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
| Antigen | Significant Titre (single test) | What it suggests |
|---|---|---|
| TO (Typhi O) | ≥ 1:160 | Active typhoid infection |
| TH (Typhi H) | ≥ 1:160 | Past infection or vaccination |
| AO (Paratyphi A O) | ≥ 1:160 | Active paratyphoid A infection |
| AH (Paratyphi A H) | ≥ 1:160 | Past paratyphoid or vaccination |
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?
POSITIVE Titre ≥ 1:160 for TO
A raised TO titre on a single test suggests active typhoid infection, especially with fever, headache, stomach pain and constipation or diarrhoea. However, a single Widal can be misleading. A rising titre — comparing two samples taken 5–7 days apart — is far more meaningful than a single reading. A 4-fold rise in titre (e.g., from 1:40 to 1:160) strongly confirms typhoid.
The BIG problem with Widal tests in India
Why Widal results are often unreliable
The Widal test has significant limitations, especially in India: (1) High false positive rate — many people in endemic areas have background Widal titres from past exposure or previous typhoid vaccination. A titre of 1:80–1:160 can be "normal background" in many Indian populations. (2) Cross-reactions with other infections (malaria, dengue, liver disease) can raise titres. (3) A negative Widal in the first week of typhoid is common — the test becomes positive only from day 6–10. (4) Antibiotic treatment before testing suppresses antibody response. For these reasons, blood culture (gold standard) is the most reliable typhoid test, though it takes 3–5 days for results.
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?