The present invention relates to the diagnostic of inflammatory diseases. The inventors described methods using NET biomarkers as diagnostic biomarkers for inflammatory diseases. COVID-19, Lupus or mCRC are used here as illustrative models for investigating an inflammatory disease. Examples in highlighting variation of the respective correlation of NET biomarkers in this invention rely on the determination of the NET main constituents: (i), DNA as determined by examining the amount of circulating DNA (cirDNA) that corresponds to the amount of NET as being degradation by-products that are released into the circulation; (ii) NE; and (iii), MPO; as well as the detection of a blood compound being indirectly associated to NET formation like the anti-cardiolipin auto-antibody. The invention provides threshold values of NE, MPO, cir-nDNA, and cir-mtDNA blood concentrations and of MNR that can be combined to diagnose/screen individuals. Thus the invention relates to a method for diagnosing a subject for an inflammatory disease comprising the steps of i) determining in a sample obtained from the subject the level of at least one marker selected in the group consisting in NET protein markers, cir-nDNA, cir-mtDNA and/or a cir-DNA fragmentation index.