Background/Purpose: To establish an easy and practical assay for detection of systemic Interferon (IFN) type I activation in primary Sjögren’s syndrome (pSS). The monocyte IFN type I signature is present in over half of pSS patients and identifies a subgroup of patients with higher clinical disease activity [Z. Brkic et al., 2012]. Currently, detection of the IFN type I signature is performed via laborious mRNA expression profiles of multiple IFN type I inducible genes. Methods: In a cohort of 35 pSS patients Myxovirus resistance protein A (MxA) was tested as potential biomarker for systemic IFN type bioactivity. An MxA-enzyme immunoassay (EIA) on whole blood was compared with flow cytometric detection of MxA in CD14⁺ monocytes. In addition CD64 (Fcγ RI), CD169 (Siglec-1) and BAFF (B-cell activating factor), previously described as biomarkers for IFN type I in other systemic autoimmune diseases, were assessed in CD14⁺ monocytes using flow cytometry. The IFNscore, a measure for total IFN type I activation, was calculated using expression values of the IFN type I signature genes – IFI44, IFI44L, IFIT3, LY6E, MX1 – in CD14⁺ monocytes, determined by real-time quantitative PCR. pSS patients were stratified in an IFN-positive (IFNscore>10) and IFN-negative pSS subgroup (IFNscore<10). Results: Twenty-one out of 35 pSS patients were IFN-positive (IFNpos), whereas HC were negative for the IFN type I signature. Significant correlations were found between IFNscores and CD14+ monocyte protein expression of MxA (p<0.001;r=0.733), CD64 (p=0.007;r=0.436), CD169 (p<0.001;r=0.495) and MxA protein expression in whole blood (p<0.001;r=0.717). MxA assessed by the MxA-EIA showed significantly elevated levels in IFNpos patients with a median of 212.5 (10-1295) µg/l, whereas IFNneg patients [1µg/l(1-330)] expressed MxA at levels equal to HC [15µg/l(1-150)] (p<0.001). Similar results were obtained for MxA assessed by flow cytometry (p<0.001). MxA-EIA protein levels correlated with the EULAR Sjögren’s syndrome Disease Activity Index score; Immunoglobulin levels; rheumatoid factor; haemoglobin levels and neutrophil counts. Conclusion: MxA protein analysis is a promising tool for assessment of IFN type I activation in pSS. MxA levels determined by MxA-EIA correlated with features of disease activity. The EIA was shown to be a functional assay and could contribute to future studies on disease pathogenesis and pSS subclassification, possibly leading to more targeted treatment strategies.