CD74 and red color blindness: These analyses yielded very similar results to the primary analyses, indicating that it is unlikely that diagnostic biases explain the differences in anti-CD74 IgG between established AS and early axSpA, the relatively high prevalence of anti-CD74 IgG and IgA antibodies in patients with CBP, or the limited PPV and NPV of anti-CD74 IgA in patients with early back pain.