. Creatinine and BUN are reported as numbers. eGFR is reported as a single estimate. Urine albumin may be reported as a ratio. Instead of comparing your numbers with someone else's, look at how they fall within your own report's ranges and how they change over time. Trends are often the most informative part of kidney monitoring.

Why Trends Matter More Than One Number

Kidney markers can move for temporary reasons, including dehydration, a recent illness, or a new medication. That is why a single result is rarely enough to define kidney health. Clinicians often compare results over months or years, watching whether values are stable, improving, or drifting.

For people with diabetes or high blood pressure, regular monitoring is especially useful because it can catch changes before symptoms appear. A steady trend within the expected range is usually reassuring, while a consistent change in one direction is what prompts a closer look.

Preparing for the Test

Ask whether fasting or a specific collection schedule applies. Some kidney tests are combined with a metabolic panel that may require fasting. Tell the lab about all medications, since certain drugs can affect kidney markers. If you are being monitored over time, bring prior results so the trend can be reviewed.