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Diagnosis of Salmonella Infection
Many cases are first suspected when a physician or public-health official recognizes a group of patients who ate food from a similar source have developed fever and diarrhea. The definitive diagnosis of all Salmonella infections is based on culture of the organisms from the patient and occasionally from a source (for example, food or water). In most cases, the feces of the patient is cultured on agar media that is selective for Salmonella spp. Unfortunately, these culture results can take three to seven days to obtain. Although rapid tests such as polymerase chain reaction (PCR) that detect the genetic material of the bacteria have been attempted, PCR does not seem sensitive enough to detect the organisms in feces (about 47% sensitive). Investigators suggest that the sensitivity of PCR is good when performed on a blood sample rather than feces (84%-95% after five days of infection), but this test is not widely available. Blood cultures are sometimes done in patients suspected of having typhoid and paratyphoid fevers; they are positive in about 50%-70% of patients. Culture results, when positive, help the physician to differentiate Salmonella infections from other organisms or infections that may have similar initial symptoms (for example, E. coli, Campylobacter, shigellosis, brucellosis, amebic dysentery, botulism). Often rectal or fecal exams are done to determine if the patient has blood in the feces. Many physicians will empirically treat patients with antibiotics if they show blood in the feces because some physicians consider the infection more likely to be caused by an aggressive bacterial agent. If the patient's infection is severe, other ancillary tests may be done (CT scan, ultrasound, CBC count, liver function tests, biopsy) to help determine the extent of the infection.
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