Red blood cell methotrexate levels in rheumatoid arthritis: Correlations with changes of class specific circulating rheumatoid factors

Red blood cell (RBC) methotrexate (MTX) levels have been studied in rheumatoid arthritis (RA) in attempt to correlate them with efficacy or toxicity, but the relationship with humoral immune system has not been analyzed. We determined the RBC concentrations of MTX in 27 RA patients treated with low-dose MTX and we studied the correlations with changes of immunological, clinical and biological parameters from baseline. After 12 months of therapy, erytrocyte MTX levels, ranging fram 20 to 248 nM/l (m ± sd = 50 ± 58 nM/l), varied considerably among patients and were significantly correlated only with changes from baseline of IgA-RF (p < 0.05). Changes of clinical and biological parameters from baseline were not correlated with MTX levels of RBC and the lowering of IgA-RF during the treatment. Thus we believe that the meaning of this correlation between erytrocyte MTX levels and changes of IgA-RF is unclear and does not seem clinically relevant.