STUDY 4 This study will assess the effect of acetylcysteine on renal function in stable stage III CKD patients receiving 100 ml of iodixanol during routine coronary angiography. Recruited patients will not receive multiple angiographies so study 4 will not be a crossover comparison but a randomised controlled study of oral acetylcysteine vs. IV acetylcysteine vs. placebo. Results from studies 1−3 will inform the analysis and interpretation of this study.