Question
Name four tube agglutination tests. Describe in detail any one of them.
Answer
Four tube agglutination tests:
- Widal test — for enteric fever (agglutination of Salmonella Typhi/Paratyphi O and H antigens).
- Standard Agglutination Test (SAT) — for brucellosis.
- Weil-Felix test — for rickettsial infections (using Proteus OX antigens).
- VDRL/RPR-type flocculation tests — non-treponemal tests for syphilis (technically flocculation, closely related to agglutination in principle).
(Other valid examples: tube agglutination test for typhoid using Salmonella antigens, or brucella tube agglutination.)
Detailed description — Widal test The Widal test is a tube (or slide) agglutination test used for the serodiagnosis of enteric (typhoid/paratyphoid) fever, detecting agglutinating antibody in patient serum against the O (somatic) and H (flagellar) antigens of Salmonella Typhi (and Paratyphi A/B).
Principle/procedure: patient serum is serially diluted in a series of tubes and mixed with standardized suspensions of killed Salmonella O and H antigens; the tubes are incubated (O antigen suspensions typically at 37°C, H antigen suspensions similarly, with characteristic differences in the pattern of agglutinate formed — O antigen gives a compact, granular/disc-like button of agglutination that is somewhat difficult to disperse, while H antigen gives a looser, flocculent, “cotton-wool”-like clump that disperses more easily on shaking); the highest dilution showing visible agglutination is recorded as the antibody titre.
Interpretation: a rising titre (≥4-fold increase) between paired acute and convalescent sera taken 7–10 days apart is the most reliable evidence of current infection; a single high titre above the locally established baseline is only suggestive, since background titres are common in endemic populations due to prior subclinical exposure or vaccination. O antibody typically appears earlier and falls faster after recovery, while H antibody appears later but persists longer.
Limitations: cross-reactions with other Salmonella serotypes and unrelated febrile illnesses; the anamnestic reaction (a non-specific rise in titre triggered by an unrelated current infection in a person previously exposed/vaccinated) can cause false positives; reduced sensitivity if the patient received antibiotics early or was tested too early in the illness before antibody has risen.