Objective: To study the correlation between antiperinuclear factor (APF) titer and disease activity variables in patients with rheumatoid arthritis (RA) treated with methotrexate (MTX) or azathioprine (AZA) and to investigate whether changes are dependent on the drug used.
Methods: Serial measurements of APF titers (2-fold dilutions) and disease activity variables in a 48-week double blind trial comparing MTX and AZA in 64 patients with RA. APF titers at baseline and during followup, and correlations between APF titers and disease activity variables and their changes from baseline were studied in the patient group as a whole and in the 2 treatment groups.
Results: The prevalence of the APF at baseline in the MTX group and in the AZA group with undiluted serum was 15/31 (48%) and 19/33 (58%), respectively. With serum diluted 1:10 this was about 25% higher. The APF titer ranged from 1/10 to 1/640. No sustained changes in APF titers were observed during followup. Statistically significant correlations were found between APF titers and 2 of the 4 disease activity variables, as well as for their changes from baseline at some time points and were most pronounced in the AZA group. However, no consistent correlation between APF titers and disease activity variables could be established. APF changed from negative to positive during followup in 4 patients (6.3%) and from positive to negative in 4 (6.3%). Changes in APF titer between 2 consecutive measuring points did not exceed 2 dilution steps.
Conclusion: The APF titer showed no sustained change during the followup period. There was no consistent correlation between APF titer and disease activity variables. We conclude that serial measurements of the APF in longitudinal studies do not give additional information.