Typhidot – A Reliable Test against Typhoid Fever
A common disease which needs rapid diagnosis and is inherently endemic in Indian settings is Typhoid Fever. In this respect Typhidot (Typhi IgM antibody) is an valuable tool in diagnosis of acute stage in typhoid fever.
Typhoid fever is widely recognized as a major public health problem in our country. In the wake of emerging multidrug-resistant strains of bacteria causing typhoid fever, the disorder is known to be associated with significant morbidity and mortality. It is also recognized that a delay in diagnosis and institution of appropriate therapy may significantly increase the risk of adverse outcome and mortality. Although the isolation of Salmonella typhi on blood culture remains the gold standard for diagnosing typhoid fever, the widespread availability and use of antibiotics in the community makes it frequently difficult to isolate the organism on blood cultures and alternative methods such as bone marrow cultures may be required. However, the latter are invasive and difficult to obtain routinely in pediatric patients.
Despite improved methods of bacteriologic isolation, there is a real need for rapid serologic diagnostic tests for typhoid fever.
Accurate diagnosis of typhoid fever at an early stage is not only important for etiological diagnosis but to identify and treat potential carriers and prevent acute typhoid fever outbreaks. Early rising antibodies to lipopolysaccharides O are predominantly IgM in nature. Detection of S.Typhi specific IgM antibodies instead of IgG or both IgG and IgM (as measured by WIDAL TEST) serves as a marker for recent infection.
The dot-enzyme immunoassay (EIA) – TYPHIDOT -is a relatively newer serologic test based upon the presence of specific IgG and IgM antibodies to a specific 50-kD outer membrane protein (OMP) antigen on S. typhi.
The test incorporates nitrocellulose strips impregnated with the OMP antigen and separately identifies IgM and IgG antibodies.This test qualitatively detects the presence of IgM class antibodies to Lypopolysaccharide Specific to S. typhi in human serum/ Plasma or whole blood specimens.
Preliminary data have shown sensitivity and specificity of 95% and 86%, respectively.
There are reports that support the contention that the Widal test has poor diagnostic value in children with typhoid fever. Antibiotic therapy has been shown to alter the antibody response to S. typhi infection (titers against O antigens). The Typhidot has been reported as significantly more sensitive than the Widal test. The relative low sensitivity of the blood culture in diagnosing typhoid fever is understandable in the wake of widespread antibiotic use in our country and the difficulties of obtaining large enough blood volumes for cultures from children.
Although bone marrow cultures significantly increase the yield from cultures, they are invasive and difficult to obtain. It must be emphasized that although cultures are associated with a lag period of at least 48 hr for preliminary confirmation of infection, with the recent emergence of drug resistance among S. typhi, they remain an essential investigation.
In many circumstances, especially among partially treated cases presenting to health facilities, combining cultures with a rapid serologic test may reduce the diagnostic difficulty in typhoid fever. Recent data indicate that combining the blood/bone marrow cultures with a Typhidot-Mt will significantly improve the diagnostic yield of these investigations among children who have previously received antibiotics.
We conclude with the thought that neither the Widal nor Typhidot tests are a substitute for cultures in typhoid fever but using these tests in combination increases the negative predictive value.
References
- Rapid and reliable serological diagnosis of enteric fever: comparative sensitivity and specificity of Typhidot and Typhidot-M tests in febrile Malaysian children. Acta Trop. 1999; 72(2):175-83 (ISSN: 0001-706X).
- Rapid serologic diagnosis of pediatric typhoid fever in an endemic area: a prospective comparative evaluation of two dot-enzyme immunoassays and the widal test. Zulfiqar Ahmed Bhutta and Naseem Mansurali, Am. J. Trop. Med. Hyg., 61(4), 1999, pp.654–657.
