Rapid tests for the diagnosis of Streptococcal infections: Viable alternative or sorely overrated?

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By Sarah Garland and Effie Helis

Probably one of the most relatable illnesses is the sore throat, also known as pharyngitis. Many of us have experienced the sensation of a burning throat — when eating, drinking, and even talking becomes painful due to the inflamed, red tissue at the back of our mouths. Most of the time, a sore throat is the result of a viral infection; but in some cases, it’s caused by a bacterial infection.

Group A Streptococcus, also called GA Strep, are bacteria that can cause a variety of conditions, notably pharyngitis and skin infections, and it can sometimes lead to serious complications. It’s estimated that 20 to 40 per cent of sore throats in children, and five to 15 per cent of sore throats in adults, are caused by GA Strep.

If pharyngitis is caused by GA Strep, treatment with antibiotics is often effective; but antibiotics will not be effective if it’s caused by a viral infection. Additionally, it’s important to prescribe antibiotics only when the infection is bacterial, as the inappropriate use of antibiotics contributes to the problem of antimicrobial resistance. To avoid the overuse of antibiotics and avoid possible complications from untreated infections, GA strep should be diagnosed correctly.

However, based on clinical presentation alone, it can be difficult to determine whether a sore throat is caused by GA Strep or a viral infection. The gold standard for diagnosing GA Strep is a diagnostic test based on a throat culture. The main limitation of traditional, laboratory, culture-based tests is the time it takes between obtaining the culture (i.e., throat swab) and receiving the test results. This process may take several days, and patients may not always be able to return to the clinic for treatment.

In response to the need for more timely diagnostic tests for GA Strep, several non-culture based tests have been developed; these are often referred to as rapid tests. The two main categories of rapid tests are those based on immunoassays and those based on molecular assays. These tests may still require a throat swab, but due to different diagnostic techniques, the results are obtained much sooner than traditional culture-based tests – some can provide results in a matter of minutes! However, it’s uncertain how accurate of a diagnosis they provide compared with culture-based methods and whether they improve health outcomes for patients.

When questions like these arise, the health care community turns to CADTH — an independent agency that finds, assesses, and summarizes the research on drugs and medical devices. CADTH undertook a Rapid Response report to look for evidence on the diagnostic accuracy and clinical utility (i.e., changes in the duration of symptoms, severity of condition, and antibiotic prescribing practices) related to rapid diagnostic tests for GA Strep. Learn more about the methods and scope of CADTH’s Rapid Response Service at: https://www.cadth.ca/about-cadth/what-we-do/products-services/rapid-response-service.

In terms of diagnostic accuracy, the CADTH review found that the sensitivity, or ability of the tests to correctly identify people with GA Strep, varied between 93% and 99% for molecular tests and between 55 and 94 per cent for immunoassays. The specificity, or ability of the tests to correctly identify people without GA Strep, varied between 91% and 99% for molecular tests and between 81 and 100 per cent for immunoassays. This suggests that for rapid tests, molecular-based techniques may be better able to identify GA Strep. Additionally, it appears that there is no test that distinguishes between GA Strep carriers and an active GA Strep infection.

Often, the decision to prescribe an antibiotic is made during the patient’s initial visit to the doctor and is based on a clinical score — an assessment of a patient’s symptoms and medical history. The CADTH review looked for evidence regarding the clinical utility of rapid tests compared with clinical scores, but the evidence that was found was limited and conflicting. There was, however, limited evidence to suggest that the use of a rapid test might decrease the number of antibiotic prescriptions written for patients with sore throats.

So what’s the bottom line? Overall, rapid tests for diagnosing Group A Streptococcus offer good diagnostic accuracy, and molecular-based tests are likely to be more sensitive than immunoassay-based tests; however, the evidence regarding clinical utility of rapid tests is limited. While rapid detection tests could be used for obtaining a clinical diagnosis for Group A Streptococcus faster, the use of these tests will depend on patient and practitioner factors – such as familiarity and skill with the tests, feasibility of each approach, type of test used, and severity of patient symptoms.

If you’d like to read CADTH’s full report on rapid tests for GA Strep, it’s freely available at https://www.cadth.ca/rapid-tests-diagnosis-group-streptococcal-infection-review-diagnostic-test-accuracy-clinical-utility. To learn more about CADTH, visit www.cadth.ca, follow us on Twitter: @CADTH_ACMTS, or talk to our Liaison Officer in your region: www.cadth.ca/contact-us/liaison-officers.

Sarah Garland and Effie Helis are Knowledge Mobilization Officers at CADTH.