eGFR and creatinine: kidney filtration, estimated from muscle waste

Creatinine is a waste product of creatine in muscle. It is made at a fairly steady rate and cleared almost entirely by the kidneys, so when filtration falls, blood creatinine rises. Labs turn creatinine, age and sex into an estimated glomerular filtration rate (eGFR), in mL/min/1.73 m². US labs now use the 2021 CKD-EPI equation, which dropped the race term used in earlier versions. Creatinine is reported in mg/dL or µmol/L (1 mg/dL is 88.4 µmol/L).

The guideline numbers

KDIGO defines chronic kidney disease as an eGFR under 60 for more than three months, or signs of kidney damage such as albumin in the urine (a urine albumin-to-creatinine ratio of 30 mg/g or more) at any eGFR. A G2 result on its own, without damage markers, is not kidney disease. MedlinePlus gives typical creatinine ranges of 0.7 to 1.3 mg/dL for men and 0.5 to 0.95 mg/dL for women, though labs differ.

What raises and lowers it

• Creatinine up, eGFR down, kidneys fine: lots of muscle, creatine supplements, a large cooked-meat meal in the hours before, hard training, dehydration, and some drugs that block creatinine secretion, such as trimethoprim.
• Creatinine down, eGFR up, kidneys not necessarily fine: low muscle mass, muscle loss with ageing or illness, amputation, and large weight loss.
• Real falls in filtration: diabetes, high blood pressure, kidney disease, acute kidney injury from dehydration, some drugs such as regular high-dose NSAIDs.

When creatinine is likely to mislead, the 2024 KDIGO guideline recommends adding cystatin C, a marker that does not depend on muscle, and using the combined equation, which the 2021 CKD-EPI study found more accurate than either marker alone.

What it predicts

The CKD Prognosis Consortium pooled general population cohorts in the Lancet in 2010. In 105,872 people with urine albumin measured, risk of death did not change between eGFR 75 and 105 and rose below that. Compared with eGFR 95, the adjusted hazard ratio for death from any cause was 1.18 at eGFR 60, 1.57 at 45 and 3.14 at 15. Urine albumin added risk independently. This is the evidence base for the KDIGO staging, and the reason a urine test belongs next to the blood test.

On GLP-1 drugs and weight loss

FLOW (NEJM, 2024) was the first kidney outcome trial of a GLP-1 drug: 3,533 people with type 2 diabetes and chronic kidney disease, semaglutide 1 mg weekly or placebo, median 3.4 years, stopped early for efficacy. Major kidney events fell 24% (HR 0.76, 95% CI 0.66 to 0.88), and eGFR declined more slowly by 1.16 units a year. In SELECT, without diabetes, eGFR at 104 weeks was 0.75 units higher on semaglutide 2.4 mg than placebo, 2.19 in those who started below 60. The kidney study page has the detail.

Two cautions on the other side. The labels warn of acute kidney injury from dehydration after vomiting or diarrhoea, and a big weight loss that takes muscle with it can make creatinine-based eGFR look better than the kidneys are. The muscle loss page explains why cystatin C helps here.

How to test

• Avoid a large meat meal in the 12 hours before, and hard exercise for a day or two.
• Be hydrated, and note any recent illness with vomiting or diarrhoea.
• Tell the lab or clinician about creatine supplements. They raise creatinine without harming the kidneys.
• Repeat a low result before calling it kidney disease. KDIGO requires three months.
• Add a urine albumin-to-creatinine ratio and, if muscle mass is unusual, cystatin C.

Tracking it in Peptide AI

In Peptide AI, eGFR and creatinine sits in the metabolic panel. The app reads it from a lab PDF or manual entry, scores the panel from 0 to 100 alongside an overall grade, flags values that warrant prompt physician consultation, and plots each result against your dose timeline. Log the draw date, whether you fasted, and anything unusual in the days before, such as illness, a hard session or a dose step, so the next result is compared like with like. Bloodwork tracking shows the series next to the dose log, weight and side effects, and the panels page lists every marker the app groups. More markers are in the bloodwork guides.