Rapid coagglutination test for the direct detection of group A streptococci from throat swabs

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Abstract
A one-minute antigen detection test was compared with a conventional culture method for detecting group A β-hemolytic streptococci. The test detects coagglutination between protein A and streptococcal antigen extracted directly from throat swabs. Of the 307 specimens tested, 66 (21.5%) were positive for group A streptococci by culture and 16 specimens (5.2%) were positive for other β-hemolytic streptococci. The direct test agreed with the culture in 274 of 307 specimens (accuracy 89.3 %). The sensitivity of the test was 86.4 % (57/66), the specificity 90 % (217/241), the positive predictive value 70.4 % and the negative predictive value 96 %. If only throat cultures with more than 100 colonies of group A streptococci per plate, were considered, the sensitivity of the direct test rose to 96 %. If only a strong agglutination was considered positive, the specificity of the direct test rose to 98 %. Further studies are needed to determine whether this test could be used alone or in addition to culture.