Widal Test (Typhoid)
Widal· also: Typhoid Test, TO, TH, Widal agglutination
Clinical Overview
The Widal test is an agglutination test that detects antibodies (IgM and IgG) against Salmonella typhi and Salmonella paratyphi antigens — specifically the O (somatic/cell wall) and H (flagellar) antigens. It has been used for over 130 years to diagnose typhoid fever in endemic areas where blood culture is unavailable. However, it has significant limitations in both sensitivity and specificity.
Why This Test Matters
The Widal test is widely used in low-resource settings due to low cost and availability, but its diagnostic accuracy is limited. A single Widal test is unreliable — a fourfold or greater rise in titer between acute (early illness) and convalescent (2–3 weeks later) serum samples is considered significant. A single elevated titer (≥1:160 for O antigen in non-endemic areas) may be meaningful, but in endemic areas background seroprevalence is high and titers of 1:160 or even 1:320 may reflect prior infection or vaccination rather than current disease. Blood culture remains the gold standard for typhoid diagnosis — it is positive in 60–80% of cases in the first week. The Widal test is gradually being replaced by more specific rapid diagnostic tests (Typhidot, Tubex) and blood culture.
Reference RangesWHO/IFCC standards
| Reference Range | Unit | Notes |
|---|---|---|
| < 1:80 | titer | O antigen <1:80 considered normal in non-endemic areas; significant ≥1:160 (or ≥1:320 in endemic areas) |
What Causes Abnormal Results?
High Widal Causes
- Active typhoid fever (Salmonella typhi infection)
- Paratyphoid fever (Salmonella paratyphi A, B, or C)
- Prior typhoid infection (antibodies persist for months to years)
- Typhoid vaccination (whole-cell vaccine induces both O and H agglutinins)
- Cross-reactivity with other Salmonella species and non-typhoidal Salmonella
- Background seroprevalence in endemic areas (even healthy individuals)
Low Widal Causes
- Early acute typhoid (first 5–7 days — antibodies have not yet developed)
- Typhoid treated with antibiotics before serological response develops
Signs & Symptoms to Watch For
How to Prepare for This Test
Collect acute serum during the first week of illness and convalescent serum 2–3 weeks later for most reliable interpretation. A single titer alone is difficult to interpret. Blood culture (3 separate samples in the first week before antibiotics) provides definitive diagnosis.
Factors That Can Affect Results
- Prior typhoid infection (elevated background titers in endemic areas)
- Typhoid vaccination (whole-cell vaccine causes both O and H titer elevation)
- Cross-reactions with Brucellosis, Rickettsial infections, malaria, and other Gram-negative infections
- Antibiotic treatment before blood draw suppresses antibody response and may cause false-negative Widal
- Lab technique and antigen preparation variations between manufacturers
Related Topics
Frequently Asked Questions
My Widal test came back as TO 1:160 — do I have typhoid?
A titer of 1:160 for the O antigen is in the range that some labs use as a cutoff, but interpretation depends critically on whether you live in or have visited a typhoid-endemic area. In endemic areas (South Asia, Southeast Asia, sub-Saharan Africa), titers of 1:160 or even 1:320 are common among healthy individuals due to prior exposure or vaccination — making this result difficult to interpret. In non-endemic areas (e.g., North America, Western Europe), a titer of ≥1:160 in the context of a clinical presentation of sustained fever is more suggestive of typhoid. Blood culture is the only definitive diagnostic test.
Is there a better test than the Widal for typhoid?
Yes. Blood culture is the gold standard (60–80% sensitive in the first week before antibiotics). Newer rapid diagnostic tests like Typhidot (detects IgM and IgG against a specific 50 kDa outer membrane protein) and Tubex (detects IgM anti-O9 antibodies specific to S. typhi) have better specificity than the Widal test and can give results within hours. Bone marrow culture (>90% sensitive) is the most sensitive test but invasive and rarely performed. PCR-based tests exist but are not widely available.