How to Calculate GFR: The CKD-EPI Formula Worked Through and CKD Stages Explained

Your estimated glomerular filtration rate (eGFR) is calculated from your blood creatinine, age and sex. The current standard is the CKD-EPI 2021 equation: eGFR = 142 × (creatinine ÷ κ)^α × 0.9938^age, multiplied by 1.012 if you are female, where κ is 0.9 for men and 0.7 for women, and the exponent α applies when creatinine is below κ. A 58-year-old man with a creatinine of 1.4 mg/dL has an eGFR of about 58 mL/min/1.73 m², which is stage G3a chronic kidney disease if it stays there for more than three months.

That's the formula. The numbers only mean something in context, so this guide walks through the calculation step by step, gives several worked examples, explains the five stages, shows why a urine test matters as much as the blood number, and lists what can make creatinine misleading.

What GFR is

The kidneys filter your blood through about a million tiny filters called glomeruli. GFR is the volume of blood filtered per minute. A healthy young adult filters roughly 100 to 120 mL per minute (per 1.73 m² of body surface area). GFR declines slowly with age, by roughly 1 mL/min per year after about 40.

True GFR is difficult to measure, so labs estimate it (eGFR) from creatinine, a waste product from muscle that the kidneys clear.

The CKD-EPI 2021 formula

The 2021 version, which removed the previous race coefficient, is:

eGFR = 142 × min(Scr/κ, 1)^α × max(Scr/κ, 1)^−1.200 × 0.9938^Age × 1.012 [if female]

Where:

• Scr is serum creatinine in mg/dL
• κ = 0.7 for women and 0.9 for men
• α = −0.241 for women and −0.302 for men
• min means the smaller of Scr/κ and 1; max means the larger of Scr/κ and 1

In practice, when creatinine is above κ (as in most people with reduced kidney function), the formula simplifies to:

eGFR = 142 × (Scr/κ)^−1.200 × 0.9938^Age × (1.012 if female)

If your lab reports creatinine in µmol/L, divide by 88.4 to convert to mg/dL.

Worked example, step by step

A man aged 58, creatinine 1.4 mg/dL.

1. κ for men is 0.9. Scr/κ = 1.4 ÷ 0.9 = 1.556. This is above 1, so the "max" term applies.
2. Raise to the power −1.200: 1.556^−1.2 = 0.5885.
3. Age term: 0.9938^58 = 0.6972.
4. Multiply: 142 × 0.5885 × 0.6972 = 58.3.

eGFR ≈ 58 mL/min/1.73 m²

More worked examples

These values were computed with the formula above.

| Person | Creatinine (mg/dL) | Age | eGFR (mL/min/1.73 m²) | Stage if persistent | |---|---|---|---|---| | Man | 0.9 | 45 | 107 | G1 (normal, if no kidney damage) | | Man | 1.4 | 58 | 58 | G3a | | Man | 2.1 | 62 | 35 | G3b | | Man | 3.2 | 55 | 22 | G4 | | Man | 5.0 | 60 | 12 | G5 | | Woman | 0.6 | 30 | 124 | G1 | | Woman | 1.1 | 50 | 61 | G2 | | Woman | 1.8 | 65 | 31 | G3b (borderline G4) |

Notice that the same creatinine means different eGFRs depending on sex and age. A creatinine of 1.1 is normal in a man of 40 (eGFR about 85) but corresponds to reduced function in a woman of 50.

A simpler alternative: Cockcroft-Gault

Older clinical practice, and some drug-dosing references, use the Cockcroft-Gault formula, which estimates creatinine clearance in mL/min:

CrCl = [(140 − age) × weight in kg] ÷ (72 × Scr) × 0.85 if female

For a 58-year-old man weighing 70 kg with creatinine 1.4: (140 − 58) × 70 ÷ (72 × 1.4) = 82 × 70 ÷ 100.8 = 57 mL/min. It gives a similar answer here, but it varies more with body weight and isn't indexed to body surface area. Doctors may use it for medicine dosing.

The five stages of chronic kidney disease

CKD is defined as abnormal kidney structure or function present for more than three months. Staging uses both eGFR (G categories) and albuminuria (A categories).

| Stage | eGFR (mL/min/1.73 m²) | Description | |---|---|---| | G1 | 90 or above | Normal or high; CKD only if there's other evidence of kidney damage | | G2 | 60 to 89 | Mildly reduced; CKD only if there is other evidence of damage | | G3a | 45 to 59 | Mild to moderate reduction | | G3b | 30 to 44 | Moderate to severe reduction | | G4 | 15 to 29 | Severely reduced | | G5 | Below 15 | Kidney failure |

Why the urine test matters

Kidney damage often shows up as protein leaking into the urine before eGFR falls. The urine albumin-to-creatinine ratio (ACR) is the standard test:

| Category | ACR (mg/g) | Description | |---|---|---| | A1 | Below 30 | Normal to mildly increased | | A2 | 30 to 300 | Moderately increased | | A3 | Above 300 | Severely increased |

Combining both gives a risk picture. A person with eGFR 75 (G2) and ACR 400 (A3) has a higher risk than a person with eGFR 55 (G3a) and ACR 10 (A1). That's why doctors order both a blood and a urine test.

One reading is not a diagnosis

A single low eGFR can be due to dehydration, an infection, certain medicines or heavy exercise. Doctors repeat the test and look for persistence over at least three months before labelling CKD, unless the cause is clearly long-standing.

What can make creatinine misleading

• Muscle mass. People with a lot of muscle (bodybuilders) can have a higher creatinine with normal kidneys; people with low muscle mass (elderly, very thin, amputees) can have a "normal" creatinine despite reduced kidney function.
• Creatine supplements and high-protein supplements can raise creatinine.
• A large meal of cooked meat shortly before the test can raise creatinine temporarily.
• Dehydration raises creatinine temporarily.
• Certain medicines (trimethoprim, cimetidine, some others) raise creatinine without harming kidneys.
• Hard exercise in the previous day can raise it.
• Pregnancy lowers creatinine, and the formula isn't valid in pregnancy.
• Children need a different formula.
• Acute kidney injury, where creatinine is changing rapidly, can't be assessed by these formulas.

When creatinine is unreliable, a cystatin C blood test can be used to calculate eGFR, either alone or combined with creatinine.

Common causes of CKD in India

• Diabetes (the leading cause)
• High blood pressure
• Glomerulonephritis (inflammation of the filters)
• Kidney stones and obstruction
• Recurrent urinary infections and reflux
• Long-term use of painkillers (NSAIDs) and some herbal or "ayurvedic" preparations containing heavy metals
• Polycystic kidney disease and other inherited conditions
• Chronic kidney disease of unknown cause, seen in some farming communities

Symptoms (often absent early)

CKD is usually silent until it is advanced. Possible signs: swelling of feet and face, foamy urine, passing urine more often at night, tiredness, loss of appetite, nausea, itching, muscle cramps, high blood pressure that is hard to control, and breathlessness. That's why screening is important if you are at risk.

Who should get kidney tests

• Anyone with diabetes (at least yearly)
• Anyone with high blood pressure
• People with heart disease
• Family history of kidney disease
• Long-term use of painkillers
• History of kidney stones or urinary tract problems
• Age above 60
• Obesity

What to do if your eGFR is reduced

• Confirm with a repeat test and a urine ACR.
• Control blood sugar (target HbA1c as advised) and blood pressure (often below 130/80 mmHg in CKD with diabetes, as advised by your doctor).
• Avoid NSAID painkillers such as ibuprofen and diclofenac unless your doctor says it is safe.
• Take prescribed medicines for the kidney, such as ACE inhibitors or ARBs, which reduce protein loss, and newer drugs when indicated.
• Eat less salt. Protein and potassium advice depends on your stage and your doctor.
• Stay hydrated but don't overdrink unless advised.
• Stop smoking, limit alcohol, exercise regularly, and keep your weight in check.
• Get vaccinations as advised.
• See a nephrologist for stage G3b or worse, for heavy proteinuria, or for a rapid fall.
• Review all medicines, including herbal ones, with a doctor.

Frequently asked questions

How to calculate GFR?

Use the CKD-EPI 2021 equation with your creatinine, age and sex. Most lab reports and online calculators do it for you.

What is a normal GFR?

90 or above is normal in a young adult. It declines gradually with age, and 60 or above is acceptable if there is no other sign of kidney damage.

What GFR means kidney failure?

Below 15 mL/min/1.73 m² is stage G5, called kidney failure, at which point dialysis or transplant is considered.

Is creatinine 1.4 bad?

It depends on age, sex and muscle mass. For a 58-year-old man it corresponds to an eGFR of about 58, mildly reduced. It deserves a repeat test and a urine ACR.

Can GFR improve?

If the cause is reversible (dehydration, a medicine, obstruction), yes. In established CKD, treatment aims to slow the decline.

Does drinking more water improve GFR?

Staying hydrated protects the kidneys, but drinking extra water does not reverse CKD.

Is eGFR accurate?

It is an estimate. It's good for tracking trends, and less reliable in extremes of body size, muscle mass or in acute illness.

When to see a doctor

See a doctor for an eGFR below 60, protein in your urine, swelling of the legs or face, foamy urine, persistent high blood pressure, or a rapid rise in creatinine. Seek urgent care for very little urine, severe breathlessness, confusion or chest pain.

Care at Prakash Hospital Noida

Whether you live in Sector 18, Sector 62, Greater Noida West, or anywhere nearby, Prakash Hospital Noida is a trusted name for nephrology and kidney care in Noida. Our specialists interpret kidney function tests carefully and work with you to protect your kidneys.

To book a consultation, call the number.

Sources: Inker LA et al. New Creatinine- and Cystatin C-Based Equations to Estimate GFR without Race. New England Journal of Medicine, 2021. KDIGO 2012 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease.

Tags: #GFRCalculation #CKDStages #PrakashHospitalNoida