IMSE: Febrile Antigens

What test measures antibodies associated with typhoid fever?

Widal test

What test detects antibodies associated with rickettsial infections using Proteus antigens?

Weil-Felix test

What are febrile antigens?

Stained bacterial antigen suspensions used to detect corresponding antibodies in patient serum

What is the principle of the Widal and Weil-Felix tests?

Direct agglutination

What is the heat-stable somatic antigen?

O antigen

What is the heat-labile flagellar antigen?

H antigen

What antigen is associated with the bacterial capsule and virulence?

K antigen; Vi is a capsular antigen

What is the causative agent of typhoid fever?

Salmonella enterica serovar Typhi

How is typhoid fever commonly transmitted?

Ingestion of food or water contaminated with feces from an infected person

What test is associated with typhoid fever?

Widal test

What are the three methods of the Widal test?

Rapid slide screening; rapid slide titration; tube agglutination

Which Widal method is used for screening?

Rapid slide screening method

Which Widal method estimates titer using different serum volumes?

Rapid slide titration method

Which Widal method determines antibody titer using serial dilutions?

Tube agglutination test

What indicates a positive rapid slide screening result?

Presence of agglutination

What indicates a negative rapid slide screening result?

Absence of agglutination

What is the serum volume corresponding approximately to a 1:20 tube dilution?

0.08 mL

What is the serum volume corresponding approximately to a 1:40 tube dilution?

0.04 mL

What is the serum volume corresponding approximately to a 1:80 tube dilution?

0.02 mL

What is the serum volume corresponding approximately to a 1:160 tube dilution?

0.01 mL

What is the serum volume corresponding approximately to a 1:320 tube dilution?

0.005 mL

What percentage of organisms is agglutinated in a 4+ reaction?

100%

What percentage of organisms is agglutinated in a 3+ reaction?

75%

What percentage of organisms is agglutinated in a 2+ reaction?

50%

What percentage of organisms is agglutinated in a 1+ reaction?

25%

What does a 1/+ reaction indicate?

Less than 25% of organisms are agglutinated

What is the prozone effect?

A phenomenon in which high serum antibody concentrations may produce a negative reaction that becomes positive after dilution

What is the endpoint in tube agglutination?

The last tube showing agglutination

What is the significant Widal titer stated in the handout?

1:80

What is the significant Weil-Felix titer stated in the handout?

1:160

What is somatic agglutination?

Agglutination producing compact aggregates that are not easily dispersed

What is flagellar agglutination?

Agglutination producing fluffy, floccular aggregates that are easily dispersed

What does an increased O (OD) antigen titer suggest according to the handout?

Acute or recent typhoid fever

What does an increased H (HD) antigen titer suggest according to the handout?

Previous typhoid infection, convalescence, or vaccination

What do increased OA, OB, and OC titers suggest?

Acute or recent paratyphoid infection

What may an increased O and H antigen titer indicate?

Mixed infection, according to the handout

What is the Weil-Felix test used to investigate?

Rickettsial infections

What is the nature of the Weil-Felix test?

A nonspecific serological test

Which Proteus antigens are used in the Weil-Felix test?

OX-2, OX-19, and OX-K

OX-2 is derived from which Proteus species in the handout?

Proteus vulgaris

OX-19 is derived from which Proteus species in the handout?

Proteus vulgaris

OX-K is associated with which Proteus species?

Proteus mirabilis

Why can Proteus antigens react in the Weil-Felix test?

Cross-reactivity between rickettsiae and certain Proteus serotypes due to shared antigenic determinants

Which Weil-Felix antigen is associated with scrub typhus in the handout?

OX-K

Which Weil-Felix antigens may react in endemic typhus according to the handout?

OX-19 and OX-2

What is the incubation condition for Salmonella O antigens in the tube test?

50°C for 4 hours, according to the handout

What is the incubation condition for Salmonella H antigens in the tube test?

50°C for 2 hours, according to the handout

What should the saline control show in tube agglutination?

No change in appearance and a characteristic swirl when flicked

What should be done before testing febrile antigens?

Bring reagents to room temperature and mix antigen suspensions well

What PPE is required during the laboratory activity?

Laboratory gown, eye protection, mask, gloves, and hair net

What does a single negative Widal or Weil-Felix test mean?

It does not exclude infection

Why must serological results be interpreted cautiously?

False positives, cross-reactions, absent agglutinin production, and clinically insignificant titers may occur