Determinants of red blood cell methotrexate polyglutamate concentrations in rheumatoid arthritis patients receiving long‐term methotrexate treatment

Large interpatient variability in MTXGlun concentrations can be explained, at least in part, by a combination of factors, particularly age, MTX dosage, and renal function. There are complex interactions between smoking, RBC folate levels, and concentrations of MTXGlun.