From the Guidelines
For a throat culture, a single swab is sufficient to collect an adequate sample from the throat, with a sensitivity of 90%–95% in detecting the presence of group A streptococci. The swab should be obtained from the surface of both tonsils (or tonsillar fossae) and the posterior pharyngeal wall, as these are the most acceptable sites for sampling 1. Other areas of the oropharynx and mouth are not acceptable sites for sampling, and these sites should not be touched before or after the appropriate areas have been sampled. The use of anaerobic incubation and selective culture media may increase the proportion of positive culture results, but the increased cost and effort associated with these methods are difficult to justify, particularly for physicians who process throat cultures in their own offices 1.
The clinical significance of the number of group A streptococcal colonies present on the throat culture plate is problematic, and there is too much overlap to permit accurate differentiation between patients with true acute GAS pharyngitis and patients who are streptococcal carriers on this basis alone 1. The duration of incubation is also an important variable, with cultures typically incubated at 35°C–37°C for 18–24 hours before reading, and additional incubation overnight at room temperature may identify a number of additional positive throat culture results 1.
Key considerations for throat culture include:
- Using a single swab to collect a sample from the throat
- Obtaining the swab from the surface of both tonsils (or tonsillar fossae) and the posterior pharyngeal wall
- Avoiding the use of antiseptic mouthwash or antibiotics before the sample is collected
- Incubating the culture at 35°C–37°C for 18–24 hours before reading, with additional incubation overnight at room temperature if necessary. The most recent and highest quality study, published in 2012, supports the use of a single swab for throat culture, with a sensitivity of 90%–95% in detecting the presence of group A streptococci 1.
From the Research
Throat Culture Swabs
- The number of swabs for throat culture is not explicitly stated in most studies, but it can be inferred from the methods used in each study.
- In the study by 2, triple throat swabs were used to determine the accuracy of a rapid latex agglutination test and backup throat culture.
- The study by 3 used a single swab for sequential plating and rapid antigen testing, and found that the recovery of streptococci from swabs plated sequentially on five plates did not vary with the order of plating.
- Another study by 4 collected two throat swabs from each patient, one for the rapid antigen-detection test and office culture, and the other for laboratory blood agar plate culture.
Implications for Throat Culture
- The use of multiple swabs may increase the sensitivity of throat culture, as seen in the study by 2 which used triple throat swabs.
- However, the study by 3 found that the actual proportion of organisms recovered from a swab on a single plate was only 1%, suggesting that multiple swabs may not be necessary.
- The choice of diagnostic test and the number of swabs used may depend on the clinical setting and the pretest likelihood of group A streptococcus pharyngitis, as seen in the study by 4.