Pneumococcal sepsis with false-negative blood cultures

Most physicians recognize the false-positive blood cultures (generally due to contamination) are common. Bacteria such as pneumococci, however, may rapidly die in broth cultures, and viable bacteria may not be identified. Several patients were observed with pneumococcal infections that had false-negative blood cultures 24 hours after inoculation. Hemolysis and methemoglobin formation in the bottle suggested bacterial growth within 12 hours after incubation, and blind subcultures at that time yielded pneumococci. Pneumococcal antigen could be detected in the blood culture bottles using counter-immunoelectrophoresis, even though subculture at 24 hours yielded no growth. Physicians and laboratory personnel should be aware that false-negative blood cultures may occur, particularly with certain bacteria such as Streptococcus pneumoniae, and that the blood culture bottles should be observed visually for the presence of brown sediment or hemolysis.