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