ATM and systemic sclerosis: To address these questions, we compared the accuracyof conventionalIP, ELISA, DID, and IP-to-MS in the detection of ATA in SSc patients.Sera derived from a group of SSc patients from the University of Pittsburghlongitudinal cohort previously classified as ATA positive (ATA+) byconventional IP, DID* (calf thymus extract used as substrate), orcommercial ELISA were reassessed by custom ELISA and DID assays usingrecombinant human topoisomerase I as substrate antigen.