Blood culture is considered the gold standard and confirmatory test for diagnosing Salmonella typhi infection because it directly detects the presence of the bacteria in the bloodstream during the early phase of illness. It has higher specificity and allows for antibiotic sensitivity testing, which guides appropriate treatment. Other tests like the Widal test are less specific and may produce false positives due to previous vaccination or exposure. Urine culture and stool microscopy are less reliable in the acute phase and serve more as adjuncts in the later stages or carriers.

Reference: Harrison's Principles of Internal Medicine, J. Larry Jameson, 20th Edition.