Enteric Fever:- Part 2 – Typhoid Fever Diagnosis, Widal test, Its Procedure and Interpretations
Sample for Enteric fever
- It is done on the serum of the patient for the widal test.
- A random sample can be taken.
- For blood, culture collects the blood in sterile test tubes or specialized culture tubes.
- For urine culture, clean the area and then collect the mid-stream sample.
- Stool culture samples may be collected in sterile containers.
- Bone marrow, if needed, will be done with all precautions and sterilely.
Purpose of the test (Indication)
- This test is done to diagnose enteric fever (Typhoid and paratyphoid fever).
History of the Salmonella infection
- Diseases caused by salmonella are divided into two groups:
- Typhoidal infection by S.typhi and S.paratyphi.
- Humans are the only host of these pathogens, spreading person-to-person or via contaminated foodstuffs and water.
- Non-typhoidal infection from all other organisms.
- The most common non-typhoidal organisms in the USA are S.typhimurium and S.enteriditis.
- Of these infections, 95% are food born.
- Typhoidal infection by S.typhi and S.paratyphi.
- History of widal test:
- In 1896 widal test was discovered by George-Fernand Widal for the diagnosis of enteric fever.
- Enteric fever includes typhoid and paratyphoid fever.
- Paratyphoid fever is milder than typhoid fever.
- Enteric fever is also called typhoid fever.
- Typhoid fever is caused by the bacterium Salmonella Typhi.
- Salmonella typhi has an incubation period of 14 to 21 days. Sometimes may even be longer.
- 30% of the cases become chronic carriers due to persistent gallbladder infection.
Antigens and the antibodies of the salmonella typhi:
- H antigen: This is flagellar H antigen.
- Vi antigen is a polysaccharide capsule that surrounds the O antigen.
- It will protect the bacteria from the antibody attack on the O-antigen.
- This antigen is also called Vi for virulence.
- O-antigen is also called a somatic antigen.
- The Lipopolysaccharides antigen is somatic or O antigen.
- When there is a high titer of this antigen-antibody indicates acute fever.
- This will rise early and disappear early before the disappearance of the H-antigen.
- There are three respective antibodies:
- Anti-O antibody.
- Anti-H antibody.
- Anti-Vi antibody.
Diagnosis of Typhoid fever:
- WBCs decrease and may range from 4000 to 6000/cmm during the first two weeks.
- 3000 to 5000/cmm during the next 2 weeks.
- WBCs count around 10,000/cmm or more may suggest perforation of the intestine or suppuration.
- The peripheral blood smear shows normocytic anemia.
- In case of bleeding, anemia may become hypochromic and microcytic.
- Blood culture is positive during the first 10 days of fever in 90% of the cases.
- <30% positive after the third week of illness.
- Stool cultures are positive after the 10th day of illness, increasing frequency up to the 4th or 5 the week.
- Stool culture-positive after the 4 months indicates carrier state.
- Urine culture is positive during the second to the third week in 25% of the cases, even if the blood culture is negative.
- The widal test is positive after 7 to 10 days of infection. O and H antigens are found.
- There is an increase and a peak in the third to the fifth week of infection.
- After that, the titer starts to fall for several weeks.
- O-antigen appears before the appearance of the H- antigen.
- The rising titer of O-antigen is very important for the diagnosis.
- During the convalescent stage, the H-antigen is higher than the O- antigen.
- Positive widal test may be seen in:
- Typhoid fever.
- After the vaccination.
- In the case of a previous infection.
- Early infection treatment will not raise the titer or remain negative or low.
- H-antigen has little value in the diagnosis of typhoid fever.
The procedure of the Widal test:
- Serial dilution of the patient’s serum is taken from 1:40 to 1:320
- Now add an equal volume of Salmonella antigen.
- This can be done as a Slide method or as a Tube method.
- When running in the tube, then incubate tubes for 12 hours or overnight.
- Prepare the serial dilution, as shown in the diagram.
Widal test reading:
It is done as follows and can read the result where the agglutination starts, as shown in the following diagram.
The drawback of the Widal test:
- This test will be positive after 7 to 10 days of Enteric fever.
- Salmonella has five serogroups: A, B, C, D, and E, based on O somatic antigen.
- Based on H-antigen are 1200 serotypes.
- 7 to 10 days, antibody to D-somatic antigen appears.
- These antibodies against O antigens reach their peak within 3 to 5 weeks.
- H-flagellar antibodies appear later on.
- The titer of O-antibodies 1:80 is suspicious in unvaccinated patients.
- The titer of 1:160 is strongly suggestive of infection in unvaccinated individuals.
- The titer of 1:40 for Antibody to flagellar-antigen (H) are suspicious in unvaccinated individual.
- Titers are much higher in the vaccinated individual.
- Vi antigens for S.typhi is used to screen the carrier. Antibodies to Vi-antigen are positive in the carrier state.
Interpretation of the widal test:
- The highest dilution of the serum is noted where there is agglutination.
- If it ends at 1:320, then that is the titer.
- The Widal test interpretations:
The titer of the antibody Interpretations of the Widal test O-antigen = >1:160 Active infection H-antigen = >1:160 Past infection or immunized Rising titer = 1:40 to 1:80 to 1:160 Diagnostic
- Antigen-O will be positive in the acute stage, and the titer will be more than 1:160.
- O-antigen appears early and also disappears early.
- H antigen rises late and disappears late.
- It is positive in the recovery stage.
- This is an indicator of the carrier stage.
The false-positive test:
- It may be seen due to cross-reacting infections, including malaria.
Other diagnostic tests for the diagnosis of Typhoid (Enteric) fever are:
- Blood culture will be positive in the first week of the negative Widal test.
- Stool culture.
- 60 to 70% of cases are negative during the first week, and more patients are positive in the third week of infection.
- Stool, 90% of the cases, are cleared of the bacteria by the 8th week of infection.
- Urine culture.
- Bone marrow culture. This is a very sensitive test.
- In the bone marrow, 90% of the cases are positive despite antibiotic therapy if these are ≤5 days.
- Another serological test is Typhidot.
- In this test, the serum of the patient is taken.
- This kit claims that enteric fever may be diagnosed in the first week of fever.
Interpretations of the typhidot test:
- The gold standard is the blood culture, where 90% of cases are positive in the first week.
- 50% of cases are positive by the third week.
- For better yield, centrifuge the blood and take a buffy coat for culture.
- Other samples are stool, urine, Rose spot, bone marrow, and gastric or intestinal secretions.
- When culture is done simultaneously on blood, bone marrow, and intestinal secretions, positivity is >90%.
- A small % becomes the carrier, and their stool culture remains positive for at least one year.
Summary of the Culture in Enteric fever patients from a different site:
|Time period||Clinical S/S||Presence of Rose spot||Blood||Stool||Urine||Bone marrow|
||Negative||Positive in 80 to 90% of the cases||Negative||Negative||Negative|
||Positive in 60% of the cases||Positive in 80 to 90% of the cases||Positive in 80% of the cases||Positive in 25% of the cases||Positive in 90% of the cases|
The patient is in recovering stage
|Positive cases start decreasing.||
||Positive in 50% of the cases||Positive in 10% of the cases||Positive cases start decreasing|
||Decreasingly positive||Decreasingly positive||Negative|
For Nonmedical (non-specialist) person explanation of Widal test (Enteric fever):
- Please advise the Widal test if the fever persists for more than 5 days and is constant. It does not touch the normal level.
- The titer of O antigen 1:40 is considered negative.
- Just see the O antigen titer if it is 1:160, indicating Enteric fever.
- If the O antigen is 1:80, repeat the test after 5 to 7 days, and now if the titer is 1:160, the rising titer again indicates acute infection.
- While the H antigen does not indicate acute infection or acute enteric fever.