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APPARENT FALSE POSITIVE DETECTION OF GROUP A STREPTOCOCCUS ANTIGEN RESULTING FROM PHARYNGEAL INFECTION WITH A NONHEMOLYTIC STREPTOCOCCUS PYOGENES

2000; Lippincott Williams & Wilkins; Volume: 19; Issue: 7 Linguagem: Inglês

10.1097/00006454-200007000-00026

1532-0987

Lorry G. Rubin, Gary S. Mirkin,

Antimicrobial Resistance in Staphylococcus

Rapid tests for detection of Streptococcus pyogenes [group A beta-hemolytic streptococci (GAS)] directly from throat swabs are frequently used for the presumptive diagnosis of GAS pharyngitis. Most of these tests are based on detection of the Lancefield group A-specific carbohydrate. The sensitivity of these tests varies, and it is generally recommended that throat cultures be performed on patients with negative rapid tests.1, 2 In contrast the specificity of these tests is high, ranging from 95 to 99% in most studies,3–7 and a positive antigen test is considered diagnostic for GAS, obviating the performance of a throat culture.1, 2 False positive detection may occur: we previously reportedly a child with false positive S. pyogenes as a result of pharyngeal carriage with a Lancefield group A-producing Streptococcus intermedius which was detected in the antigen detection test.8 False positive detections could be true positives explained by nonviable GAS (such as might occur after antibiotic therapy) or the presence of GAS without beta-hemolysis. We report a patient infected with nonhemolytic S. pyogenes resulting in an apparently false-positive antigen detection test. Case report. A previously healthy 7-year-old girl visited her pediatrician with a complaint of sore throat and cough. Physical examination revealed pharyngitis without exudate. A throat swab tested positive for GAS antigen (CARDS Q. S. Quick View Strep A; Quidel Corp., San Diego, CA) and amoxicillin was prescribed. The swab was cultured on Group A Selective Strep Agar (Becton Dickinson Microbiology Systems, Cockeysville, MD). No beta-hemolytic colonies were identified, and amoxicillin was discontinued. Five days later the patient developed a left earache. Left otitis media was diagnosed, and she was treated with a 10-day course of amoxicillin. One and two months later repeated throat swabs tested positive for GAS antigen; cultures were performed. Methods and results. Colonies from the primary culture of the throat swab on selective sheep blood agar were swept with a swab. The swab tested positive for GAS antigen in three GAS antigen detection kits designed for throat swabs (CARDS Q. S. Quick View Strep A; Abbott Testpack Plus Strep A from Abbott Laboratories, Abbott Park, IL; and Strep A OIA from Biostar, Inc., Boulder, CO) using the manufacturers' instructions for testing throat swabs. Individual colonies were picked, subcultured and tested for GAS antigen by preparing a suspension of colonies swabbed from the surface of the blood agar plate and testing a swab moistened with this suspension according to the manufacturers' instructions for testing throat swabs. One colonial morphology that was nonhemolytic, 1 mm in size, gray and nonmucoid tested positive for GAS antigen with either of two GAS detection tests (Abbott Testpack Plus Strep A and Strep A OIA). Examination of a Gram-stained smear of these colonies revealed Gram-positive cocci. The isolate was identified as S. pyogenes using the API 20Strep kit (bioMerieux-Vitek, Inc., Hazelwood, MO). Serologic group determination, performed with Streptex latex typing reagents (Murex Diagnostics, Norcross, GA) was positive for Group A, weakly positive for group G and negative for groups B, D and F. There was no zone or a minute zone of inhibition (after anaerobic incubation) surrounding a bacitracin (0.04 unit, "A") disk. No beta-hemolysis around colonies was observed after incubation in ambient air, under anaerobic conditions or in CO2-enhanced conditions. In vitro antibiotic susceptibility testing, performed using the disk diffusion method on Mueller-Hinton agar containing 5% sheep blood, showed the isolate to be resistant to erythromycin and trimethoprim-sulfamethoxazole and susceptible to penicillin, rifampin, vancomycin and several cephalosporin antibiotics. Identical isolates were found on cultures of throat swabs obtained 1 and 2 months later. The isolate was typed at the Streptococcus Laboratory, Centers for Disease Control and Prevention as group A Streptococcus, T-type 6, emm type 6, opacity factor-negative.9 Discussion. This patient appeared to have a false-positive test for direct detection of S. pyogenes because there were no beta-hemolytic colonies on a culture of a swab obtained simultaneously. The detection of beta-hemolytic colonies is fundamental to the identification of S. pyogenes from throat cultures in clinical microbiology laboratories. The isolation of nonhemolytic S. pyogenes demonstrates that the positive result found on direct testing of the throat swab was a true positive. The occurrence of infection with nonhemolytic S. pyogenes led to an error in management of our patient, namely the discontinuation of antibiotic therapy for GAS pharyngeal infection. However, the patient received adequate therapy for GAS infection because several days later she was treated with a 10-day course of amoxicillin for acute otitis media. The persistence of the nonhemolytic S. pyogenes in subsequent throat cultures in the absence of symptoms indicates that she became a carrier; we cannot rule out the possibility that the initial isolation reflected a carrier state rather than acute infection. Streptolysin S, an oxygen-stable toxin (in contrast to streptolysin O, which is reversibly oxygen-labile), is the GAS hemolysin responsible for the beta-hemolytic reaction on blood agar. The occurrence of nonhemolytic GAS presumably results from an absence of a functional streptolysin S enzyme. Although streptolysin S lyses both erythrocytes and leukocytes, its activity does not appear to be central to the pathogenicity of S. pyogenes. Nonhemolytic S. pyogenes are rarely encountered clinically. This may be in part because of the difficulty in identifying such strains in specimens such as throat swabs that contain other bacterial species. Pharyngeal infections with nonhemolytic streptococci were associated with an outbreak of pharyngitis and acute rheumatic fever at a military base in 1968.10 The six cases of acute rheumatic fever were the first occurrences at that base in 10 years. The outbreak isolates formed mucoid colonies, were inhibited by a bacitracin disk and were typed as M type 18, T type 18. In 1941 Coburn and Pauli11 reported that 3 of 38 people infected with type 12 GAS had nonhemolytic forms of the organism (although beta-hemolysis was observed around colonies that grew on inoculated plates incubated at 22°C). In 1942 Colebrook et al.12 reported recovery of 13 isolates of nonhemolytic GAS from infected patients with wounds and/or sepsis. Eleven of the isolates were type 12; the remaining 2 were type 11 or 25. Physicians should be aware that isolates of GAS might occasionally not produce beta-hemolysis on blood agar and thus be difficult to detect. As stated by James and McFarland10: "Without its beta-hemolytic handle, the Group A streptococcus is like a rattlesnake without a rattle." Occurrences of apparently false positive direct detection of GAS from pharyngeal swabs should be further investigated. Acknowledgments. We thank Bernard Beall and Richard Facklam at the Centers for Disease Control and Prevention for typing the isolate.