Myeloperoxidase-Antibody Positivity and Progression to Microscopic Polyangiitis in Interstitial Lung Disease: A Case Series.

Microscopic polyangiitis (MPA) is associated with myeloperoxidase-antibody positivity and pulmonary complications. However, these antibodies may be found incidentally in some people with interstitial lung disease (ILD). As a result, management of people with ILD and myeloperoxidase-antibody positivity remains unclear. Our case series describes the natural history of this group of patients over a mean follow-up period of 4 years. Using data from a public sector pathology provider in Western Australia, we identified 17 people with a positive myeloperoxidase-antibody titre and who attended our tertiary Respiratory outpatient clinic. Eventual or concurrent diagnosis of MPA was more common in those with radiological usual interstitial pneumonia pattern, higher baseline and peak myeloperoxidase-antibody titres, lower gas transfer capacity and lower distances achieved on 6-min walk testing. These features may be used by clinicians during multidisciplinary team discussion to identify those at high-risk of developing MPA.