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What is Typhidot test?
Typhoid is an enteric fever that is caused due to the bacteria Salmonella typhiIt mainly spreads through contaminated food and water and manifests in the form of fever, loss of appetite, headache and stomach pain.

A typhidot test is a rapid serological test helps to identify IgG and IgM antibodies released by the immune system against Salmonella antigens. Along with typhoid, it is used in the early diagnosis of paratyphoid fever caused by the bacteria S. paratyphi. 

  1. Why is a Typhidot test performed?
  2. How do you prepare for a Typhidot test?
  3. How is a Typhidot test performed?
  4. What do Typhidot test results indicate?

A blood culture test and Widal test are the standard tools used in the diagnosis of typhoid. The disease-causing organisms are detected in the blood after one week of the onset of symptoms. Results of a blood culture test take 2-3 days, which delays the diagnosis of typhoid and paratyphoid. Identification of the bacteria in a bone marrow sample is the confirmatory test for correct diagnosis of typhoid. However, it is difficult to perform the test as a routine laboratory procedure.

In contrast, a typhidot test is helpful in detecting antibodies in the first week of illness. It helps in the early diagnosis of the condition, enabling doctors to initiate prompt treatment of typhoid and paratyphoid fever. This test is performed when individuals present with the following symptoms:

  • Fever with pain in the body, particularly in legs and head
  • Fever with increased sleepiness and weakness in the body
  • Bloating sensation with pain in the abdomen
  • Slow rise in body temperature (stepladder pattern) for 4-5 days
  • Relatively slow pulse in comparison to the rising body temperature
  • Red, small boils or rash on the abdomen and back appearing after one week of fever.

Fasting is not essential for typhidot test. No special restrictions need to be followed before the test.

A typhidot test is performed using a ready-to-use kit in which blood sample is placed in a test holder and combined with reagents or chemicals provided in the kit. The result or the turn-around time for the typhidot test is 1-3 hours. Test results are compared with a control provided in the kit to get a diagnosis.

A typhidot test is an enzyme immunoassay that detects IgG and IgM antibodies against the salmonella bacterial antigen- a 50 kilodaltons outer membrane protein.
A typhidot-M test is another variation of the typhidot test that helps in the diagnosis of the acute phase of typhoid infection.

Normal results: A negative result for anti-salmonella IgG and IgM antibodies indicates no infection.

Abnormal results: 

  • A positive result for IgM antibodies indicates acute infection.
  • A positive result for IgG antibodies indicates past infection or chronic infection. 

Since this test gives a positive result for IgG antibodies present in the blood due to past infection, it cannot differentiate between the infection that occurred previously and a current infection or chronic infection.
Additionally, various research studies have shown that the efficacy of a typhidot test is low or poor in the detection of both positive and negative cases of typhoid compared to a blood culture test. 

A typhidot M test is superior compared to a typhidot test, as it detects only the IgM antibodies present in blood during the acute phase of the illness. However, both these tests are much more effective in the diagnosis of typhoid at an early stage compared to a blood culture test.

According to WHO, a typhidot test can be used for the early detection of typhoid, particularly in areas where typhoid is frequently seen. Also, due to the low cost of the test, typhidot test can be performed even in laboratories with insufficient staff and equipment.
However, rapid antibody test (typhidot test, typhidot M test, Tubex test, etc.) are not recommended in routine laboratory investigations for the diagnosis of typhoid due errors in the result. This is because it may lead to a complete lack of treatment to those who are diagnosed as false negative and needless treatment to those who are diagnosed as false positive with typhoid.

Blood culture, though a bit slow, is an accurate method for diagnosis of typhoid and paratyphoid.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational point of view and is in no way a substitute for medical advice by a qualified doctor.

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References

  1. Sir Stanley Davidson. Davidsons Principles And Practice Of Medicine . 21st Edition China: Elsevier Publishing, 2010, Page no: 334
  2. Jacob John, Carola J. C. Van Aart, Nicholas C. Grassly. The Burden of Typhoid and Paratyphoid in India: Systematic Review and Meta-analysis PLoS Negl Trop Dis. 2016 Apr; 10(4) PMID: 27082958
  3. World Health Organization [Internet]. Geneva (SUI): World Health Organization; Typhoid
  4. World Health Organization [Internet]. Geneva (SUI): World Health Organization; Sensitivity and specificity of typhoid fever rapid antibody tests for laboratory diagnosis at two sub-Saharan African sites
  5. Asma Ismail. New Advances in the Diagnosis of Typhoid and Detection of Typhoid Carriers Malays J Med Sci. 2000 Jul; 7(2): 3–8 PMID: 22977383
  6. World Health Organization [Internet]. Geneva (SUI): World Health Organization; Sensitivity and specificity of typhoid fever rapid antibody tests for laboratory diagnosis at two sub-Saharan African sites
  7. Sonja J. Olsen et al. Evaluation of Rapid Diagnostic Tests for Typhoid Fever J Clin Microbiol. 2004 May; 42(5): 1885–1889 PMID: 15131144
  8. KJ Prasad, JK Oberoi, N Goel, C Wattal. Comparative evaluation of two rapid Salmonella-IgM tests and blood culture in the diagnosis of enteric fever Indian Journal of Medical Microbiology, 2015, Volume 33, Issue 2, Page no: 237-242