How common is methotrexate pneumonitis? A large prospective study investigates.

The current literature states the prevalence of methotrexate pneumonitis (MTX-P) to be 3.5-7.6%. This is based on retrospective data. Consequently, clinicians remain cautious in using methotrexate especially in patients with pre-existing lung disease. To get a true idea of the incidence of MTX-P we designed an ongoing prospective study, which is the largest to date. We recruited all patients starting low-dose methotrexate in our department, and followed them up for 2 years or until development of MTX-P. All patients had their pulmonary spirometry checked at baseline. Patients were excluded if they did not give consent for methotrexate therapy, or had a forced expiratory volume in 1 s (fev1) or full vital capacity (FVC) of less than 1 l. So far, 223 patients have been recruited of whom 223 have completed 6 months and 185 have finished 2 years of follow-up from commencing methotrexate. Only two patients developed MTX-P. This gave an incidence of one case every 192 patient years of MTX-P. The results of this ongoing prospective study suggest that MTX-P when diagnosed using Carson's criteria and Chest HRCT scanning, does not occur as often as previously thought. Also it would appear from our data that baseline spirometry rather than full pulmonary function tests can be used routinely as an immediate screening of lung function prior to commencement of methotrexate. Interestingly the patients who developed MTX-P did not have any specific abnormalities at baseline.