In screening specimens of sputum, not discard them as unsatisfactory because of the absence of polymorphonuclear leukocytes; further, the specimens should be cultured for Legionella even though they do not meet criteria of Murray and Washington for being of high quality. The increased use of culture techniques together with serologic methods, urinary antigen detection and gen-probe systems will help to provide more accurate data on the incidence of pneumonia due to L pneumophila.
Another interesting observation is that L pneumophila was recovered more frequently in samples of patients with respiratory failure. Kroboth and colleagues observed that the severity of the radiographic findings correlated significantly with the presence of L pneumophila in sputum by direct immunofluorescence. In 74 percent of their patients with bilobar or bilateral infiltrates, direct immunofluorescence was positive as compared with none with unilobar involvement. It is possible that patients with respiratory failure and with severe radiographic involvement had a greater inoculum of bacteria and that would explain why L pneumophila is isolated more frequently in these patients. In this way, isolation of L pneumophila in sputum would be an indicator of severity of illness.