Rapid Instrumental Testing of Bacterial Strep A Infections
C, F and G streptococci (S. milleri, S. anginosus, S. constellatus, S. intermedius), S. salivarius, Enterococcus faecalis, E. casseliflavus, Staphylococcus aureus or Arcanobacterium haemolyticum were observed, indicating the high specificity of the test. ReaScan values ≤5 (= negative) were observed with all these organisms. The results showed that the tests meet the requirements for diagnostic use.
The test performance was compared with streptococcal culture and with another commercial antigen test (TestPack Strep A test) for the parallel throat samples. The ReaScan cut-off value 20 was used in the discrepant analyses. A reading value below this cut-off value 20 represents a negative result and a value above this cut-off means a positive result. ReaScan Strep A test performed in good concordance with the culture method (see Table 2A) showing high sensitivity and specificity as well as high positive predictive value (PPV) and negative predictive value (NPV). The corresponding values were somewhat lower for the TestPack Strep A test (see Table 2B). The difference between these two methods was not, however, statistically significant when tested according to the procedures of Hawass,9
perhaps due to
the limited number of patients tested. Figure 1C shows a comparison of the present test reader values with those of streptococcal culture. A reasonable correlation line was achieved.
The test performance of ReaScan Strep A in the present study is very similar to that of the LightCycler® polymerase chain reaction (PCR) against bacterial culture in the study of Uhi et al.10
Uhi et
al. compared the performance characteristics of a realtime PCR method, the LightCycler Strep A assay (Roche Applied Science, US), with those of a rapid antigen immunoassay, the Directigen 1-2-3 Group A Strep test kit (BD Diagnostic Systems, US) and a standard culture method for detection of GAS, in 384 throat swabs. In this study, the LightCycler PCR produced more positive results (n=58) than either culture (n=55) or the Directigen immunoassay (n=31). The results of the LightCycler PCR and the Directigen method were independently compared with the results of the accepted ‘gold standard’ bacterial culture. The sensitivities, specificities, PPVs and NPVs for this comparison were as follows: for the Directigen method, 55, 99, 97 and 93 %, respectively; for the LightCycler PCR, 93, 98, 88 and 99 %, respectively.
Conclusion
In conclusion, ReaScan Strep A is a novel rapid test for reliable and quick diagnosis of GAS tonsillitis and pharyngitis. The ReaScan Strep A is suitable for point-of-care testing of GAS with minimum training and its performance is comparable to the standard laboratory method for GAS. An additional advantage of this novel instrumental Strep A assay is the possibility of receiving numerical test results
1. Wannamaker LW, Perplexity and precision in the diagnosis of streptococcal pharyngitis, Am J Dis Child, 1972;124:352–35.
2. Shaikh N, Leonard E, Martin JM, Prevalence of streptococcal pharyngitis and streptococcal carriage in children: A meta-analysis, Pediatrics, 2010;126:e557–e64.
3. Armengol CE, Schlager TA, Hendley JO, Sensitivity of a rapid antigen detection test for group A streptococci in a private pediatric office setting: answering the red book’s request for validation, Pediatrics, 2004;113:924–6.
4. Bisno AL, Gerber MA, Gwaltney JM, et al., Practice guidelines for the diagnosis and management of group A streptococcal pharyngitis, Clin Infect Dis, 2002;35:113–25.
Objective interpretation of rapid test results with the aid of an instrument has not been widely used. Reagena International Ltd has now introduced the ReaScan reader to provide a more objective tool and to overcome hurdles of visual interpretation. The ReaScan reader is easy to use and inexpensive and together with dedicated tests, can be used in several clinically important applications.11
n
5. Roe M, Kishiyama C, Davidson K, et al., Comparison of BioStar Strep A OIA Optical Immune Assay, Abbott TestPack Plus Strep A, and culture with selective media for diagnosis of group A streptococcal pharyngitis, J Clin Microbiol, 1995;33:1551–3.
6. American Academy of Pediatrics Committee on Infectious Diseases, Group A streptococcal infections. In: The Red Book, Elk Grove Village, IL: American Academy of Pediatrics, 2000;526–36.
7. Faclam RR, Specificity study of kits for detection of group A streptococci directly from throat swabs, J Clin Microbiol, 1987;23:504–8.
8. Spellerberg B, Brand C, Chapter 29: Streptococcus. In:
Murray PR, Baron EJ, Jorgensen JH, et al. (eds), Manual of Clinical Microbiology, 9th edition, Washington, DC: American Society for Microbiology Press, 2007;418.
9. Hawass NED, Comparing the sensitivities and specificities of two diagnostic procedures performed on the same group of patients, Br J Radiol, 1997;70:360–6.
10. Uhl JR, Adamson SC, Vetter EA, et al., Comparison of LightCycler PCR, Rapid Antigen Immunoassay, and Culture for Detection of Group A Streptococci from Throat Swabs, J Clin Microbiol, 2003;41:242–9.
11. Parviainen M, Kelo E, Sirola H, et al., Detection of Acute Hantavirus Infections Using Novel Instrument-readable Rapid Tests, Eur Infect Dis, 2011;5:35–7.
ReaScan Strep A Positive
Culture positive Culture negative n
Specificity 96.9 % Sensitivity
92.1 % Prevalence 16.5 %
Table 2B: TestPack Strep A Test against Culture TestPack Strep
Culture positive Culture negative n
Specificity 93.8 % Sensitivity
86.8 % Prevalence 16.5 %
which are directly proportional to the amount of GAS antigen in the swab samples.
A Positive 33 12 45
TestPack Strep A Negative
5
181 186
Negative predictive value 97.3 % Positive predictive value
73.3 % n 38
193 231
35 6
41
ReaScan Strep A Negative
3
187 190
Negative predictive value 98.4 % Positive predictive value
85.4 % n 38
193 231
Figure 1C: Comparison of the Test Results Obtained by the ReaScan Strep A Rapid Test and by Culturing Group A Streptococcus in the Present Study
Reader value versus culture result (+, ++, +++)
100 150 200 250
50 0 Negative + Culture result
The reader values (mean ± standard error of the mean [SEM]), as measured and the corresponding culturing results (negative, +, ++ and +++) are shown.
Table 2A: ReaScan Strep A Test against Culture, Test Reader Cut-off Value 20
++ +++
EUROPEAN INFECTIOUS DISEASE
145
Reader value
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