Inter-slide correlation between sample types. (A) Scatter plot correlation of antibody-antigen spot signal intensity between plasma (PL) and synovial fluid (SF) samples on a patient-by-patient basis. Across the study cohort, antibody prevalence in plasma is evident from the increased signal scale. Antibodies detected at ≤45% higher levels within plasma are indicated by green circular markers; those detected at ≤45% higher levels within synovial fluid are indicated by yellow triangular markers; a representative set detected at similar mean levels in both sample types are indicated by black circular markers. This subset of antibody targets is summarized in Table 1. (B) Overall mean correlation between fluids across all antibodies detected within all samples is r = 0.961 (range of r = 0.859 to 0.981).