Kidney function tests, line by line

In shortThe kidney block answers two questions: how well the kidneys filter (creatinine, estimated GFR, the Rehberg clearance test) and whether they leak protein (microalbuminuria, 24-hour proteinuria). Urea and uric acid are supporting lines. The main reading principle: filtration and protein are independent axes, and kidneys cannot be judged by creatinine alone.

Creatinine and GFR: filtration in numbers

Creatinine is a product of muscle metabolism excreted by the kidneys at a nearly constant rate. From its level the laboratory calculates the estimated glomerular filtration rate (eGFR) — the main integral assessment of kidney work. Chronic kidney disease guidance builds staging precisely on eGFR combined with albuminuria.

Creatinine has blind spots: it depends on muscle mass, so in a frail elderly person filtration can be reduced at a formally normal creatinine, and in an athlete the reverse. The estimated GFR partially smooths this, but at extreme body builds the clinician relies on additional methods.

The Rehberg test: measured filtration

When the estimated GFR is not enough, filtration is measured directly with the Rehberg test: urine is collected over 24 hours, creatinine is measured in urine and blood, and clearance is computed — the volume of plasma the kidneys clean per minute. It is laborious but a more direct way to estimate filtration, especially at the edges of the range.

The test's accuracy lies entirely in collection: a missed portion or a wrongly recorded volume distorts the calculation. The rules are the same as for any 24-hour collection: all portions into one container, kept cool, a poured-off portion with the total volume noted.

The protein axis: microalbuminuria and 24-hour proteinuria

A healthy kidney filter barely lets protein through. Microalbuminuria catches the earliest albumin leaks — years before creatinine rises, especially in diabetes and hypertension. Chronic kidney disease guidance makes albuminuria the second staging axis alongside GFR: risk is read from the intersection of the two axes.

Twenty-four-hour proteinuria is the quantitative version of the same idea: how much protein is lost per day. It is used when the leak is already significant and for therapy monitoring. An intermediate option is the protein- or albumin-to-creatinine ratio in a spot portion — it approximates the 24-hour collection without collecting for a day.

Urea and uric acid: the supporting lines

Urea is a product of protein breakdown, and unlike creatinine it is sensitive to diet, hydration and liver state. A rise in urea with normal creatinine speaks more often of dehydration or protein load than of kidney disease; the pair is always read together.

Uric acid is the end product of purine metabolism, excreted renally. Its sustained rise (hyperuricemia) is a risk factor for gout and kidney stones, described in gout guidelines. In the kidney block it is more of a companion: on its own it says little about filtration.

When the kidney block makes sense without complaints

Diabetes and hypertension — regular monitoring per guidelines
Long-term use of medications that affect the kidneys
Swelling, changed urine colour, foamy urine
A family history of kidney disease

Markers covered in this article

Creatinine BUN (blood urea nitrogen)Creatinine clearance Microalbuminuria 24-hour urine protein Uric acid Zimnitsky test

Several markers on your report?

See how MedProtect explains results in the app: the whole report, a check of the recognized values, and a clear breakdown.

Get your results explained See an example

Frequently asked questions

Creatinine is normal — are the kidneys healthy?

Not necessarily. Creatinine depends on muscle mass and lags behind: filtration can be reduced at a formally normal value. Full assessment is estimated GFR plus albuminuria, with a clearance test in doubtful cases.

What matters more — GFR or urine protein?

They are two independent axes of risk. GFR shows how much the kidneys filter; albuminuria shows how damaged the filter is. Guidelines assess their intersection, not one line.

Urea is high, creatinine normal — what is it?

Most often dehydration, protein load or medication effects. Such a combination is rechecked after normalizing fluid intake and reviewed with the clinician alongside the other lines.

Why a clearance test if estimated GFR exists?

Estimated GFR is a formula from blood creatinine; it is imprecise at extreme body builds and at the edges of the range. The Rehberg test measures clearance directly from 24-hour urine — a second instance for disputed cases.

How to prepare for kidney tests?

Blood is drawn fasting, without alcohol the day before and without hard workouts for a couple of days — exertion raises creatinine. Timed collections follow the laboratory's instructions, with no missed portions.

Foamy urine — is it always protein?

Not always: foam occurs with rapid urination and concentrated urine. Persistent foam is a reason to check protein: first the routine analysis, and if confirmed, microalbuminuria or 24-hour proteinuria.

Sources

Chronic kidney disease — clinical guideline context — Minzdrav of Russia (cr.minzdrav.gov.ru). Accessed: August 29, 2026.
Creatinine clearance test: MedlinePlus Medical Encyclopedia — MedlinePlus, US National Library of Medicine. Accessed: August 31, 2026.
BUN - blood test: MedlinePlus Medical Encyclopedia — MedlinePlus, US National Library of Medicine. Accessed: August 30, 2026.

Kidney function tests, line by line: creatinine, BUN, GFR and urine protein