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January 4, 1985

False-Positive Direct Fluorescent Antibody Testing for Legionella

Author Affiliations

Portsmouth Naval Hospital Portsmouth, Va

JAMA. 1985;253(1):40-41. doi:10.1001/jama.1985.03350250048013

To the Editor.—  Direct fluorescent antibody (DFA) testing of lung tissue or respiratory tract secretions has proved to be an expedient, reliable means for establishing the diagnosis of legionnaires' disease. The specificity of the DFA test on sputum is about 95% using the polyvalent reagent supplied by the Centers for Disease Control (CDC) in Atlanta.1 False-positives with DFA have been described in cases wherein filter water and/or reagents for rinsing contain Legionella.2 False-positive cross-reactions have been reported with isolates of Pseudomonas, Flavobacterium- xanthomonas group, and Bacteroides fragilis.3 We have observed two separate instances when Hemophilus influenzae in the sputum cross-reacted with pool C of the polyvalent assay (Legionella bozemanii, Legionella micadadei, Legionella gormanii, and Legionella longbeachae), resulting in 4+ fluorescence (≥5 bacilli per high-power field, magnification × 1,000) consistent with the CDC criteria for a positive test result. Sputum for culture and monovalent DFA test results were