eGFR and Creatinine: What Your Kidney Function Numbers Mean

Your blood test shows an eGFR of 58, and a note says it is low. Or your creatinine is flagged high. You may worry that something is wrong with your kidneys. This guide explains what these numbers measure, how clinicians stage kidney function, and what usually happens next.

What creatinine and eGFR measure

Your kidneys filter waste from your blood and remove it in urine. Creatinine is a waste product from normal muscle use. Healthy kidneys clear it steadily, so the level in your blood stays fairly stable [2].

The estimated glomerular filtration rate (eGFR) is a calculation. It uses your creatinine, age, and sex to estimate how much blood your kidneys filter each minute [2]. It is reported in mL/min/1.73 m2, which adjusts for body size.

• A higher eGFR means better filtering.
• A lower eGFR means less filtering.
• eGFR naturally declines slowly with age.

The race-free equation

Older eGFR equations included a correction for Black race. In 2021, researchers published new equations that do not use race [4]. US kidney organizations recommended them, and most Canadian and US labs now report the 2021 CKD-EPI equation. If your past results came from an older equation, your number may have shifted a little when your lab switched.

Why these numbers matter

Kidney disease often causes no symptoms until it is advanced. That is why blood and urine tests matter. Lower eGFR and higher urine albumin are each linked to a higher risk of heart attack, stroke, and kidney failure [3]. Finding a problem early gives you and your clinician time to protect your kidneys, often by controlling blood pressure and blood sugar and by choosing medicines that are safe for your kidneys.

What your doctor looks at

Kidney health is judged by two tests together, not eGFR alone [1][3].

• eGFR from a blood test.
• Urine albumin to creatinine ratio (ACR) from a urine sample. Albumin is a protein that should mostly stay in the blood. Albumin in urine is an early sign of kidney damage [5].

The definition of chronic kidney disease

The international KDIGO 2024 guideline, used in both Canada and the US, defines chronic kidney disease (CKD) as either of these lasting more than 3 months [3]:

• An eGFR below 60.
• Signs of kidney damage, such as an ACR of 3 mg/mmol (30 mg/g) or higher, blood in the urine from the kidney, or changes on imaging.

The "more than 3 months" part matters. One low result is not enough for a diagnosis.

eGFR stages (G categories)

• G1: 90 or higher. Normal or high.
• G2: 60 to 89. Mildly decreased.
• G3a: 45 to 59. Mild to moderately decreased.
• G3b: 30 to 44. Moderately to severely decreased.
• G4: 15 to 29. Severely decreased.
• G5: below 15. Kidney failure.

A G1 or G2 eGFR alone is not CKD. It counts only when there are other signs of kidney damage [3].

Albumin stages (A categories)

Canadian labs usually report ACR in mg/mmol. US labs usually use mg/g [3][5].

• A1: below 3 mg/mmol (below 30 mg/g). Normal to mildly increased.
• A2: 3 to 30 mg/mmol (30 to 300 mg/g). Moderately increased.
• A3: above 30 mg/mmol (above 300 mg/g). Severely increased.

Your risk depends on both letters. For example, G3a with A1 carries much lower risk than G3a with A3. KDIGO uses a color chart (green, yellow, orange, red) that combines the two [3].

Confirming and following

• Clinicians often repeat creatinine, and sometimes add cystatin C, a second blood marker that is less affected by muscle mass [3].
• They look for causes such as diabetes, high blood pressure, medicines, or blockage.
• They check how fast eGFR is changing over time.
• KDIGO suggests referral to a kidney specialist (nephrologist) when eGFR is below 30, ACR is above 30 mg/mmol (300 mg/g), or kidney function is dropping fast, among other reasons [3].

Things that can affect creatinine

Creatinine depends on muscle as well as kidneys. These can shift it without kidney damage [2]:

• Very high or very low muscle mass.
• A large meal of cooked meat before the test.
• Creatine supplements.
• Hard exercise the day before.
• Dehydration.
• Some medicines, such as trimethoprim, that change how creatinine is handled.

Some blood pressure and diabetes medicines can cause a small, expected drop in eGFR when started. Your clinician watches for this. Do not stop any medicine on your own; talk with your clinician or pharmacist first.

Questions to ask your clinician

• Is my eGFR stable, or is it changing over time?
• What is my urine ACR, and has it been checked?
• What is my combined G and A category?
• Could anything have affected this test, such as diet, exercise, or a medicine?
• Should we confirm with a repeat test or cystatin C?
• Do any of my medicines need a dose change for my kidney function?
• Do I need to see a kidney specialist?

When to get care quickly

Go to the emergency department or call 911 if you have:

• Very little or no urine for a day.
• Sudden swelling of the legs or face with shortness of breath.
• Confusion, severe weakness, or chest pain.
• Severe vomiting or diarrhea and you cannot keep fluids down, especially if you have known kidney disease.

Bottom line

eGFR is an estimate of kidney filtering, based mainly on creatinine. A single low result is common and often changes on repeat. Clinicians diagnose chronic kidney disease only when changes last more than 3 months, and they always pair eGFR with a urine albumin test. Together, the two numbers show your real risk and guide next steps.

Primary AI can show you the guideline and study sources behind this topic in plain language, so you can bring better questions to your appointment.