Foals with a WBC of more than 14,000 cells/mL, no clinical signs, and normal lung sounds should be considered for additional tests such as thoracic ultrasonography.

Many diagnostic tests, including complete blood cell count, fibrinogen level, thoracic ultrasound, radiographs, and serology, have been used to distinguish R. equi pneumonia from other causes. Bacteriologic culture or PCR amplification sometimes with cytological examination of a tracheobronchial aspirate provide good diagnosis.