Rapid Streptococcus Tests Accurate

Veronica Hackethal, MD

September 08, 2014

Rapid antigen diagnostic tests (RADTs) are accurate in the diagnosis of strep throat, according to a systematic review and metaanalysis published online September 8 in Pediatrics.

"RADTs can be used for accurate diagnosis of [group A streptococcus (GAS)] pharyngitis to streamline management of sore throat in primary care," write Wei Ling Lean, MBBS, from the Department of General Medicine, Royal Children’s Hospital, Melbourne, Australia, and colleagues. "RADTs may not require culture backup for negative tests in most low-incidence rheumatic fever settings. Newer molecular tests have the highest sensitivity, but are not true point-of-care tests." Point-of-care tests ideally have a turn-around time of 1 to 3 hours, according to the authors.

Symptoms caused by GAS, the most common bacterial cause of sore throat, are often difficult to differentiate from other causes of sore throat. Throat culture, the gold standard for diagnosing sore throat, can delay diagnosis and interfere with effective care, according to the authors. At least 2 guidelines (1 in Europe and 1 in the United States) recommend using RADTs, but concerns still exist about their accuracy.

The researchers searched Medline and Embase for studies in children and adults published in English only from 1996 to 2013. They also manually reviewed bibliographies of selected articles. They limited their review to studies that used throat cultures or blood agar as a reference.

Ultimately, the investigators included 48 studies, of which 36 took place in developed countries and 12 in developing countries. The studies covered 23,934 patients. Results suggest an overall sensitivity and specificity of 0.86 (95% confidence interval [CI], 0.83 - 0.88) and 0.96 (95% CI, 0.94 - 0.97), respectively. Estimates of sensitivity and specificity in pediatric cases were similar (0.87 [95% CI, 0.84 - 0.89] and 0.96 [95% CI, 0.95 - 0.97], respectively).

Molecular-based RADTs, such as DNA probes, polymerase chain reaction, and fluorescence in situ hybridization, were the most accurate (pooled sensitivity, 0.92 [95% CI, 0.89 - 0.95]; specificity, 0.99 [95% CI, 0.97 - 0.99]).

The authors note considerable variability in the true positive rate across studies but little variability in the true negative rate. They also note considerable variability in factors such as study methods and quality, clinical severity of patients, and variations in throat swab technique. They also point out that the small number of studies limited their ability to perform further analyses to weed out potential confounders.

The results suggest a possibility of missing 14% of strep throat cases. The authors considered this an "acceptable level of risk" in developed countries that have a low risk for rheumatic fever (the most serious adverse effect of untreated strep throat). Developing countries, however, have a higher risk for rheumatic fever and could benefit from highly sensitive, inexpensive RADTs with rapid turnaround times. Results from this study, however, suggest no single test satisfies these 3 criteria.

"The high overall specificity of RADTs means that these tests can prevent unnecessary antibiotic prescription due to minimal overdiagnosis of GAS pharyngitis in the vast majority of cases [in developed countries]," the authors conclude.

Two coauthors report receiving funding from Quidel Corporation for the development of a rapid antigen diagnostic product. The other authors have disclosed no relevant financial relationships.

Pediatrics. Published online September 8, 2014.

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