Kidney function tests: creatinine, eGFR, urea and urine albumin

What your kidneys do and how they are tested

The kidneys filter waste products and excess water out of the blood and return what the body needs. They also help to control blood pressure, balance salts such as sodium and potassium, and support red blood cell production and bone health.

Kidney disease often causes no symptoms until it is advanced, which is why doctors rely on tests, especially for people with diabetes, high blood pressure, a family history of kidney disease or long-term use of certain medicines. A standard kidney assessment combines blood tests and a urine test:

• Serum creatinine, from which the eGFR is calculated.
• Blood urea or blood urea nitrogen (BUN).
• Urine albumin, often as a ratio to urine creatinine.
• Often electrolytes (sodium, potassium) and sometimes a urine routine test or ultrasound.

Creatinine and its units

Creatinine is a waste product made by muscles. Healthy kidneys remove it from the blood steadily, so when kidney filtering falls, creatinine in the blood tends to rise.

Indian laboratories usually print creatinine in milligrams per decilitre (mg/dL). Many other countries, including the UK, use micromoles per litre (µmol/L). To convert, multiply mg/dL by 88.4: a creatinine of 1.0 mg/dL is 88.4 µmol/L, and 1.1 mg/dL is about 97 µmol/L.

Typical adult reference ranges are roughly 0.7 to 1.3 mg/dL in men and 0.6 to 1.1 mg/dL in women, but they depend on the laboratory and method. The reason men have higher ranges is muscle mass, which brings us to the main limitation: creatinine reflects muscle as well as kidneys. A muscular person may have a higher creatinine with normal kidneys, while an older or frail person with little muscle may have a creatinine in the normal range despite reduced kidney function.

Other things can raise creatinine temporarily without lasting kidney damage: dehydration, a very high-protein meal or cooked meat just before the test, intense exercise, creatine supplements, and some medicines. This is one reason doctors repeat abnormal results.

eGFR: the number most doctors use

The glomerular filtration rate (GFR) is the volume of blood the kidneys filter per minute. It cannot be measured directly in everyday practice, so laboratories estimate it (eGFR) from serum creatinine, age and sex using an equation. The race-free CKD-EPI 2021 equation is widely used internationally, though some laboratories still use older equations. The report gives eGFR in mL/min/1.73 m².

Based on the international KDIGO guideline, kidney function is grouped into stages by eGFR:

Chronic kidney disease (CKD) is defined by abnormal kidney structure or function lasting more than three months, not by a single result. Stages G1 and G2 only count as CKD if there is other evidence of kidney damage, such as albumin in the urine. Because eGFR gradually declines with age, a slightly lower value in an older person is not necessarily a disease, which is why doctors interpret the number in context.

The eGFR is an estimate and can be inaccurate in people with very high or very low muscle mass, during pregnancy, in acute illness, or with certain diets and supplements. When more accuracy matters, doctors can use another blood marker, cystatin C, or other measurements.

Blood urea and BUN

Urea is another waste product, made when the body breaks down protein. In India, laboratories commonly report blood urea in mg/dL. In the United States the same waste is reported as blood urea nitrogen (BUN), which counts only the nitrogen part of urea. The two are not the same number:

• Urea (mg/dL) = BUN (mg/dL) × 2.14.
• Urea (mmol/L) = BUN (mg/dL) × 0.357.

For example, a BUN of 14 mg/dL equals a urea of about 30 mg/dL, or 5.0 mmol/L. Urea is less reliable than creatinine as a measure of kidney function, because it also rises with dehydration, a high-protein diet, gastrointestinal bleeding and some medicines, and falls with a low-protein diet or liver disease. Doctors often look at it together with creatinine rather than on its own.

Urine albumin: the early-warning test

Albumin is a protein that healthy kidneys keep in the blood. A small leak into the urine can be the earliest sign of kidney damage, often before eGFR has changed, and it is also a marker of cardiovascular risk. For this reason, guidelines recommend testing urine albumin in people with diabetes or high blood pressure.

The preferred test is the albumin-to-creatinine ratio (ACR) on a urine sample, preferably the first one in the morning. Dividing by urine creatinine corrects for how dilute or concentrated the urine is. KDIGO groups albuminuria like this:

Fever, intense exercise, a urinary infection, menstruation and uncontrolled blood pressure or glucose can all raise urine albumin temporarily, so a raised result is normally confirmed on repeat samples before it is considered meaningful.

A worked example (example values)

Suppose a person’s example results are creatinine 1.4 mg/dL, eGFR 52 mL/min/1.73 m² and urine ACR 85 mg/g. Three months earlier, creatinine was 1.3 mg/dL and eGFR 55.

The eGFR sits in stage G3a, and the ACR is in category A2. Because the abnormality has now persisted over three months and shows some albumin leakage, a doctor would take it seriously as possible chronic kidney disease and look for causes: diabetes, blood pressure, medicines, and anything blocking urine flow. They would probably repeat tests, order an ultrasound, review the medicine list and ask about pain-killers. The change between the two creatinine values is small, and what matters is the pattern and the albumin result.

Converted, 1.4 mg/dL equals about 124 µmol/L, so a report in µmol/L would show this number. A person comparing 124 with an old report showing 1.3 could wrongly think the kidneys had worsened dramatically, which is the sort of error unit checks prevent.

What can help protect results and kidneys

This guide cannot advise on treatment, but some general points are widely agreed:

• Keep blood pressure and blood glucose well controlled, as your doctor advises, because these are the two leading causes of kidney damage in adults.
• Drink fluids sensibly unless a doctor has limited them, and avoid being dehydrated when tests are taken.
• Tell every doctor and pharmacist that you have reduced kidney function, because the safe amount of many medicines changes. Ask before using regular pain-killers, herbal products or supplements.
• Keep your earlier reports so that the trend can be seen.

Other kidney-related results on your report

A kidney panel or a routine health check may include several other lines.

• Electrolytes. Sodium and potassium are controlled by the kidneys, and potassium in particular can be affected by kidney function and by some blood pressure medicines. A sample that sat for too long before testing, or in which cells were damaged, can give a falsely high potassium, and doctors may repeat the test if the value is unexpected.
• Uric acid. It is a waste product that can rise when kidney function is reduced, but it is affected by diet, alcohol, body weight and some tablets, and it is often high without any problem.
• Calcium, phosphate and bicarbonate, which doctors follow in more advanced kidney disease.
• Urine routine and microscopy. Protein, blood, glucose and cells in the urine are screened with a dipstick and a look under the microscope. A dipstick detects larger amounts of protein than the ACR test does, which is why the ACR is preferred for early detection.
• A kidney ultrasound, which shows kidney size and shape and can reveal stones or blockage.

How often are kidney tests repeated?

There is no single answer. People with diabetes or high blood pressure are usually tested at least once a year, and more often when results are abnormal or treatment is changing. KDIGO suggests that how often to test should depend on the combination of eGFR stage and albuminuria category, with more frequent monitoring when both are worse. Ask your doctor what your own schedule is, and note the date of the next test with your records.

Questions worth asking your doctor

• What stage are my eGFR and albumin results, and how confident are we?
• Are my results stable, or do we need to repeat them soon?
• Could any of my medicines be affecting my kidneys, and should they be reviewed?
• Do I need a urine albumin test or an ultrasound?
• How often should my kidney tests be repeated?

Sources

• KDIGO: CKD Evaluation and Management guideline — international guideline that defines the eGFR and albuminuria categories
• NIDDK: Chronic Kidney Disease Tests and Diagnosis — patient information on eGFR and urine tests
• MedlinePlus: eGFR and GFR Tests — how eGFR is calculated
• MedlinePlus: Creatinine Test — what creatinine results can mean
• MedlinePlus: Microalbumin Creatinine Ratio — the urine ACR test
• MedlinePlus: BUN (Blood Urea Nitrogen) — urea nitrogen explained

A note on this guide

This guide is general information about kidney reports. It is not medical advice and cannot tell you whether your own kidneys are healthy. Confirm the meaning of your results, and any change to your medicines, with your doctor.