Detection of Legionella pneumophila antigen in urine samples by the BinaxNOW immunochromatographic assay and comparison with both Binax Legionella Urinary Enzyme Immunoassay (EIA) and Biotest Legionella urinary antigen EIA

The new BinaxNOW Immunochromatographic (ICT) Assay for the detection of Legionella pneumophila antigens was used to test 535 urine specimens from patients with and without Legionnaires' disease. The specificity, calculated by testing 112 samples from patients with pneumonia of aetiologies other than Legionella infection, and 167 urine specimens from urinary tract infections, was found to be 97.1% if the manufacturer's guidelines were followed. However, it was determined that the 'false positive' results characterised by very weak bands could be discounted by reexamination of the results at 60 min, yielding a specificity of 100%, With this minor modification of the procedure applied to examination of urine samples from 117 patients with legionellosis confirmed by isolation of L, pneumophila and 70 patients who had seroconverted to L, pneumophila serogroup 1, sensitivity was calculated to be 79.7%. In comparison, the sensitivities of the Binax Urinary Antigen Enzyme Immunoassay (EIA) and Biotest Urin Antigen EIA were estimated to be 79.1 and 83.4%, respectively. Eleven cases (5.9%) were positive by BinaxNOW assay but negative by Binax or Biotest EIA, or both. The sensitivities of all assays increased to c. 94% if only diagnosis of cases confirmed by isolation of serogroup 1 L, pneumophila was considered, although the sensitivity for infections caused by L. pneumophila serogroup 1 monoclonal antibody (MAb) subgroup Bellingham was significantly lower than for other MAb subgroups. The Biotest EIA recognised 10 (45%) of the 22 cases not caused by L, pneumophila serogroup 1, whereas the two Binax kits detected only three each. The ICT assay BinaxNOW can be recommended as a rapid specific test for the diagnosis of Legionnaires' diseases caused by L, pneumophila serogroup 1, although very weak bands should be interpreted cautiously.