Diagnostics
A Novel Strep A Test for a Rapid Test Reader Compared with Standard Culture Method and a Commercial Antigen Assay
Markku Parviainen,1,2 Markku Koskela,3 Irma Ikäheimo,4 Eira Kelo,5 Helena Sirola,6 Kati Ryynänen7 and Ilkka Mononen8
1. Department of Clinical Chemistry, Faculty of Health Sciences, School of Medicine, University of Eastern Finland; 2. Managing Director, Reagena Ltd; 3. Chief Physician; 4. Microbiologist, Microbiology Laboratory, Oulu University Hospital; 5. Research and Development Manager; 6. Quality Control Manager; 7. Marketing Manager, Reagena Ltd; 8. Chief Physician, Department of Clinical Chemistry and Haematology, Turku University Central Hospital and Department of Clinical Chemistry and Joint Laboratory, University of Turku
Abstract ReaScan® Strep A is a novel rapid test for Streptococcus pyogenes (Group A Streptococcus [GAS]), which is a common cause of curable tonsillitis and pharyngitis. ReaScan Strep A is suitable for point-of-care testing for GAS with minimum training, and its performance is comparable to a standard laboratory method for GAS. The test performance was compared with streptococcal culture and with a commercial antigen test in parallel throat swab samples. ReaScan Strep A test performed in good concordance with the culture, showing sensitivity 92.1 % and specificity 96.9 %, as well as positive predictive value (PPV) 85.4 % and negative predictive value (NPV) 98.4 %, with a prevalence of 16.5 %. The corresponding values for a commercial antigen test were somewhat lower; still, the difference was not statistically significant.
Keywords Rapid antigen detection test, Group A Streptococcus, streptococcal pharyngitis
Disclosure: Markku Parviainen, Eira Kelo, Helena Sirola and Kati Ryynänen are employed by Reagena. The remaining authors have no conflicts of interest to declare. Acknowledgements: We thank the doctors and nurses at the Oulu University Hospital joint emergency department for the skilled sampling of the throat swab samples of this study and for performing the rapid antigen detection test (RADT) with the TestPack®
Strep A test. We also thank the personnel at the Oulu University Hospital
microbiology laboratory for performing the reference cultivations on the selective sheep blood agar for streptococci and for performing the RADT with the ReaScan Strep A test. We greatly appreciate the development work carried out by Ilpo Kuronen and Mikko Regina. We also thank Anne Lyytikäinen and Riikka-Liisa Miettinen for their great technical assistance. Received: 4 August 2011 Accepted: 12 August 2011 Citation: European Infectious Disease, 2011;5(2):143–5 Correspondence: Markku Parviainen, Reagena Ltd, Takojantie 18, FIN-70900 Toivala, Finland. E:
markku.parviainen@
reagena.fi
Support: The publication of this article was funded by Reagena Ltd.
Streptococcus pyogenes (Group A Streptococcus [GAS]) is a significant cause of tonsillitis and pharyngitis, accounting for approximately 30–40% of cases, while the remaining 60–70 % are mostly viral.1,2
The standard
method for establishing the aetiological diagnosis of GAS pharyngitis is culture of a pharyngeal specimen obtained with a throat swab and cultivated on selective sheep blood agar for streptococci. The throat culture requires 24–48 hours to interpret, thus delaying the diagnosis.3
Rapid antigen detection tests (RADT), e.g. Strep A rapid tests, help in early diagnosis of and immediate intervention in, GAS infection, since RADT provide results much more quickly than the standard throat swab plate culture does. RADT generally have specificity of 95 %, and therefore false-positive results are rare. The primary advantage of RADT over throat swab cultures is that results can be available in only 5–10 minutes and the tests are inexpensive. Treatment, if needed, can be started before the patient leaves the doctor’s office.4
The sensitivity of RADT has been questioned since rapid antigen tests have shown a significant percentage of false-negative results, resulting in recommendations to back up negative rapid antigen tests with a culture.5
Guidelines from the American Academy of Pediatrics © TOUCH BRIEFINGS 2011
recommend that the diagnosis of GAS infection should only follow laboratory confirmation and that for any rapid antigen test that is negative for GAS infection, a follow-up culture should be carried out.6 According to Armengol et al.,3
the measured sensitivity of RADT
depends to some degree on the culture method with which they are compared, but yet they conclude that a negative RADT result in their paediatric practice still requires a confirmatory throat swab culture.
Materials and Methods ReaScan Instrument The ReaScan® rapid test reader (Reagena International Ltd, Toivala, Finland) is based on the measurement of light reflection from a test membrane at the maximum intensity level (background) and the intensity of the test line. Reflected light is measured by a photodetector at a 45° angle. The measurement time is five seconds and the ReaScan reader automatically subtracts the background intensity R from the test-line intensity T. The net intensity of the test line (R-T) appears as a numerical value on the display. This numerical value is the basis for the final result that is determined to be positive, weakly positive or negative. The reference value ranges are given on the lot-specific ReaScan result labels. The measurement is performed
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