Letter to the Editor Variability of the RNase L Isoform Ratio (37 Kilodaltons/83 Kilodaltons) in Diagnosis of Chronic Fatigue Syndrome

Chronic fatigue syndrome (CFS) is a disorder characterized by debilitating fatigue whose etiology and pathophysiology re-main unclear. Previous studies showed abnormalities of the RNase L pathway in peripheral blood mononuclear cells (PBMC) from patients with CFS (1, 2). The ratio of RNase L isoforms (37 kDa/83 kDa ratio [37/83 R]) has therefore been proposed as a potential biochemical marker of CFS, with a sensitivity of 91 % and a specificity of 71 % when the cutoff ratio was 0.4 (3). In order to evaluate the variability and reproducibility of the 37/83 R in the diagnosis of CFS, we assayed it in duplicate as described by Demettre et al. (2) once a month for 3 months with nine patients fulfilling CFS criteria (mean age standard deviation 43.1 10.4 years). The accuracy of this ratio in the diagnosis of CFS was also tested and correlated with a clinica