Group A Streptococcus Rapid Immunoassay Evaluated

By LabMedica International staff writers
Posted on 27 Apr 2016

Clinical reasoning utilizing certain symptoms and scores has not proven to be a reliable decision-making tool to determine whether or not to suspect a group A Streptococcus (GAS) infection in the patient presenting with a sore throat.

An excellent solution for this dilemma would be a bedside rapid antigen detection test (RADT) for GAS that provides a positive or negative result within five to 10 minutes, thus directly influencing the treatment decision. Such RADTs have been evaluated previously using throat culture as the gold standard and initially showed insufficient sensitivity and specificity.

Image: The TestPack Strep A test is a simple rapid immunochromatographic assay for the qualitative detection of Group A Strep antigen from throat swabs (Photo courtesy of Alere).

Scientists at North West Hospital and Health Service (Mount Isa, QLD, Australia) collected throat swab samples from patients attending the Mount Isa Hospital emergency department for a sore throat; these samples were compared to swab samples collected from healthy controls that did not have a sore throat and both groups were aged three to 15 years. None of these children had previously had rheumatic fever.

The study team processed the swabs within 72 hours using the Alere Test Pack +Plus with OBC Strep A kit (Alere, Waltham, MA, USA). This test kit had shown a 95% to 97% sensitivity and a specificity of 95% to 100% to detect GAS. In this study the outcomes were compared with the assumption that the sensitivity of the point-of-care test was 95% and with the assumption that the sensitivity was 97%. The treating clinician was blinded to the point-of-care test result.

From June 30, 2014 to February 27, 2015, 248 throat swabs were collected and examined within 72 hours. Out of the 101 patients presenting with a sore throat, 26 (26%) tested positive for GAS. Only one (0.7%) of the 147 control patients had a positive test result. Statistical analysis showed both the positive and negative etiologic predictive value (EPV) to be high. The 95% confidence interval for positive EPV was 88% to 100% and for negative EPV was 97% to 99%, depending on assumptions made.

The authors concluded that their study demonstrated that the point-of-care test Alere Test Pack +Plus Strep A has a high positive predictive value and was able to rule in GAS infection as long as the proportion of carriers is low. Also the negative predictive value for ruling out GAS as the etiologic agent is very high irrespective of the carrier rate. Hence, this test is always useful to rule out GAS infection. The study was published in the April 2016 issue of the International Journal of Infectious Diseases.

Related Links:
North West Hospital and Health Service
Alere