Abstract & Commentary Importance of Culture for Group A Strep Pharyngitis after a Negative Rapid Test
Study raises serious questions about current IDSA guidelines
By Richard R. Watkins, MD, MS, FACP
Division of Infectious Diseases, Akron General Medical Center, Akron, OH; Associate Professor of Internal Medicine, Northeast Ohio Medical University, Rootstown, OH.
Dr. Watkins reports no financial relationships in this field of study.
SOURCE: Dingle TC, et al. Reflexive culture in adolescents and adults with group A streptococcal pharyngitis. Clin Infect Dis 2014;59:643-650.
Pharyngitis is a very common infection resulting in more than 6 million annual office visits by adults in the United States. The ability to distinguish pharyngitis caused by group A streptococci (GAS) from other etiologies (e.g. viruses) is important because untreated GAS can have serious consequences including peritonsillar abscess and rheumatic fever. However, over-prescribing antibiotics is one of the main causes for the spread of antibiotic resistance. Several guidelines on the diagnosis and treatment of GAS pharyngitis have been published but unfortunately they disagree on the need for reflexive culture after a negative RADT. Because of this controversy, Dingle and colleagues sought to determine the utility of reflexive culture after a negative RADT in adult and adolescent patients suspected of having GAS pharyngitis.
The study was a retrospective analysis from two hospitals in Seattle, Washington. Included were all patients 13 years and older who between 1 January 2000 and 31 December 2011 had a negative RADT and a positive GAS culture. It is the policy of both microbiology labs in the study that all negative RADTs be reflexively followed by a backup throat culture. Subjects were eliminated from the analysis if they had a missing note corresponding to the respective date or inadequate clinical symptoms. Out of 1,023 patients identified, 726 patients underwent analysis (71%).
The authors calculated the modified Centor score (0 to 4) for each patient by assigning 1 point for each of the following symptoms: swollen or tender anterior cervical lymph nodes; absence of cough; tonsillar swelling or exudate; and fever. They added 1 point for ages 13-14 and deducted 1 point for age ≤ 45. After a negative RADT, a second throat swab was anaerobically cultured and growth was scored semiquantitatively, from 1+ representing growth in the first streak area to 4+ representing growth on the whole plate.
The sensitivity of the RADTs performed during the study was 76.3%. Of the 21,284 negative RADTs that were reflexively cultured, 1023 (4.8%) were positive for GAS. RADT missed 29 patients (4.0%) with peritonsillar abscess and 2 (0.28%) with acute rheumatic fever. Moreover, RADT failed to detect some patients with high modified Centor scores; 55% of patients with negative RADT and positive GAS culture had modified Centor scores ≤ 2. Interestingly, modified Centor scores did not correlate with culture quantities of GAS and RADT missed some patients with substantial quantities of GAS, as 77% of cultures had ≤ 2+ growth.
No significant differences were found in the clinical parameters between adolescents and adults, nor were there any differences in antibiotic treatment or incidence of peritonsillar abscesses between the two groups. Finally, bacterial quantities were similar in patients with peritonsillar abscesses and the overall study population, reflecting how bacterial burden did not correlate with modified Centor scores.
The rationale for not doing a follow up culture for GAS after a negative RADT in the current IDSA guidelines is that GAS pharyngitis is rare in adults and patients
with negative RADTs have both mild disease and low complication rates.1
The results of the study by Dingle and colleagues call this recommendation into serious question. Notably, RADT failed to detect 55% of patients with a modified Centor score ≤ 2. This means that many patients would likely not have been treated had a back-up throat culture not been obtained.
The authors found no differences in the clinical presentations or complication rates between adults and adolescents, which suggests that separate recommendations for the two groups are superfluous.
An important limitation to the study was its retrospective design. Also, it is difficult to ascertain how many of the positive GAS cultures represented colonization and not true infection. While all of the patients in the study had sore throat as their primary reason for seeking medical attention, it is possible other etiologies (e.g. viruses) were the true culprits for their symptoms.
It is clear that patients with GAS pharyngitis need to be identified because antibiotic therapy reduces communicability, improves symptoms and prevents both recurrences and rheumatic fever.
The present study by Dingle and colleagues is important because it raises questions about an authoritative treatment guideline. It seems reasonable for microbiology laboratories that routinely perform back up cultures for negative RADTs to continue this practice, which is the current policy at my institution. In order to remain relevant, guidelines must be a continuous work in progress.
As new evidence becomes available, the expert panels who decide guideline content should carefully consider these data and modify their recommendations when necessary.
- Shulman ST et al. Clinical practice guideline for the diagnosis and management of group A pharyngitis: a 2012 update by the Infectious Diseases Society of America. Clin Infect Dis 2012; 55:e86-102.