Widal Test

What is the Widal test?

The Widal test measures antibodies against Salmonella Typhi (typhoid) and Salmonella Paratyphi (paratyphoid) bacteria in your blood. It detects four antibodies: TO (Typhi O, body antigen), TH (Typhi H, flagellar antigen), AO (Paratyphi A O), and AH (Paratyphi A H). Results are expressed as "titres", a dilution ratio showing how concentrated the antibodies are in your blood. The higher the titre, the more antibody is present.

Widal Test: How to read titres

Note: Significant titre cutoff varies between regions. Some labs use 1:80 or 1:320. Always ask your doctor what cutoff their lab uses.

What does a positive Widal mean?

The big problem with Widal tests

Questions to ask your doctor

• Should we do a blood culture to confirm typhoid rather than relying on Widal alone?
• What is the background Widal titre in this region?
• Should we repeat the test in 5 days to look for a rising titre?
• Do I need antibiotics now or should we wait for culture results?

References

Sources cited on this page. PubMed links open the original abstract.

1. Olopoenia LA, King AL. Widal agglutination test – 100 years later: still plagued by controversy. Postgrad Med J. 2000;76(892):80–84. PMID 10644383 · doi:10.1136/pmj.76.892.80

Why the Widal test has significant limitations

The Widal test – developed in 1896 – detects agglutinating antibodies against Salmonella Typhi (O and H antigens) and S. Paratyphi A. Despite its longevity, it is considered unreliable by modern infectious disease standards and is no longer recommended by WHO as the primary diagnostic test for enteric fever in well-resourced settings.

The core problem is low specificity:

• Antibody titres are affected by prior vaccination with typhoid vaccine (raises baseline titres), prior exposure to non-typhoid Salmonella strains (cross-reacting antibodies), and prior typhoid infection (persistent elevated titres for months after recovery – making reinfection difficult to diagnose).
• Titre thresholds vary by region and local baseline. A titre of 1:160 for anti-O may be considered significant in a non-endemic area but meaningless in a hyper-endemic area where baseline titres are universally elevated. There is no universal threshold.
• Antibody production takes 7–14 days from illness onset – the Widal test may be negative in the first week of typhoid fever, precisely when treatment decisions matter most.

Better alternatives to the Widal test

• Blood culture: The gold standard for diagnosing typhoid fever. Salmonella Typhi is isolated from blood in 40–80% of cases in the first week, with sensitivity highest in the first 2–3 days of fever onset. Bone marrow culture is more sensitive (up to 90%) but impractical. Blood culture takes 48–72 hours for a result.
• Typhoid IgM/IgG rapid tests (Typhidot, TUBEX): These detect IgM antibodies against specific S. Typhi O9 antigen (TUBEX) or outer membrane proteins (Typhidot-M). Sensitivity and specificity are better than the Widal test. TUBEX has sensitivity around 65–75% and specificity around 80–90% in endemic areas. Rapid tests provide results within 2 hours – valuable where culture facilities are limited.
• Widal in resource-limited settings: Where blood culture is unavailable, the Widal test may still be used alongside clinical scoring. NICE and WHO both emphasise that a positive Widal must never be used as the sole basis for typhoid treatment – the diagnosis should be supported by compatible clinical features (fever for more than 5 days, relative bradycardia, rose spots, constipation or diarrhoea, splenomegaly).

Antibiotic resistance in typhoid – a critical concern

Fluoroquinolone resistance in S. Typhi (including extensively drug-resistant – XDR – typhoid, first detected in Pakistan in 2016) has made empirical treatment difficult. Azithromycin remains active in non-XDR typhoid; XDR typhoid requires third-generation cephalosporins (ceftriaxone) or carbapenems. Culture and sensitivity testing is therefore essential for all confirmed cases – not just for diagnosis but for selecting effective therapy.