This week on For Kidneys Sake, Prof Jeremy Levy and Dr Andrew Frankel celebrate an unlikely centenary: 100 years since creatinine was first recognised as a marker of kidney function. From the early days of serum creatinine and the Cockcroft-Gault formula to today’s eGFR reporting, the duo unpack how kidney function testing evolved and why the numbers we rely on still come with important caveats.

The conversation explores why muscle mass can dramatically distort creatinine-based kidney estimates, when cystatin C offers a better alternative, and why gold-standard nuclear medicine tests remain impractical for routine care.

Along the way, they tackle common misconceptions around eGFR, explain why urine ACR tells a different story entirely, and remind listeners that numbers should never replace clinical judgement. A witty, practical, and surprisingly celebratory deep dive into the science behind every kidney health check.

Top 5 Takeaways

1. Creatinine has been used for 100 years — Serum creatinine was recognised as a marker of kidney function in 1926, making 2026 the centenary year of one of medicine’s most widely used blood tests.

2. eGFR is helpful — but imperfect — eGFR improves on creatinine alone by incorporating age and sex into mathematical formulas, but it still struggles in people with unusually high or low muscle mass.

3. Muscle mass matters more than many realise — A muscular person may appear to have “worse kidneys” on paper, while frail patients with low muscle mass can have deceptively normal creatinine levels despite significant kidney disease.

4. Cystatin C is an underused alternative — Unlike creatinine, cystatin C is not heavily influenced by muscle mass and can provide a more accurate estimate of kidney function in selected patients.

5. Kidney health is more than eGFR — Urine ACR measures kidney damage rather than filtration and can be abnormal even when kidney function appears normal. Both tests matter.

The purpose of this podcast is to inform and educate health care professionals working in the primary care and community setting. The content is evidence based and consistent with NICE guidelines and North West Guidelines available at the time of publication.

The content of this podcast does not constitute medical advice and it is not intended to function as a substitute for a healthcare practitioner’s judgement.

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