An integrative pharmacokinetic/pharmacodynamics (PK/PD) ESA-EpoR mathematical model calculates the binding behavior of erythropoiesis stimulating agents (ESA). The invention provides methods for the determining of ESA binding sites in cells or patients suffering from anemia. Knowing the amount of ESA binding sites enables the clinical practitioner to optimize the dosage regimen during a treatment of anemia, in particular in patients suffering from a cancerous disease. Further provided are methods for screening ESAs which have a higher specificity for cells strongly expressing the EPO receptor such as colony forming units-erythroid (CFU-E) cells, and not to cells with a low level of EPO receptor cell surface expression, which is the case in cancer cells. Also provided is a computer implemented method, comprising the use of the mathematical model of the invention.