The eGFR-C study will assess the accuracy of current and alternative tests of kidney function against a reference test in people with moderate (stage 3) chronic kidney disease (CKD).

The eGFR-C study will assess the accuracy of current and alternative tests of kidney function against a reference test in people with moderate (stage 3) CKD. Participants will be recruited from hospital clinics and General Practitioner (GP) practices at six major United Kingdom (UK) centres.

The best measure of kidney function is accepted to be the glomerular filtration rate (GFR), which measures the ability of the kidney to filter blood and is widely used in clinical practice. A low GFR suggests poor kidney function. An estimate of GFR can be obtained from a simple blood test.

Participants will undergo reference GFR testing at study entry with a second follow-up reference test three years later. The reference test involves injecting a small amount of iohexol into a vein and taking blood samples over the next 4 hours to see how quickly the iohexol disappears from the blood stream as a result of glomerular filtration. The rate at which iohexol disappears is equivalent to the level of kidney function. Blood tests for monitoring kidney function, including testing for creatinine and cystatin C, and measurement of urinary albumin will be done every six months during the study period.

Iohexol measured GFR will be accepted as the reference ('gold standard') measure of kidney function against which each GFR-estimating equations will be compared. The alternative estimated measures of GFR, derived from measuring substances (creatinine and cystatin C) in the blood, will be compared against the reference test. An important outcome is how much the reference test changes over the three years of the study, and how well the surrogate measures reflect this change.

The investigators will also collect accurate test cost data for subsequent cost-effectiveness analysis (e.g. do the relative costs of the tests justify any change in practice due to improved performance of one test compared to another?).

Inclusion Criteria:

• Patients with stage 3 CKD (GFR 30-59 mL/min/1.73 m2) as defined internationally, diagnosed using MDRD/CKDEPI eGFR (at least two consecutive test results in this range at least 90 days apart, with the most recent test in the last 12 months)
• Aged 18 years or over
• Written informed consent

Exclusion Criteria:

• History of untoward reactions to iodinated contrast media or allergy to topical iodine
• Episode of acute kidney injury in previous 6 months (as defined by the Acute Kidney Injury Network criteria)
• Amputation of whole or part-limb
• Pregnant or breastfeeding
• Known current alcohol or drug abuse
• Kidney transplant recipient
• Any condition with an expected survival of less than study duration
• Inability to comply with study schedule and follow-up
• Inability to provide informed consent e.g. due to cognitive impairment

Adults with stage 3 CKD.