Urinary antigen detection is a valuable tool in the early presumptive diagnosis of Legionnaires’ disease. Legionella pneumonia can often be difficult to diagnose due to nonspecific presentation and traditional methods of culture may take up to five days. Urinary antigen immunochromatographic assays are easy to use, fast and do not require specialised equipment. This assay is routinely used to obtain results when the Legionella EIA is not available. Due to the ease of use and rapid detection, urinary antigen testing has become one of the most used tools in the diagnosis of Legionnaires’ disease. It allows for early antibiotic intervention, aids in the recognition of outbreaks and enables prompt epidemiology investigation.
In this study, we evaluated the sensitivity of the BinaxNOW Legionella urinary antigen immunochromatographic assay. According to the manufacturer’s guidelines, the assay is incubated and the result read after a fifteen minute period. It is not recommended by the manufacturer to incubate the assay or obtain results past this time due to the possibility of obtaining false positive results. However, recent studies suggest a correlation between incubation time and increased sensitivity of the assay.
We collected and analysed three hundred and forty urine samples from patients with symptoms of pneumonia. The BinaxNOW ICT assay was incubated and the results recorded over a period of fifteen minutes to twenty four hours. 1.5 percent of the urines analysed were positive after fifteen minutes. The results obtained from the BinaxNOW ICT assay were confirmed by EIA. We found that by extending the incubation time of the assay up to four hours could increase the sensitivity of the assay. However, positive results obtained after four hours were likely to be false positive.