The eGFR-C protocol paper was published in January 2014:
Lamb EJ, Brettell EA, Cockwell P, Dalton N, Deeks JJ, Harris K, Higgins T, Kalra PA, Khunti K, Loud F, Ottridge RS, Sharpe CC, Sitch AJ, Stevens PE, Sutton AJ, Taal MW; eGFR-C study group. The eGFR-C study: accuracy of glomerular filtration rate (GFR) estimation using creatinine and cystatin C and albuminuria for monitoring disease progression in patients with stage 3 chronic kidney disease - prospective longitudinal study in a multiethnic population. BMC Nephrol. 2014 Jan 14;15(1):13. The full paper is available here.