The majority of cardiac stem cell trials to date used autologous bone marrow stem cells. Yet, functional properties of bone marrow cells vary across patients, and comorbidities negatively affect cell characteristic. Chronic heart failure also negatively affect bone marrow cells function, with lower rates of colony formation units and response to erythropoetin stimulation versus age-matched controls. Although patients may receive the same number (ie. dose) of autologous bone marrow cells, the therapeutic potency of those  cells can vary widely from patient to patient, and this likely accounts, in part, for the observed variation in effect.