AT PAS 17
SAN FRANCISCO (FRONTLINE MEDICAL NEWS) – The diagnosis of pharyngitis due to infection by group A streptococcus (GAS) based on the detection of nucleic acid is fast and accurate. But this benefit brings the possibility of overuse of antibiotics, with treatment offered to those who, in the era of growth-based detection of the bacteria, would not have received treatment, according to Robert R. Tanz, MD .
Dr. Tanz, professor of pediatrics at Northwestern University, Chicago, delivered this cautionary note at the meeting of the Pediatrics Academics Societies.
“The short turnaround time, high sensitivity, and high specificity of newer molecular tests for GAS will make their use increasingly common. Many of the additional patients identified by molecular testing may not have an illness attributable to GAS. The increase in positive tests will probably be associated with increased antibiotic prescribing. This may not be beneficial, especially in areas with low rates of acute rheumatic fever,” said Dr. Tanz, who practices at Ann & Robert Lurie Children’s Hospital of Chicago.
For over a half century, the diagnosis of bacterial infections including GAS pharyngitis relied on the growth of the bacteria. The process is tried and true, Dr. Tanz said. But growth of the bacteria to form a visible colony on the growth medium takes time – typically 18-48 hours. The development of tests capable of rapidly and selectively detecting bacterial components – first proteins and more recently target sequences of genetic material – has changed the landscape of diagnosis, he added.
Nowadays, diagnosis of GAS at Ann & Robert Lurie Children’s Hospital uses the Illumigene system. Its use has been approved by the Food and Drug Administration for diagnosis without the need for backup culture of throat swabs. Results are available in about 1 hour.
Dr. Tanz and his colleagues took a retrospective look at patient records during 2013, when testing was still growth-based, and in 2014 and 2015, after the hospital had shifted to the molecular analysis of throat swabs. The aim was to determine the proportion of tests positive for GAS prior to and after the switch.
The positive detection rate of 9.6% (96 of 997 samples) in 2013 climbed to 17% (152 of 894) in 2014 and 16% (138 of 859) in 2015. The difference was highly significant (P less than .00001).
Sore throats increase in the colder months when people tend to be indoors more often, and this seasonality was evident in 2013. However, detection was more consistent throughout the 12 months in 2014 (2013 vs 2014, P less than .000001) and 2015 (2013 vs 2015, P less than .00001). The detection rates in 2014 and 2015 were similar (P equal to .59), according to Dr. Tanz.
The new era of molecular testing circumvents what is known as the spectrum effect, in which culture-based tests are more often positive in patients with symptoms that are consistent with the infection, he said. In contrast, molecular tests can increase the identification of the target bacterium, here GAS, in patients who have sore throat caused by a viral infection.
Preliminary results presented by Dr. Tanz indicated a significantly greater detection of GAS in children not displaying symptoms of infection. In decades past, these children would not have been treated.
“The increase in positive tests for group A strep is likely associated with increased antibiotic prescribing. This may or may not represent a benefit to patients, as the additional identified patients may not have an illness actually attributable to group A strep,” said Dr. Tanz.
“Our findings support rigorous selectivity in choosing which patients to test, specifically excluding those with overt viral symptoms, as recommended in the guidelines from the [Infectious Diseases Society of America], [American Academy of Pediatrics], and other groups,” he said, adding that clinician guidance on the use of molecular diagnostic tests for pharyngitis caused by group A strep is a prudent step.
Features suggestive of GAS may include sudden onset of sore throat, age 5-15 years, fever, headache, nausea, vomiting, and abdominal pain, tonsillopharyngeal inflammation and patchy tonsillopharyngeal exudates, palatal petechiae, tender nodes, and scarlatiniform rash. Viral pharyngitis features include conjunctivitis, coryza, cough, diarrhea, discrete ulcerative stomatitis, viral exanthema, and hoarseness, according to the 2012 Infectious Diseases Society of American guidelines (Clin Infect Dis. 2012. doi: 10.1093/cid/cis629).
For most people, a sore throat is more of a temporary inconvenience rather than a looming health threat, but GAS easily spreads from person to person and can lead to the more serious condition of acute rheumatic fever, Dr. Tanz said. Hence the concern with diagnosing the cause of sore throat and, when the cause is bacterial, alleviating the infection using antibiotics.
The study was conducted at Ann & Robert H. Lurie Children’s Hospital of Chicago and was not funded. Dr. Tanz reported having received support from Meridian Bioscience, manufacturer of the Illumigene Group A Streptococcus assay. Meridian Bioscience did not support this study.