Comparative Study Of Typhidot Biology Essay

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Object: The study was conducted to evaluate the Dot-EIA (typhidot) for the diagnosis of typhoid fever and its usefulness for an early diagnosis, its sensitivity and specificity as compared to blood culture and Widal test.

Study Design: A comparative case control study.

Place & Duration of Study: Department of Medicine, Liaquat University of Medical and Health Sciences Jamshoro/Hyderabad, Sindh, form April 2008 to August 2009.

Patients & Methods: The study included 160 patients who presented with fever and satisfied the inclusion criteria. Patients were divided in two groups; Group I included 112 patients with clinical diagnosis of typhoid fever and Group II included 48 patients with non-typhoid febrile illness. Full blood cell counts, blood culture, liver function test, chest X-ray, Widal test and typhidot test were ordered. Results of blood culture, Widal test and typhidot were compared in all patients for their sensitivity and specificity. The data was analyzed using student t-test on SPSS 10.0 for Widows. A p-vlaue of <0.05 was taken statistically significant.

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Results: Mean age was 34±1.7 years (18-40 years). Out of 112 patients in Group I, 76 (67.5%) were positive for blood culture, 64 (57.14%) were Widal test positive, and 88 (78.57%) were positive for typhidot test. All of 48 patients of Group II were sterile on blood culture, 8 (16.66%) were Widal test positive, and 6 (12.5%) tested positive for typhidot (p = 0.01). Amongst 76 culture positive cases in Group-I, typhidot was positive in 70 patients, and Widal test was positive in 56 patients, giving sensitivity of 92% and specificity of 87.5% as compared to Widal test which had sensitivity of 74% and specificity of 83% (p = 0.02).

Conclusions: Dot-EIA (typhidot) is more sensitive and specific test in diagnosing typhoid fever. It is a rapid, easy to perform and more reliable test compared to Widal test and can be useful in early institution of therapy.

Key Words: Typhidot Widal test Blood culture Typhoid fever

INTRODUCTION

Typhoid fever is an important cause of morbidity in many regions of the world, with an estimated 12 to 33 million cases occurring annually1. Typhoid fever is a systemic infectious disease characterized by an acute febrile illness, the first typical manifestations of which are fever, headache, abdominal pain, relative bradycardia, splenomegaly, and leukopenia. Salmonella.typhi (S.typhi) is the etiological agent of typhoid fever. Pakistan is a hyper-endemic area for typhoid fever, and according to WHO 2008 report the incidence of typhoid fever in 5-15 years children was 412 per million in 20022, 3, 4. A definitive diagnosis typhoid fever can be made by isolation of S.typhi from blood or bone marrow by culture, which is regarded as "gold standard method", but bacterial culture facilities are often unavailable, expensive, time consuming and usually negative because of prior antibiotic use. The sensitivity of blood culture alone is 50-70% because fewer that 15 organisms are present per millimeter of blood in patients with typhoid fever. When patients have already received antimicrobial therapy, blood cultures may be positive in only 40% of the cases. The Widal test has been used for almost more than 100 years, is widely available in developing countries, and is still regarded as a useful test in endemic areas5. In the original format, the Widal test required acute- and convalescent-phase serum samples taken approximately 10 days apart. More recently, the test has been adapted for use with a single, acute-phase serum sample. It is a tube dilution test which measures agglutinating antibodies against the lipopolysaccharide O (TO) and protein flagellar H (TH) antigens of S. typhi. The value of the test for the diagnosis of typhoid fever has been debated for as many years as it has been available. There is, however, considerable interest in newer methods of diagnosis of typhoid fever such as latex agglutination, coagglutination, and the polymerase chain reaction6. The dot-enzyme immunoassay (EIA) 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 strains 7 and has been commercially marketed as a dot-EIA (typhidot). The test incorporates nitrocellulose strips impregnated with the OMP antigen and separately identifies IgM and IgG antibodies. The test has shown promising results in preliminary studies from Malaysia 8 and the Philippines9. But the interpretation of IgG response in highly endemic areas remains problematic. There is concern that in such endemic populations pre-existing IgG antibodies to S. typhi may increase rapidly following reinfection and potentially mask a concomitant IgM response10. Although the tests have shown promising results in trials from Southeast Asia, given the genetic diversity and plasticity of S. typhi strains, it is unknown if the test would be of comparable sensitivity in other regions5, 7, 8. We prospectively conducted this study to evaluate the sensitivity and specificity of typhidot in terms of diagnostic yields as compared to Widal test.

SUBJECTS & METHODOLOGY:

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This comparative case control study undertaken at Department of Medicine, Liaquat University of Medical and Health Sciences Jamshoro/Hyderabad, Sindh, covered the period from April 2007 to August 2008. Patients were selected through non-probability purposive sampling. The data was collected on a structured proforma. Blood culture and typhidot tests for typhoid fever and suspects were performed for the subjects admitted to the hospital. The subjects were selected who satisfied the criteria of; ages 18-40 years, fever ≤14 days, clinical manifestations suggestive of typhoid fever, and no history of typhoid immunization in the recent past. Patient's history, physical examination findings, diagnostic studies, and results of blood culture, Widal test and typhidot test were recorded. The blood cultures, Widal test and typhidot tests were analyzed at the end of study period. Patients were divided in two groups. Group I included 112 patients with clinical diagnosis of typhoid fever, and Group II included 48 patients' with non-typhoid febrile illnesses. The validity of typhidot test was evaluated by determining the sensitivity, specificity, positive and negative predictive values in the diagnosis of culture positive typhoid fever and non-typhoid febrile illnesses. The full blood cell counts, chest X-ray, & Widal tests were also ordered. Results of blood culture, Widal test and typhidot were compared in all patients for their sensitivity and specificity. The study was approved by the ethics committee of institute. The data was analyzed using student t-test on SPSS 10.0. A p-value of 0.05 was taken significant.

RESULTS:

A total of 160 patients with febrile illness were studied from April 2008 to August 2009, including 96 (60%) were male and 64 (40%) were female. Mean age was 34±1.7 years (range 18-40 years). Out of 112 patients in Group I, 76 (67.5%) patients were positive for blood culture, 64 (57.14%) were Widal test positive, and 88 (78.57%) were positive for typhidot test. The 48 patients of Group II were suffering form; pneumonia (n=7), pharyngiitis (n=5), Cholecystitis (n=5), dysentery (n=3), otitis media (n=5), malaria (n=10), urinary tract infection (n=7), acute viral hepatitis (n=6). Group-II (n=48) had all patients sterile on blood culture, 8 (16.66%) were Widal test positive, and 6 (12.5%) tested positive for typhidot (p = 0.01). (Table I). On comparative evaluation, the sensitivity and specificity of Widal test was; 57% and 83% respectively, whereas blood culture showed a sensitivity of 68% and specificity of 100% and typhidot test showed a sensitivity 79 % and specificity 87.5%(p = 0.001). Amongst all the 76 patients in Group I, who were having positive blood culture, typhidot test was positive in 70 patients, giving a sensitivity of 92%, a specificity of 87.5%, and a positive predictive value of 92% as compared to Widal test which was positive in 56 patients with a sensitivity, specificity, and positive predictive value of 74%, 83%, and 87.5% respectively (p = 0.01) (Table II).

DISCUSSION

Typhoid fever is a systemic illness with a significant morbidity and mortality in developing countries. Poor sanitation, overcrowding, low standard of living, lack of medical facilities, and indiscriminate use of antibiotics lead to endemicity of typhoid fever and multi-resistant strains of Salmonella typhi in developing countries13,14. Blood culture has remained the gold standard test in diagnosis of typhoid fever, but its utility in early diagnosis is limited in early phase of illness thereby making the isolation of the organism difficult. Widal test has been used for over a century in developing countries for diagnosing typhoid fever but it has a low sensitivity, specificity and positive predictive value, which changes with the geographical areas. Sharing of O and H antigens by other Salmonella serotypes and other members of Enterobacteriaceae makes the role of Widal test even more controversial in diagnosing typhoid fever15. However, modified Widal test particularly when used along with conventional Widal test has a greater sensitivity16. Typhidot is a new and reliable serodiagnostic test recently available commercially and studied in many endemic areas with reports of higher sensitivity and specificity. We studied typhidot test for its usefulness in patients of typhoid fever presenting to our hospital and observed that it has a sensitivity of 92% and specificity of 87.5%, which was higher than that of Widal test. The study of Jessudason et al 17 carried out in the southern part of India reported typhidot of having a sensitivity of 100% and a specificity of 80% and was recommended for its utility in conjunction with the Widal test for an early diagnosis of typhoid fever. In another study of typhoid patients in Pakistan conducted by Butta et al5, typhidot test had a comparable sensitivity of 94% and specificity of 77%, while Widal test had a sensitivity and specificity of 63% and 83% only. The study of Sherwal BL et al 18 showed the sensitivity of 92% and specificity of 87.5% for typhidot as compared to Widal test, which had sensitivity of 74% and specificity of 83%. The effectiveness of typhidot test in early diagnosis of typhoid fever patients was also studied in two different studies in Malaysia. The Choo et al19 reported sensitivity and specificity of 90.3% and 91.9% respectively, while the study of Gopalakrishan20 showed a sensitivity and specificity of 98% and 76.6% respectively. Both the Malaysian studies showed it to be a better test in contrast to Widal test for rapid diagnosis as well as for its simplicity of ease in use.

CONCLUSION

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Typhidot test is more sensitive and specific test in diagnosing typhoid fever. It is a rapid, easy to perform and more reliable test compared to Widal test and can be useful in early institution of therapy. However, a larger prospective study would be required to fully evaluate the usefulness of this test in countries endemic to typhoid fever.