The goal of this observational study is to assess if new ways of measuring kidney function can better predict when individuals will become symptomatic due to kidney failure, and whether residual kidney function can be accurately measured in those already on dialysis. The main questions the study is trying to answer is whether measuring kidney function by clearance of iohexol is comparable to the current standard of care methods. In addition to routine care, participants will: * undergo a brief clinical assessment * be given an injection of iohexol and asked to perform 4 finger prick blood tests over 24 hours, recording the time of each sample. Samples will be returned in person or posted back * be asked to complete a questionnaire on their experience
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Iohexol measured glomerular filtration rate
Timeframe: Assessed at each study visit through study completion, up to 6 months.
Creatinine based estimated glomerular filtration rate (Standard of care measure for kidney function in those not receiving renal replacement therapy)
Timeframe: Assessed at each study visit while the participant has not yet started renal replacement therapy through study completion, up to 6 months.
Creatinine clearance corrected for body surface area (Standard of care measure for kidney function in those receiving peritoneal dialysis)
Timeframe: Assessed at any study visit where the participant is established on peritoneal dialysis through to study completion, usually 48 hours after this visit.
Residual renal clearance of creatinine and urea (Standard of care measure for kidney function in those receiving haemodialysis)
Timeframe: Assessed at any study visit where the participant is established on haemodialysis dialysis through to study completion, usually 48 hours after this visit.