Methods for increasing red blood cell levels and treating anemia using a combination of GDF traps and erythropoietin receptor activators

A method for increasing red blood cell levels or treating anemia in a patient, the method comprising administering to a patient in need thereof:

• (a) an erythropoietin receptor activator, wherein the activator is an agent that i) increases proliferation of immature erythroblasts, ii) increases differentiation of immature erythroblasts, and/or iii) decreases apoptosis in erythroid progenitor cells;
• and
• (b) a polypeptide comprising an amino acid sequence that is at least 95% identical to the sequence of amino acids 29-109 of SEQ ID NO:
• 1, and wherein the polypeptide comprises an acidic amino acid at the position corresponding to position 79 of SEQ ID NO:
• 1, wherein the polypeptide binds to GDF11;
• and
• wherein the erythropoietin receptor activator and the polypeptide are administered in an effective amount.