What does an elevated O titer in the Widal test suggest?

An elevated O titer in the Widal test indicates an increase in agglutinins to Salmonella typhi, the bacterium responsible for typhoid fever. The Widal test is designed to detect specific antibodies in the patient's serum against the antigens of Salmonella species, and the O antigen is associated with the somatic structure of Salmonella typhi.

When a patient has been infected with Salmonella typhi, the immune system responds by producing antibodies to the O antigen. A significant rise in the O titer suggests recent or ongoing infection with this organism. The measurement of these titers is especially important during diagnostic evaluation, as it helps in confirming the presence of typhoid fever.

This context makes it clear that the test has a specific relationship with Salmonella typhi and is not indicative of agglutinins for other organisms like E. coli or Shigella, nor does it suggest a lack of change in antibody levels. The Widal test primarily assesses the immune response specifically to Salmonella species, particularly of clinical relevance in the diagnosis of typhoid fever.