Identification of patients undergoing chronic kidney replacement therapy in primary and secondary care data: validation study based on OpenSAFELY and UK Renal Registry by Santhakumaran, S, Fisher, L, Zheng, B, Mahalingasivam, V, Plumb, L, Parker, EP, Steenkamp, R, Morton, C, Mehrkar, A, Bacon, S, Lyon, S, Konstant-Hambling, R, Goldacre, B, MacKenna, B, Tomlinson, LA, Nitsch, D