Hidoc Bulletin

Typhoid can be dreadful if kept untreated or diagnosed lately. It is essential to diagnose the disease at an earliest so that the patients can be treated properly. In order to investigate or diagnose typhoid laboratory studies is the primary option as the disease is mainly caused by the Salmonella typhi bacterium. Therefore, clinical studies are done to diagnose this enteric fever. Since, the sensitivities of different tests for S typhi vary widely as observed in the articles and respected journals, culture isolation of the organism is considered to be a trustable option and it is 100% specific. Culture can be done of the bone marrow aspirate which is found to be 90% sensitive until at least 5 days after commencement of antibiotics. However, this technique is extremely painful and thus less preferred.

Culturing blood samples, intestinal secretions (vomitus or duodenal aspirate) and stool can be helpful if done within the first week of onset, otherwise, they decline at a rate of 20%-30% later in the disease course. In particular, stool culture may give positive results even after several days of ingestion of the bacteria as the bacteria shed through the gallbladder. Further, culturing the punch-biopsy samples of rose spots may show positive results even after administration of antibiotics. Moreover, performing a single rectal swab culture upon hospital admission can detect S typhi in 30%-40% of patients. The organism can also be isolated from the cerebrospinal fluid, peritoneal fluid, mesenteric lymph nodes, resected intestine, pharynx, tonsils, abscess, and bone.

Apart from culturing, a widal test can be another best option. It measures the levels of agglutinating antibodies against O and H antigens both qualitatively and quantitatively and requires weeks to give results. The test is usually performed by collecting at least 1 ml of blood and extracting the serum. Furthermore, the test has only moderate sensitivity and specificity. On the other hand, S. typhi shares O and H antigens with other Salmonella serotypes and has cross-reactive epitopes with other Enterobacteriacae, which can lead to false-positive results. Such results can be seen in conditions like. malaria, typhus, bacteraemia caused by other organisms, and cirrhosis. In order to get quick results, a slide agglutination widal test can be done where agglutination against O, H, AH, BH antigens can be observed within a minute.

Recently other advanced tests are included, the IDL Tubex® test can detect IgM O9 antibodies from patients within a few minutes, another rapid serological test,is Typhidot® which takes three hours to perform and detects specific IgM and IgG antibodies against a 50 kD antigen of S. typhi. A newer version of the Typhidot-M® test specifically detects IgM antibodies only. Further, tropical PCR can detect salmonella enteric serotype Typhi, Paratyphi A,B & C from the blood, bone Marrow, urine, stool and serum samples with the sensitivity of 69-85% and specificity of 98-100%.