Evaluation of a Typhoid/Paratyphoid Diagnostic Assay (TPTest) Detecting Anti-Salmonella IgA in Secretions of Peripheral Blood Lymphocytes in Patients in Dhaka, Bangladesh.

Background: Rapid and reliable diagnostic assays for enteric (typhoid and paratyphoid) fever are urgently needed. We report the characterization of novel approach utilizing lymphocyte secretions, for diagnosing patients with enteric fever by the TPTest procedure. Methodology: TPTest detects Salmonella-specific IgA responses in lymphocyte culture supernatant. We utilized TPTest in patients with suspected enteric fever, patients with other illnesses, and healthy controls. We also evaluated simplified modifications of TPTest for adaptation in laboratories with limited facilities and equipment. Principal Findings: TPTest was positive in 39 (27 typhoid and 12 paratyphoid A) patients confirmed by blood culture and was negative in 74 healthy individuals. Among 32 individuals with other illnesses, 29 were negative by TPTest. Of 204 individuals with suspected enteric fever who were negative by blood culture, 44 were positive by TPTest and the patients were clinically indistinguishable from patients with confirmed bacteremia, except they were more likely to be under 5 years of age. We evaluated simplifications in TPTest, including showing that lymphocytes could be recovered using lysis buffer or buffy coat method as opposed to centrifugation, that incubation of cells at 37°C did not require supplemental CO2, and that results were available for majority of samples within 24 hours. Positive results by TPTest are transient and revert to negative during convalescence, supporting use of the test in endemic areas. The results can also be read using immunodot blot approach as opposed to ELISA. Since no true gold standard currently exists, we used a number of definitions of true positives and negatives. TPTest had sensitivity of 100% compared to blood culture, and specificity that ranged from 78–97% (73–100, 95% CI), depending on definition of true negative. Conclusion: The TPTest is useful for identification of patients with enteric fever in an endemic area, and additional development of simplified TPTest is warranted. Author Summary: Salmonella enterica serotype Typhi and Paratyphi A are responsible for typhoid and paratyphoid fever respectively and the disease caused is known generally as enteric fever. Appropriate and early diagnosis of the disease is important for initiation of treatment of the patient with a suitable antibiotic. The performance of the available diagnostic methods take time and as well as have low sensitivity and specificity. We describe here an immunodiagnostic assay, the TPTest, which is based on the use of secretions of antibodies from peripheral blood lymphocytes. We describe simplifications of the procedure and show that the assay has as a sensitivity of 100% with a specificity that ranges from 78–97% (73–100, 95% CI) for detecting patients with typhoid and paratyphoid fever in an enteric fever endemic zone. The TPTest uses a small blood volume, and reverts to negative by convalescence, supporting its further development as a relatively low cost assay to diagnose patients with enteric fever in endemic zones. [ABSTRACT FROM AUTHOR]