New rapid test for paratyphoid a fever: usefulness, cross-detection, and solution

We described a 5-min colorimetric test for paratyphoid A fever, which detects anti-Salmonella O2 antibodies by inhibiting the binding between 2 types of reagent particles. This test (TUBEX-PA) is based on that (TUBEX-TF) used for typhoid fever, which detects anti-O9 antibodies. TUBEX-PA showed a sensitivity of 81.0% (47/58 culture-confirmed patients) to 93.3% (14/15) and was 98.1% (52/53) specific for healthy subjects. However, TUBEX-PA also detected 50% (7/14) to 81.8% (9/11) of typhoid patients, and conversely, TUBEX-TF detected 46.7% (7/15) to 73.3% (11/15) of paratyphoid A cases. This cross-detection could be abrogated in both tests by adding a blocker (heterologous antigen) to remove the antibodies responsible, which presumably bind to a common antigen (O12) located close to O2 and O9. The presence of anti-O12 antibodies in typhoid (9/12 or 75.0% sensitive) and paratyphoid A (22/33 or 66.7%) patients was demonstrated directly using a prototypic TUBEX test designed specifically to detect these antibodies. Thus, using TUBEX-PA and TUBEX-TF together can increase the diagnostic accuracy of detecting both typhoid and paratyphoid A fever, while the further use of differential tests allows possible immediate discrimination between these diseases.