Original Research

The BinaxNOW pneumococcal antigen test: An adjunct for diagnosis of pneumococcal bacteraemia

Hafsah D. Tootla, Colleen Bamford, Chad M. Centner, Clinton Moodley
Southern African Journal of Infectious Diseases | Vol 36, No 1 | a244 | DOI: https://doi.org/10.4102/sajid.v36i1.244 | © 2021 Hafsah D. Tootla, Colleen Bamford, Chad M. Centner, Clinton Moodley | This work is licensed under CC Attribution 4.0
Submitted: 04 September 2020 | Published: 15 April 2021

About the author(s)

Hafsah D. Tootla, Division of Medical Microbiology, Faculty of Health Sciences, University of Cape Town, Cape Town, South Africa; and, National Health Laboratory Service, Microbiology, Groote Schuur Hospital, Cape Town, South Africa
Colleen Bamford, Division of Medical Microbiology, Faculty of Health Sciences, University of Cape Town, Cape Town, South Africa; and, National Health Laboratory Service, Microbiology, Groote Schuur Hospital, Cape Town, South Africa
Chad M. Centner, Division of Medical Microbiology, Faculty of Health Sciences, University of Cape Town, Cape Town, South Africa; and, National Health Laboratory Service, Microbiology, Groote Schuur Hospital, Cape Town, South Africa
Clinton Moodley, Division of Medical Microbiology, Faculty of Health Sciences, University of Cape Town, Cape Town, South Africa; and, National Health Laboratory Service, Microbiology, Groote Schuur Hospital, Cape Town, South Africa


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Abstract

Background: Culture remains the diagnostic standard for Streptococcus pneumoniae bacteraemia but is limited by time to identification, prior antibiotics and bacterial autolysis. Culture-independent methods for detecting S. pneumoniae include PCR and antigen tests. We evaluated an antigen test on blood culture broth for the rapid detection of S. pneumoniae bacteraemia.

Method: We collected 212 signal-positive blood cultures, with gram-positive cocci in pairs, chains or with uncertain morphology. The BinaxNOW S. pneumoniae urinary antigen test, Gram stain, culture and lytA PCR were performed on all samples. Diagnostic accuracy of the antigen test and Gram stain with gram-positive cocci in pairs were compared with culture, polymerase chain reaction (PCR) and the composite of culture and PCR.

Results: Streptococcus pneumoniae was isolated in 26% of samples, 66% cultured other gram-positive organisms and 8% of samples had no growth. Sensitivity and negative predictive values of the antigen test were 100%, specificity and positive predictive values were 87% – 88% and 76% – 81%, but increased to 93% – 96% and 96% – 98% when applied to subsets with gram-positive cocci in pairs, or history compatible with respiratory illness or meningitis. Sensitivity (69% – 75%) and specificity (81%) of Gram stain (gram-positive cocci in pairs) were lower than the antigen test even when applied to the same subsets.

Conclusion: Accurate and rapid diagnosis of S. pneumoniae bacteraemia is challenging. Specificity of this antigen test is limited by cross-reactivity with other gram-positive organisms, but could be improved if Gram stain morphology and clinical history are available. The antigen test is a useful adjunct for rapid diagnosis of S. pneumoniae bacteraemia.


Keywords

Streptococcus pneumoniae; bacteraemia; BinaxNOW; antigen; pneumococcal diagnosis; test

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