A method is disclosed for diagnosing multiple myeloma or myelodysplastic syndrome (MDS) in a subject and/or predicting the responsiveness of a patient with a multiple myeloma or MDS to lenalidomide treatment, erythropoietin treatment, or a combination thereof. The method involves assaying a biological sample from the subject, such as a blood, serum, or plasma sample, for s100A9 protein levels, TNF? levels, or a combination thereof wherein elevated levels of s100A9 in the biological sample and/or reduced levels of TNF? levels is an indication of MDS in the subject and/or that the patient will be responsive.