Commercial antigen kits and a single blood agar plate culture were less sensitive for detecting streptococcal pharyngitis than a 2-plate culture method
ACP J Club. 1992 July-Aug;117:22. doi:10.7326/ACPJC-1992-117-1-022
Wegner DL, Witte DL, Schrantz RD. Insensitivity of rapid antigen detection methods and single blood agar plate culture for diagnosing streptococcal pharyngitis. JAMA. 1992 Feb 5;267:695-7.
To compare 5 commercially available antigen detection kits and a single bacterial culture with a 2-plate culture method for the diagnosis of streptococcal pharyngitis.
Independent comparison of antigen detection kits and both single and 2-plate cultures.
1 family practice and 4 pediatric office practices.
755 consecutive outpatients suspected of having streptococcal pharyngitis were included in the 1-month study. At least 100 patients were included from each practice.
Description of test and diagnostic standard
2 throat swabs were taken from each patient. 1 swab was tested immediately for streptococcal antigen by an office nurse, using an antigen detection kit: Abbott Testpack Strep A (Abbott Laboratories), Culturette Brand 10 Minute Group A Strep ID (Marion Scientific), Reveal (Wellcome Diagnostics), Pacific Biotech Cards Strep A (Pacific Biotech), or Ventrescreen Strep A (Ventrex Laboratories). Each office used a different kit. The second swab was sent to the laboratory in Stuart transport media and inoculated onto both a trimethoprim-sulfamethoxazole 5% sheep blood agar plate (incubated anaerobically) and a 5% sheep blood trypticase soy agar plate (incubated aerobically in 5% CO2). The plates were read after incubating for 1 and 2 days. A culture with any growth of group A β-hemolytic streptococci growth was considered positive.
Main outcome measures
Sensitivities were calculated for the 5 commercial antigen kits, and a single culture was read at days 1 and 2. The 2-plate culture method read at day 2 was considered the gold standard.
261 cases of group A streptococcal pharyngitis were diagnosed in 755 patients. Sensitivity for the anaerobically incubated plate at day 2 was 94%. Sensitivity for the single blood agar plate culture incubated aerobically in 5% CO2 at day 1 was 58% and at day 2 was 72%. For the antigen kits, sensitivities were 31% (Testpack), 36% (Culturette), 44% (Reveal), 47% (Cards), and 50% (Ventrescreen). Sensitivities decreased with decreasing colony counts. Except for the Ventrescreen kit at 72%, specificities for the antigen kits ranged from 95% to 100%.
Commercial antigen kits and a single blood agar plate culture were less sensitive for detecting streptococcal pharyngitis than a 2-plate culture method.
Source of funding: Not stated.
Address for article reprint: Dr. D.L. Wegner, Ottumwa Regional Health Foundation, 1001 East Pennsylvania Avenue, Ottumwa, IA 52501, USA.
With the recent resurgence in the rate of serious streptococcal sequelae, it is increasingly important for the clinician to accurately diagnose group A streptococcal disease so that appropriate treatment is given. In epidemic situations it may be prudent and cost effective to treat all patients suspected of having streptococcal pharyngitis without ordering confirmatory laboratory tests. In other situations laboratory testing is recommended when clinical and epidemiologic data cannot identify the patients likely to have the disease. The advent of rapid detection methods for group A antigen has increased the clinician's options for such testing.
The study by Wegner and colleagues, in a setting that approximates a good community clinician's office, makes a valuable contribution by showing that the rapid antigen detection methods are less sensitive than previous studies have shown. On the positive side, most of the rapid antigen kits are quite specific. A positive antigen test, therefore, can be assumed to be the same as a positive culture result, and treatment can be started without delay. Negative antigen tests require a confirmatory culture. This investigation, along with previous work, has shown that cultures incubated aerobically with added CO2 provide lower yields of group A streptococcus than do other culture methods and are not recommended. The authors' conclusion that the 2-plate method used in this study should be the "standard of practice" in evaluating streptococcal disease is not warranted and would substantially increase the cost of this test. Good evidence exists that several different single-plate methods should reliably detect 90% to 95% of group A streptococci from symptomatic patients (1).
Paul G. Shekelle, MD, MPH
Veterans Affairs Medical CenterLos Angeles, California, USA
1. Kellogg JA. Suitability of throat culture procedures for detection of group A streptococci and as reference standards for evaluation of streptococcal antigen detection kits. J Clin Microbiol. 1990;28:165-9.