Baseline titres of O, H and AH agglutinins to Salmonella Typhi and Paratyphi A in blood donors in Sri Lanka.

Background: Sri Lanka is considered as an endemic country for enteric fever. Due to difficulties in performing blood cultures, the Widal test is still commonly carried out for the diagnosis of enteric fever. However, there are no published data on current baseline Widal titres in the country. This study was carried out to determine the baseline titres of O, H and AH agglutinins among the Sri Lankan population.

Method: Five hundred and one (501) serum samples of blood donors from 31 blood banks in Sri Lanka were collected during 2012 and 2013 and were screened for Salmonella O, H and AH agglutinins using the Widal tube test. A titre of 20 and above was considered positive. Age and gender of the study participants were recorded.

Results: Of the 501 sera tested, 58% were positive for at least one of the O, H and AH agglutinins. Salmonella O, H and AH agglutinins were positive in 46.1%, 26.5% and 8.4% of the study population respectively. Of the study population, 97.5% had O, H and AH agglutinin titres less than or equal to 80, 160 and 80 respectively. A significantly higher percentage of females (H-36.7%, p=0.019; AH-15.3%, p=0.15) were positive for H and AH agglutinins than males (H-24.9%; AH-7.4%). The baseline titre of AH agglutinins was higher in females (160) than males or the total population (80). Highest test positivity (40%) was seen among the 31-40 year age group. A significant number of donors below 20 were negative for Salmonella O agglutinins (p=0.024).

Conclusion: We recommend baseline titres of 80, 160 and 80 for Salmonella O, H and AH agglutinins respectively to be used in Sri Lankan settings. As there is a variation in baseline titre with age and gender it is necessary to consider both when interpreting Widal test results.