Reality check – the low positive predictive value of positive autoimmune markers in CTD ILD/myositis

Current guidelines recommend performing routine serological autoimmune markers for patients newly diagnosed with ILD (Interstitial Lung Disease) (doi.org/10.1164/rccm.201807-1255ST). There is no consensus regarding specific tests to perform. <bold>Aim and objectives:</bold> To evaluate the diagnostic utility of positive autoimmune markers in ILD and clarify whether presence of myositis-specific antibodies confers a diagnosis of myopathy. <bold>Methods:</bold> Data for myositis-specific antibodies requested from our hospital between 2019-2023 were collected and analysed. Where the result was positive the case notes and blood results were reviewed. <bold>Results:</bold> Myositis-specific tests were requested for 804 patients. 125/804 (15%) patients had positive results. 6/125 (4.8%) with positive results also had evidence of myositis, i.e, a raised CK, and/or myalgia. Of these, 4 were diagnosed with anti-synthetase syndrome (ASSD), 1 with polymyositis, and 1 with dermatomyositis, as summarised in the table below. <table-wrap><object-id>erj;64/suppl_68/PA4286/T1</object-id><object-id>T1</object-id><object-id>t1</object-id><table><colgroup><col></col><col></col><col></col><col></col></colgroup><tbody><tr><td>Diagnosis</td><td>CK</td><td>CT</td><td>Antibody</td></tr><tr><td>ASSD</td><td>1783</td><td>NSIP</td><td>EJ, Anti-NXP2, Anti-RNP, M2</td></tr><tr><td>Polymyositis</td><td>5208</td><td></td><td>PMSCL-100 and 75, Ro52</td></tr><tr><td>ASSD</td><td>5255</td><td></td><td>Jo-1, Ro-52</td></tr><tr><td>ASSD</td><td>956</td><td></td><td>PL-7, Ro-52</td></tr><tr><td>ASSD</td><td>3112</td><td></td><td>Jo-1</td></tr><tr><td>Dermatomyositis</td><td>352</td><td></td><td>Anti-MDA5, Ro-52</td></tr></tbody></table></table-wrap> <bold>Conclusions:</bold> Autoimmune markers are an essential diagnostic investigation for those with ILD, however, this large comprehensive review of real-world experience suggests that routine serological testing yielded limited diagnostic results. We suggest a more selective approach to autoimmune markers, tailored to individual cases based on clinical suspicion. Clinicians should avoid relying too much on serology to prevent unnecessary investigations and should appreciate that a positive test is not diagnostic.