Reactive knee and ankle joint arthritis: abnormal manifestation of Legionella pneumophila
In atypical pneumonia, causative organisms are difficult to isolate, so careful clinical assessment is essential in arriving at a working diagnosis. Definitive diagnosis through serologic testing is usually retrospective. Either a high initial titer or a fourfold or greater rise between the acute and convalescent titer is considered diagnostic in a patient with compatible illness. Legionella and mycoplasma organisms may be cultured from respiratory secretions if plated on appropriate culture media. Using a syndromic approach, physicians can almost always differentiate typical from atypical community-acquired pneumonia and narrow diagnostic possibilities among the atypical pathogens, making possible institution of early, possibly lifesaving, empirical therapy.