Widal test Flashcards
gold standard for detection of salmonella
blood culture for isolation
typhidot qualitative testing
widal test is to diagnose enteric or typhoid fever that is caused by what micoorganisms?
- S. typhi
- S. paratyphi A, B, and C
principle of widal test
agglutination (clumps)
composed to proteins. its antibody formation is rapid and sustained, highly immunogenic and its antibody levels persists for a longer period
heat — and —
Flagellar antigen
H antigen
heat labile and sensitive
composed of polysaccharide, is heat — and alcohol —. its antibody formation is rapid but only on an early phase, — immunogenic, and its antibody levels falls off quickly
Somatic “O” antigen
heat stable and alcohol resistant
less immunogenic
it is the surface polysaccharide envelope covering the O antigen. this antigen is expressed only on a few serotypes and renders the Bacilli inagglutinable with O anti-serum. the name is believed to be related in virulence
Surface Vi antigen
what is being detected by the widal test?
O and H antigen
widal uses — and was discovered by — in 1896
uses direct agglutination
discovered by Fernand Widal
enumerate the clinical manifestations
- chest congestion
- constipation
- diarrhea
- gastroenteritis
- headache
- body malaise
- high temperature
- loss of apetite
- stomach pain
mode of transmission
- ingestion of contaminated food or water
- person to person
- fecal-oral route
- carriers from food handlers
enumerate the procedures that can be used to detect typhoid infections
- widal test (rapid slide)
- widal test (tube agglutination)
- typhoid test (IgG or IgM rapid test)
- ELISA
- electrochemiluminiscent immunoassay
- PCR
— stained organisms are specific to somatic (O) antigen while the — stained organisms are specific to flagella (H) antigen
blue - O antigen
red - H antigen
widal kit inclusions
- antigen suspension
- positive control vial
- negative control vial
- package insert
- white glass slide
- stirring sticks
positive control contains ready to use — with polyspecifc antibodies having specific reactivity towards O and H antigens, useful in the validation of the performance of widal reagents
standardized goat antiserum with polyspecific antibodies
this control does not contain antibodies against the specific bacteria
widal negative control
why widal is difficult to interpret for diagnosis of typhoid fever?
because cross-agglutinating antibodies remaining from the past infection with related salmonella giving FALSE POSITIVE results
advantages of widal
cheap
accessible
simple
enumerate the causes of false positives
- previous immunization with salmonella antigen
- cross-reaction with non-typhoid salmonella
- variability and poor standardized commercial antigen preparation kit
- infection with malaria
- other enterobacteriaceae with the same endotoxins or lipopolysaccharides
