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Widal Test (Slide Agglutination) test helps to detect typhoid fever and paratyphoid fever, collectively called enteric fever, which is caused by the consumption of food or water contaminated by Salmonella bacteria. These microorganisms, like Salmonella typhi and Salmonella paratyphi A, B, and C cause this. Typhoid fever, or enteric fever, is an acute, life-threatening, fever-causing illness that is transmitted by the ingestion of food or water that has been contaminated with the faeces of a person who has been infected with the bacteria Salmonella typhi. In this test, the blood of an individual reacts with the Salmonella antigens on the slide and either forms agglutination (clumping) or not.
The other names are the Typhoid Test and the Enteric Fever Test.
There is only one parameter.
Widal test analyses the titres of antibodies against the bacteria that cause enteric fever.
Typhoid and paratyphoid fever are generally acquired when you consume food or water contaminated by the faeces of an acutely infected or convalescent person recovering from the disease or a chronic carrier. Usually, the incubation period, i.e. time interval between the exposure to an infection and the appearance of the first symptoms of enteric fever, is 6–30 days. Paratyphoid fever is very similar, but it is less severe than typhoid fever.
Widal test is a slide agglutination test to find the antibodies (agglutinins) in a blood sample against the two antigens (O & H) of the bacteria Salmonella enterica. Agglutination refers to the visibility of the clumping of particles when they have antigen and mix with their antibody in the presence of optimum conditions for the reaction of antigen and antibody. When this test is processed on a slide, it is called a "slide agglutination test," and when it is carried out in a test tube, it is called a "tube agglutination test." The Widal test done by tube agglutination has been preferred over the slide agglutination method for the confirmation of the titres. The antigens used in the test are the “H” and “O” antigens of Salmonella Typhi and the “H” antigen of S. Paratyphi.
A Widal test should be performed after the first week of the infection. The reason is that the antibody testing against "O" and "H" antigens of Salmonella starts appearing in serum at the end of the first week of the fever. Therefore, it is advisable to perform the test with two blood samples at an interval of 7 to 10 days to demonstrate the increasing antibody titres.
Positive or Negative.
It is in the early stages of the disease, symptoms include abdominal pain, fever, and a general feeling of being unwell. The initial symptoms are similar to those of other illnesses.
As typhoid fever worsens, the symptoms often include:
A very high fever is up to 104 degrees Fahrenheit.
Usually, abdominal pain, constipation, and perhaps diarrhea later.
A small red spot over the abdomen or on the chest (rose-colored spots)
Loss of appetite and weakness
Other symptoms of Widal (typhoid fever) include:
Blood in the stools
Difficulty in paying attention
The agitation, confusion, and hallucinations (seeing or hearing things that are not real)
There is no special preparation required, and fasting is not required.
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This test requires a blood sample.
A healthcare provider, also called a phlebotomist, usually performs blood draws, including those for Widal slide agglutination tests. Any healthcare provider trained in drawing blood might perform this task. These samples are sent to the lab where a medical laboratory scientist prepares the samples and processes the tests on analysers or manually.
You may expect to experience the following during the blood test or a blood draw:
You have to sit comfortably on the chair, and a healthcare provider will check your arms for an easily accessible vein. This is on the inner side of your arm, on the other side of your elbow.
Once the vein is located, they will clean and disinfect the area with an alcohol swab.
A vacutainer needle will be inserted into the vein to draw a blood sample. This may feel like a small pinch.
After the vacutainer needle is inserted or pricked, the required amount of blood will be collected into a test tube.
When they have drawn enough blood for the test, they will remove the needle and hold a cotton ball or gauze on the site to stop any bleeding.
They will apply a band-aid over the pricked site, and the blood collection is finished.
This process takes less than five minutes.
Once the blood is collected by the healthcare provider, the blood sample, will be sent to the laboratory for processing or testing. When the reports are ready, the healthcare provider will share the reports with you.
Usually, blood tests are common and don’t carry any significant risks. You may have a slight pain like an ant bite when the needle gets inserted, and a small bruise can develop there.
Reports are available via email or WhatsApp within 6 hours of the collection of the blood sample.
If antibodies against Salmonella typhi are present in the patient’s serum, they will react with the respective antigen in the reagent and give visible agglutination (clumping) on the slide test. This antibody titre is the highest dilution of the patient’s serum sample that gives a clear visual agglutination with the Widal test antigen suspensions. The titre means the serum sample will be diluted by adding 20 ul of blood serum and 80 ul of normal saline, or it is 1:160, meaning 10 ul of blood serum and 160 ul of normal saline are mixed with it, and to this, adding the O antigen and H antigen to check for the titres.
The serum (blood) sample shows a titre of 1:80 or more for "O" antigen agglutination and 1:160 or more for "H" antigen agglutination, which indicates active infection. An increase in the titres (4-fold or more) in two blood samples that have been taken 10 days apart confirms the diagnosis. Please note that different labs may have different reference ranges for the interpretation of the Widal test.
This interpretation of a Widal test is greatly influenced by the nature and extent of the patient’s previous contact with the typhoid antigens. This can be either due to clinical or subclinical infection with typhoid or related organisms or from the typhoid vaccination. Usually, the lesser the extent of the previous contact, the greater the usefulness of the Widal test.
Positive or Negative
Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or may test different samples. Talk to your health care provider about the meaning of your specific test results.
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A Widal test positive level indicates that the person has Salmonella enteric fever for typhi bacterial infection. In the typhoid report, it is mentioned as positive, for both O and H antigen titres are present. S. typhi O positive means an active infection of typhoid fever.
The Widal test is done after having a continuous fever for 4 to 6 days. The doctor recommends performing the Widal test. The widal test is an agglutination test that detects the presence of serum agglutins of O antigen and H antigen in patients with typhoid and paratyphoid fever.
The Widal test is one method that can be used to help make a presumptive diagnosis of enteric fever, also called typhoid fever. Enteric fever is an illness caused by infection with the bacterium Salmonella enterica serotype Typhi (S. typhi).
The normal range for the widal test is that the titres of O and H antigens are less than 1:160 and are considered negative. The titres of O and H antigens above 1:160 to 1:320 or above are considered positive. The reference range slightly varies from lab to lab.
In the Widal test, the antigen O is considered IgM somatic and appears first and represents the initial response to acute typhoid fever, while the IgG flagellar H antigen usually develops more slowly but it persists for longer.