The diagnosis of typhoid paratyphoid is based on the presence of compatible clinical disease and the isolation of Salmonella typhi or Salmonella paratyphi from culture samples. The rate of positive blood cultures in patients with typhoid fever is 50-70%, depending on the specific study and the culture technique used. Blood cultures may need to be incubated for several days. Diagnosis can also be made by culture of feces, urine, roseola specimens, or duodenal contents (by pulling a thread capsule). Stool cultures are positive in as many as 30-40% of cases, but are usually negative until the patient is seen for systemic symptoms. Bone marrow culture is the most sensitive diagnostic method, but the test is rarely needed in routine clinical practice. Serologic tests (e.g., the Fertilizer test) have limited clinical utility in endemic areas because a positive result may represent a previous infection. The fiducial test detects anti-Salmonella typhi antibodies, and the minimum titers defined as O (surface polysaccharide) antigen and H (flagellar) antigen positivity must be determined regionally; the minimum titers are higher in developing countries than in developing countries.