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

Hafsah D Tootla,Clinton Moodley,Colleen Bamford,Chad M Centner

doi:10.4102/sajid.v36i1.244

Hafsah D Tootla, Clinton Moodley + Show 2 more

Open Access

https://doi.org/10.4102/sajid.v36i1.244

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Abstract

BackgroundCulture 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.MethodWe 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.ResultsStreptococcus 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.ConclusionAccurate 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.

Highlights

Invasive pneumococcal disease (IPD) remains a cause of significant morbidity and mortality worldwide despite the introduction of pneumococcal conjugate vaccination programmes.[1,2,3]The diagnosis of IPD from sterile sites traditionally relies on microscopy indicating gram-positive cocci in pairs and culture of the organism.[4]
A cycle threshold (CT) cut-off value of ≤ 22 was used to determine Polymerase chain reaction (PCR) positivity. This cut-off value was experimentally determined by receiver operating characteristic (ROC) curve analysis (Figure 1-A1) and inspection of CT-value distributions of signal-positive samples (Figure 2-A1)
Receiver operating characteristic curve analysis suggested that the optimal CT cut-off value for determining a positive PCR result was between 17 and 22 and the upper value of CT ≤ 22 was selected

Summary

Introduction

Invasive pneumococcal disease (IPD) remains a cause of significant morbidity and mortality worldwide despite the introduction of pneumococcal conjugate vaccination programmes.[1,2,3]. The diagnosis of IPD from sterile sites traditionally relies on microscopy indicating gram-positive cocci in pairs and culture of the organism.[4] Identification of Streptococcus pneumoniae from culture is confirmed by phenotypic methods, such as susceptibility to optochin (ethylhydrocupreine) and bile solubility. Streptococcus pneumoniae is a fastidious organism, inhibited by prior antibiotic exposure and prone to autolysis under laboratory conditions. Culture remains the diagnostic standard for Streptococcus pneumoniae bacteraemia but is limited by time to identification, prior antibiotics and bacterial autolysis. Cultureindependent 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

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