Additional reference GFR measurements will be undertaken after @ and @ years to enable a model of disease progression and error to be built @ ) A biological variability study to establish reference change values for reference and test measures @ ) A modelling study of the performance of monitoring strategies on detecting progression , utilising estimates of accuracy , patterns of disease progression and estimates of measurement error from studies @ ) , @ ) and @ ) @ ) A comprehensive cost database for each diagnostic approach will be developed to enable cost-effectiveness modelling of the optimal strategy.The performance of the estimating equations will be evaluated by assessing bias , precision and accuracy .