Equal performance of self-collected and health care worker-collected pharyngeal swabs for group a streptococcus testing by PCR

Abstract

A process employing patient- or parent-collected pharyngeal swabs for group A Streptococcus (GAS) testing would expedite diagnosis and treatment, reduce patient exposure to the health care setting, and decrease health care costs. Our aim was to determine the concordance between patient- or parent-collected (self-collected) and health care worker (HCW)-collected pharyngeal swabs for detection of GAS by PCR. From 9 October 2012 to 21 March 2013, patients presenting with a sore throat meeting criteria for GAS testing and not meeting criteria for severe disease were offered the opportunity to collect their own pharyngeal swab. The HCW also collected a swab. Paired swabs were tested by GAS real-time PCR, allowing semiquantitative comparisons between positive results. Of the 402 participants, 206 had a swab collected by the patient and 196 a swab collected by the parent. The percent positivity results were 33.3% for HCW-collected swabs and 34.3% for self-collected swabs (P = 0.41). The overall concordance between the two collection strategies was 94.0% (95% confidence interval [CI], 91.3 to 96.0). Twenty-four of the paired swabs had discordant results, with 10 and 14 positives detected only with the HCW- and self-collected swabs, respectively (P = 0.41). The person collecting the swab in the self-collected arm, the order of collection, and prior swab collection training did not influence results. Among the 124 specimens that were positive by both collection methods, the amount of GAS DNA was higher in the self-collected versus the HCW-collected swabs (P = 0.008). Self-collected pharyngeal swabs provide a reliable alternative to HCW collection for detection of GAS and offer a strategy for improved health care delivery.

FIG 1

Instructions for collecting a pharyngeal swab. Instructions provided to patients and parents at the time of study participation are presented.

FIG 2

Real-time PCR crossing-point values from HCW-collected versus self-collected swabs for the 124 specimens that were positive by both collection methods. The crossing point (CP), which represents the number of real-time PCR cycles at which a positive PCR is first detected, can be used as a semiquantitative measure of the amount of DNA collected by each swab. The value is inversely proportional to the amount of DNA in the specimen; lower values correlate with larger amounts of DNA. The red line indicates where the data would fall were the CPs exactly the same by the two methods.