Sponsored link The Widal test is a test procedure that involves mixing of the bacteria which cause typhoid fever to the blood consisting of certain antibodies taken from an infected person. It is an assumptive serological test used for undulant fever or enteric fever. When the Widal test is used to check of the presence of Salmonella infections, then it is a presentation of the occurrence of O-soma false-positive result. It is important to carefully and correctly interpret the test results as per the past history of enteric fever, the typical composition of antibodies in the general population living in the endemic regions of the world and typhoid vaccination. Another test which is employed to detect and diagnose typhoid fever is the Typhidot test. The other procedures that can be used to diagnose Salmonella typhi or paratyphi include options like urine, blood and stool cultures.
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Laboratory diagnosis of enteric fever includes Blood culture, Stool Culture and Serological test. Widal test is a common agglutination test employed in the serological diagnosis of enteric fever. Antibodies in serum, produced in response to exposure to Salmonella organisms will agglutinate bacterial suspension which carries homologous antigens. This forms the basis of Widal test. The organisms causing enteric fever possesses two major antigens namely somatic antigen O and a flagellar antigen H along with another surface antigen, Vi.
During infection with typhoid or paratyphoid bacilli, antibodies against flagellar antigen of S. Positive result is indicated by the presence of agglutination Absence of agglutination indicates a negative result.
The paratyphoid O antigens are not employed as they cross-react with the typhoid O antigen. If agglutination occurs with O antigen then it is considered positive for Salmonella typhi. If agglutination occurs in A or B antigen then it is confirmed as positive for Salmonella paratyphi. Agglutination will occur in H antigen for all the cases of antigens like O, A, and B. Preparations of antigens Antigen suspensions may be prepared from suitable stock cultures in the laboratory.
But generally commercially prepared suspensions are used. Salmonella typhi is used to prepare S. O antigens for S. H antigen suspension is prepared by treating overnight broth culture or saline suspension of Salmonella with 0. For preparing O antigen suspension, Salmonella are grown on phenol agar to inhibit flagella.
The antigens are treated with chloroform preservative and appropriate dyes are added for easy identification of antigens. Procedure of Widal test The Widal test can be conducted in two ways : Slide agglutination Widal test 2. Quantitative Slide Test Tube agglutination Widal test Tube agglutination has more accuracy as compared to the slide agglutination technique. However, A slide widal test is more popular among diagnostic laboratories as it gives rapid results.
Qualitative Slide Test Procedure : Bring all reagents to room temperature and mix well. Mix it thoroughly with the aid of applicator stick and rotate the slide gently. Observe for agglutination. Positive Test : Agglutination within a minute Negative Test : No agglutination This is performed for the samples which showed positive agglutination during qualitative test.
Procedure : Bring all reagents to room temperature and mix well. Dispense one drop of saline into the first reaction circle and then place 5, 10, 20, 40, 80 ul of the test sample on the remaining circles. Add a drop of the antigen, which showed agglutination with the test sample in the screening qualitative method, to each circle. Mix the contents of each circle with the aid of applicator stick and rotate the slide gently.
Interpretation : The antibody titre of the test sample is its highest dilution that gives a visible agglutination. Agglutinin titre greater than is considered as significant infection and low titres indicate absence of infection.
Quantitative Tube Test Bring all reagents to room temperature and mix well. Prepare 4 sets of test tubes for individual antigen. Each set contains 1- 8 tubes. Add 1. To tube no. To tube No. Transfer 1 ml of the diluted serum sample from tube No. Continue this serial dilution till tube No. Discard 1. So the dilutions of the serum sample from tube No.
Tube no. To one set i. In second set i. Mix well, cover and incubate these tubes overnight at 37 degree Celcius approximately 18 hours. After incubation dislodge the sediment and observe for agglutination. Interpretaton : The antibody titre of the test sample is its highest dilution that gives a visible agglutination. Limitations of widal test Tests done within 7 days of illness and after 4 weeks are usually negative.
The local titre of the place should be known for the results interpreted correctly. This test Quantitative is highly time consumable. Previous typhoid vaccination may contribute to elevated agglutinins in the non-infected population. Other infections of non-enteric salmonella infection such as Typhus, Immunological disorders, chronic liver disease may cause false positive reaction.
Cross reaction between malaria parasites and salmonella antigens may cause false positive Widal agglutination test Related Posts:.
Widal test – Interpretation, Normal Values, Procedure
Laboratory diagnosis of enteric fever includes Blood culture, Stool Culture and Serological test. Widal test is a common agglutination test employed in the serological diagnosis of enteric fever. Antibodies in serum, produced in response to exposure to Salmonella organisms will agglutinate bacterial suspension which carries homologous antigens. This forms the basis of Widal test. The organisms causing enteric fever possesses two major antigens namely somatic antigen O and a flagellar antigen H along with another surface antigen, Vi. During infection with typhoid or paratyphoid bacilli, antibodies against flagellar antigen of S.
Widal Test- Introduction, Principle, Procedure, Interpretation and Limitation
Purpose serological test for enteric fever In and named after its inventor, Georges-Fernand Widal , is a presumptive serological test for enteric fever or undulant fever whereby bacteria causing typhoid fever is mixed with a serum containing specific antibodies obtained from an infected individual. In cases of Salmonella infection, it is a demonstration of the presence of O-soma false-positive result. Test results need to be interpreted carefully to account for any history of enteric fever, typhoid vaccination, and the general level of antibodies in the populations in endemic areas of the world. Typhidot is the other test used to ascertain the diagnosis of typhoid fever. As with all serological tests, the rise in antibody levels needed to perform the diagnosis takes 7—14 days, which limits its applicability in early diagnosis.
Widal Test : Principle, Procedure, Result Interpretation and Limitations
Widal test card or slide. Procedure of Widal Test Widal test can be done in two ways. One is rapid test on slide and another is tube test in which result may be obtained after one night of incubation. For Rapid Slide Test Clean the glass slide or test card supplied in the kit well and make it dry.