Kidney Function Tests Explained: eGFR, Creatinine and Urea

Your kidneys filter approximately 180 litres of blood daily, removing waste products, balancing electrolytes, and regulating blood pressure. Despite their importance, kidney disease often progresses silently. In Australia, chronic kidney disease affects approximately 1.7 million adults, according to Kidney Health Australia.

Creatinine

Creatinine is a waste product from muscle metabolism. Normal levels are typically 60–110 μmol/L for men and 45–90 μmol/L for women. However, creatinine alone is influenced by muscle mass and is not the best standalone measure of kidney function.

eGFR: The Gold Standard

The estimated glomerular filtration rate (eGFR) is the best overall indicator of kidney function. It estimates how much blood your kidneys filter per minute (mL/min/1.73m²). Normal is > 90. CKD stages range from Stage 1 (eGFR > 90 with kidney damage) through to Stage 5 (eGFR < 15, kidney failure).

eGFR naturally declines with age, so results should be interpreted in context. The RACGP recommends interpreting eGFR alongside urine tests and the trend over time. In 2022, Australian labs updated their eGFR calculations, moving away from race-based adjustments.

Urea

Urea (normal: 2.5–7.0 mmol/L) is produced when the liver breaks down protein. It is less specific than eGFR and can be influenced by diet, dehydration, and liver function. The urea-to-creatinine ratio can help distinguish dehydration from intrinsic kidney disease.

Why Tracking Matters

The rate of eGFR decline is one of the strongest predictors of progression to kidney failure. A stable eGFR of 55 over five years is far less concerning than one dropping from 80 to 55 over the same period. A decline of more than 5 mL/min/1.73m² per year is considered rapid progression. Regular annual blood testing makes tracking straightforward.

References

1. Kidney Health Australia. Chronic Kidney Disease Management in Primary Care, 4th Edition. Melbourne, 2020.
2. KDIGO 2024 Clinical Practice Guideline for CKD. Kidney Int Suppl. 2024;14(4):S1–S314.
3. Australasian Creatinine Consensus Working Group. Automatic reporting of eGFR. MJA. 2005;183(3):138–141.