The Widal test is used to demonstrate rising titres of antibodies to flagellar (H), somatic (O) antigens in typhoid and paratyphoid fever.
An increasing O antibody level signifies acute infection, whilst a raised H antibody level may indicate the serotype of the infecting organism.
Widal's test is of limited use because H and O antibody levels may rise non-specifically due to cross reactions with other enterobacteriacea.
The interpretation of the serology thus requires consideration of factors such as previous immunization, the stage of the illness and the effect of any antibiotic treatment, etc. Widal's test and particularly its interpretation requires an expert.
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