Multidrug resistance (MDR) protein activity is an independent prognostic marker in certain haematological malignancies. Immune activation is also linked to the expression of MDR proteins in autoimmune disorders. High activity of these transporters in target cells is the most common mechanism of resistance to chemotherapeutic and immunosuppressive agents. The SOLVO MDQ Kit™ enables a more accurate prediction of drug resistance than other methods such as quantifying the expression of the transporters at mRNA or protein level. Determination of transporter activities allows risk stratification of patients by predicting resistance to certain cytotoxic agents in hematology. In resistant cases the chances for better patient outcomes are higher by alternative treatment choices, and the costs of ineffective treatment can be saved. Furthermore, MDR protein function may predict patient response to traditional DMARD, as well as biological treatment in rheumatology, quantifying the actual disease activity of RA patients. Our recent results suggest that low BCRP/ABCG2 and MRP1/ABCC1 protein activities on CD3 cells of RA patients whose symptoms do not improve on classical DMARD treatment reflect the need to start biological therapy. Further decrease of CD3 BCRP/ABCG2 and increase in CD3 MRP1/ABCC1 activities upon follow-up may indicate a good therapeutic response to biological therapy.