Sites will be randomly assigned in 1:1 ratio to 1 of 2 different subject-reported functional capacity questionnaires. The questionnaires will be used to develop the FCST. Subjects will participate in the Modified Harvard Step Test (MHST) at required timepoints and receive darbepoetin alfa once every 2 weeks for 15 weeks. All subjects will return for a follow-up visit 2 weeks after the last dose of darbepoetin alfa.