Kidney Values Explained: Creatinine, GFR, Urea

TL;DR: Creatinine below 1.3 mg/dl (men) or 1.1 mg/dl (women), eGFR above 90 ml/min/1.73 m², BUN 7 to 20 mg/dl (US units). eGFR below 60 for more than 3 months defines chronic kidney disease. Muscle mass, protein intake, dehydration and creatine intake can distort creatinine. Cystatin C is muscle-independent and often more precise.

This article is for information only and does not replace medical advice. Intake, the dose set by a doctor, and therapy for medications and dietary supplements belong in medical hands. If eGFR is below 60, there is proteinuria or values are rising, a nephrology workup is appropriate.

What Kidney Values Actually Show

Kidney values do not measure kidney health directly. They estimate how well the kidneys filter waste products from the blood. Two layers matter: filtration and damage markers.

Filtration capacity. Each kidney contains roughly 1 million nephrons. These tiny filters remove metabolic waste from the blood and produce urine. The glomerular filtration rate (GFR) measures how many milliliters of blood are filtered per minute. Normal: above 90 ml/min/1.73 m². Creatinine and cystatin C serve as estimators for GFR.

Damage markers. When nephrons are damaged, albumin leaks into the urine. The urine albumin/creatinine ratio (ACR) detects early damage long before GFR drops. Values below 30 mg/g are normal, 30 to 300 mg/g mean microalbuminuria, above 300 mg/g macroalbuminuria.

Practical example: creatinine is 1.1 mg/dl, eGFR 82, BUN 18 mg/dl. Looks normal at first glance. But the urine albumin-to-creatinine ratio is 45 mg/g — moderately increased albuminuria (category A2). A single value is not a diagnosis yet: the test gets repeated and a doctor checks the cause. Without the urine test, the finding would stay hidden. In Lab2go all four values can be tracked over time, so the trend is at hand for the conversation with a doctor.

The 5 Key Kidney Markers

For the full picture across all markers, the guide on understanding blood values helps.

Creatinine: The Classic With Caveats

Creatinine comes from the breakdown of creatine phosphate in muscles. About 2 percent of muscle creatine converts to creatinine daily and is excreted via the kidneys. The more muscle mass, the higher the baseline.

The muscle mass problem. A 95 kg strength athlete produces roughly twice as much creatinine as a 55 kg office worker — with identical kidney function. The eGFR formula corrects for age and sex but not muscle mass. So creatinine underestimates GFR in very muscular people and overestimates it in seniors with sarcopenia.

Other influencing factors:

• Dehydration: creatinine rises 10 to 20 percent when fluid intake is low.
• Protein intake: a high-protein meal 12 hours before the test can raise creatinine by 0.1 to 0.2 mg/dl.
• Intense training: 48 hours after hard exercise, creatinine can rise up to 30 percent.
• Creatine intake: studies describe a rise in serum creatinine of 0.1 to 0.3 mg/dl. This is considered a lab artifact, not kidney damage.

Practical scenario: creatinine was 0.9 mg/dl in May and 1.2 mg/dl in October. In between came 4 months of strength training, a very high-protein diet and creatine intake. Such a change can be explained by lifestyle, but that is for a doctor to judge. For clarification, cystatin C can be measured or the test repeated under comparable conditions.

eGFR: The Number That Counts

The estimated glomerular filtration rate (eGFR) is the most important single value for kidney function. The modern formula CKD-EPI 2021 uses creatinine, age and sex. One variant uses cystatin C, a third combines both.

CKD stages by eGFR:

CKD is only diagnosed when eGFR stays below 60 for at least 3 months or additional damage markers (albuminuria, imaging findings) are present. A single low value is not a diagnosis. The trend matters.

Note: eGFR declines naturally with age. Per decade from 40 onward, about 8 ml/min/1.73 m² are lost. A 70-year-old with eGFR 68 does not necessarily have kidney disease. That can be age-physiological.

Urea and the BUN/Creatinine Ratio

Urea (in the US labeled BUN, Blood Urea Nitrogen) is the end product of protein metabolism. The liver produces urea, the kidneys excrete it. Unlike creatinine, urea fluctuates strongly with diet and hydration.

Reference range: 7 to 20 mg/dl (BUN, US units) or 17 to 43 mg/dl (urea, EU units).

The BUN/creatinine ratio is more informative than any single value. Normal is 10:1 to 20:1.

• Ratio above 20:1: prerenal cause. Dehydration, heart failure or blood loss. Kidneys are healthy but blood flow is reduced.
• Ratio below 10:1: low protein intake, liver insufficiency (less urea production) or overhydration.

Practical example: after a 24-hour fast with low fluid intake, the panel shows BUN 55 mg/dl, creatinine 1.0 mg/dl, ratio 55. That points to dehydration, not a kidney problem. With adequate fluid intake the value usually normalizes again.

For meaningful lab results, fluid intake in the 24 hours before the blood draw, high-protein meals the evening before and intense training in the last 48 hours are usually taken into account. The practice gives preparation instructions; general details are in the biomarker baseline checklist.

Cystatin C: The Better Marker for Athletes and Seniors

Cystatin C is a small protein produced by all nucleated cells. Production is largely constant and independent of muscle mass, age (across decades) or diet. The kidneys filter cystatin C and break it down — well suited for GFR estimation.

When cystatin C beats creatinine:

• Extreme muscle mass: bodybuilders, strength athletes above 85 kg lean mass.
• Sarcopenia: seniors over 70 with low muscle mass.
• Vegans and vegetarians: low creatinine baseline due to low meat intake.
• Grey zone eGFR 45 to 75: cystatin C-based formula is more precise.
• Amputees or people with neuromuscular disorders.

The CKD-EPI Cys 2021 calculator combines cystatin C with age and sex for a more precise eGFR. Cost: 15 to 30 euros as an out-of-pocket test. With diabetes, hypertension or creatine intake, cystatin C can be used as an additional marker; whether and how often is decided by the treating doctor.

Influencing Factors: What Can Move Your Values

Six factors distort kidney values most often. In Lab2go they can be logged with every measurement to avoid misinterpretation.

Muscle mass. Most important factor for creatinine. Muscle mass changes over years — and so does baseline creatinine. A strength athlete gaining 5 kg of muscle in 2 years may see creatinine move from 1.0 to 1.2 mg/dl. That is physiological.

Protein intake. A high-protein meal 12 hours before the test raises both creatinine and urea. Studies describe that after a beef steak meal, creatinine rises by 0.1 to 0.2 mg/dl and urea by 8 to 15 mg/dl.

Hydration. Dehydration raises creatinine by 10 to 20 percent and urea by up to 50 percent.

Training. Intense strength or endurance work in the last 48 hours raises creatinine through muscle breakdown. Marathon runners show creatinine 1.5 to 2 times the upper limit directly after a race. After 72 hours of rest, values normalize.

Creatine intake. Studies describe a rise in serum creatinine of 0.1 to 0.3 mg/dl. The effect is considered a lab artifact, not reduced filtration. Long-term studies of up to 5 years found no signs of kidney damage in healthy users. General background on supplements is in the supplement beginners guide.

Medications. NSAIDs (ibuprofen, diclofenac, naproxen), ACE inhibitors, ARBs, lithium, certain antibiotics and contrast agents influence kidney values. Studies describe an approximately threefold higher risk of acute kidney injury with continuous ibuprofen use for more than a week.

Supplements, Medications and Kidneys: What the Research Shows

The evidence on dietary supplements and the kidneys is mixed; the points below describe what studies report. For all of them: intake, the dose set by a doctor, and therapy belong in medical hands; this article makes no recommendation on use.

Creatine monohydrate. It can raise serum creatinine by 0.1 to 0.3 mg/dl. Actual GFR (measured via cystatin C or inulin) stayed unchanged in the studies; investigations of up to 5 years found no damage in healthy kidneys. With existing CKD (eGFR below 60), the question of creatine belongs in a nephrology consultation. For blood tests it matters that the lab or doctor knows about the intake.

High-protein diets. Studies reported no decline in GFR in healthy kidneys; the evidence differs with pre-existing conditions. The short-term GFR rise after protein meals is physiological hyperfiltration, not damage. In CKD or with a family history of kidney disease, protein intake is set by a doctor or dietitian.

NSAIDs and over-the-counter painkillers. Ibuprofen, diclofenac and naproxen are considered genuine kidney stressors. Continuous use for more than 7 days meaningfully raises the risk of acute kidney injury in studies. With regular use, checks of creatinine and eGFR should be agreed with a doctor. Acetaminophen is considered kidney-friendlier at usual use but burdens the liver. More on that in the liver values guide.

Fluid intake. Adequate hydration is considered an important factor for kidney health; individual needs depend on health status and medication and, in kidney disease, are set by a doctor.

When to See a Doctor

Four situations are a clear signal for a nephrology workup.

eGFR below 60 for more than 3 months. This defines chronic kidney disease. Cause workup needed: diabetes, hypertension, glomerulonephritis, polycystic kidney disease, NSAID abuse. Early diagnosis is the basis for medical treatment and follow-up.

Urine albumin above 30 mg/g. Microalbuminuria is often the first sign of diabetic or hypertensive nephropathy — years before eGFR drops. A simple spot urine test is enough. Cost: 10 to 20 euros.

Rising creatinine trend. A rise of more than 0.3 mg/dl over 3 to 6 months — without an obvious lifestyle cause — needs investigation. Lab2go can hold the context (training, protein, medications); the trend can be shown to a doctor.

Symptoms of kidney dysfunction. Unexplained fatigue, nausea, swelling in legs or eyelids, foamy urine, blood in urine or markedly reduced urine volume. Most CKDs remain asymptomatic for a long time. When symptoms appear, G3 or beyond has often been reached.

The combination with elevated inflammation markers deserves particular attention — chronic inflammation and kidney damage can reinforce each other.

Tracking: How Often Kidney Values Are Checked

Standard (healthy adults). Usually once a year creatinine, eGFR and urea as part of the basic blood panel. Out-of-pocket cost: 10 to 20 euros.

Risk groups. With diabetes, hypertension, cardiovascular disease or family history, closer monitoring is common, often adding the urine albumin/creatinine ratio (10 to 20 euros). The intervals are set by the doctor.

Regular medication. With NSAIDs, ACE inhibitors, lithium or creatine intake, creatinine, eGFR and cystatin C may be checked additionally; this is decided by a doctor.

Diagnosed CKD. Close monitoring of creatinine, eGFR, cystatin C, urine albumin and blood pressure tracking as directed by a doctor, combined with an annual inflammation marker check.

Documenting the circumstances of each measurement helps: fluid intake in the 24 hours before, protein intake the day before, training in the last 48 hours, current medications and dietary supplements. Without that context, values are hard to compare.

Bottom Line: Kidney Values Are Trend Data

A single kidney value tells you little. The trend across 6 to 12 months tells you much more. Creatinine has weaknesses due to muscle mass dependence — cystatin C can be added with high muscle mass, a vegan diet or older age. eGFR is the key single number, but urine albumin catches damage earlier.

Three points for interpretation:

1. Baseline. Creatinine, eGFR, urea and urine albumin/creatinine ratio form the starting point. Cost: 20 to 50 euros.
2. Cystatin C can add information when creatinine might be skewed, for example with high muscle mass, creatine intake or older age.
3. Context. Hydration, protein, training and medications influence the values — without these details they are not comparable.

For preparing a measurement, the biomarker baseline checklist gives an overview. For the digital side, take a look at the features of Lab2go or compare the plans and pricing.

This article is for information and does not replace medical advice. Intake, the dose set by a doctor, and therapy are to be clarified with a doctor; no recommendation on the use of substances is given here. If eGFR stays below 60 for more than 3 months, creatinine is rising, or there is blood in the urine or foamy urine, a nephrologist should be consulted. Self-tracking complements medicine. It does not replace it.