What is the basis of the Widal test (Widal test) for diagnosing typhoid fever?

Widal Test Basis

The Widal test detects agglutinating antibodies against O (somatic) and H (flagellar) antigens of Salmonella Typhi and Paratyphi through a serological agglutination reaction, but should not be used for diagnosing typhoid fever due to poor performance characteristics. 1, 2

Scientific Principle

The Widal test operates on the principle of bacterial agglutination, where patient serum containing antibodies against Salmonella antigens causes visible clumping when mixed with standardized bacterial antigens. 3 The test specifically measures:

• O (somatic) antigens: Lipopolysaccharide components of the bacterial cell wall 2
• H (flagellar) antigens: Protein components of bacterial flagella 2

The test can be performed as either a slide agglutination screening method or a more quantitative tube dilution method, with serial dilutions typically ranging from 1:40 to 1:1280 to determine antibody titers. 4

Why Guidelines Recommend Against Its Use

The IDSA guidelines explicitly state that serologic tests should not be used to diagnose enteric fever (strong recommendation, moderate evidence). 1 This recommendation is based on multiple critical limitations:

Poor Sensitivity and Specificity

• Sensitivity ranges from only 52-71% in acute-phase testing, meaning nearly half of true typhoid cases may be missed 5, 6
• Specificity is 68-88%, resulting in numerous false positives 5, 6
• Positive predictive value is extremely low at 5.7-54.6%, meaning most positive results do not represent true disease 5, 4
• The test performs better in convalescent-phase serum (90% sensitivity/specificity), but this timing makes it clinically impractical for acute diagnosis 6

Multiple Sources of False Positives

The CDC guidelines emphasize that false positives occur due to:

• Previous typhoid vaccination 2
• Prior typhoid infection 2
• Cross-reactions with other Salmonella species 2
• Other febrile illnesses 2
• Background antibody levels in endemic populations, where 4.6-18.5% of healthy individuals have elevated titers 4, 7

False Negatives

• Prior antibiotic treatment suppresses antibody response 2
• Immunosuppression reduces antibody production 2
• Early disease presentation before antibody development 6

Recommended Diagnostic Approach Instead

Blood culture remains the gold standard and should be performed in all patients with suspected enteric fever, with 2-3 specimens of 20 mL each collected prior to antibiotics. 8 The diagnostic algorithm prioritizes:

• First-line: Aerobic blood culture (sensitivity ~50%, highest yield in first week) 1
• Most sensitive: Bone marrow culture (particularly valuable if antibiotics already given) 1, 8
• Adjunctive: Stool, duodenal fluid, and urine cultures 1, 8
• Emerging: Culture-independent multiplex molecular diagnostics from stool and blood 1, 8

Common Pitfalls to Avoid

• Never rely on Widal test alone for diagnosis, as the negative predictive value (98.3-98.9%) is the only reliable parameter, meaning negative results help exclude disease but positive results cannot confirm it 5, 4
• Do not misinterpret results in endemic areas where background titers may be elevated in healthy populations 2, 7
• Failing to obtain blood cultures before antibiotics, which dramatically reduces sensitivity 8
• Not considering vaccination history when evaluating any serological results 2

The Widal test's continued popularity stems solely from being simple and inexpensive 3, but these practical advantages do not overcome its fundamental diagnostic inadequacy for clinical decision-making in typhoid fever.