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Blood Test Tracker

Widal, Salmonella Typhi O Titer

A Widal test titer for the typhoid bacterium's O antigen. One result cannot confirm or rule out typhoid fever.

AI-drafted. Awaiting verificationAlso called TO, S. Typhi OLOINC 34372-3

What it is

Typhoid fever is an infection caused by the bacterium Salmonella Typhi. The Widal test mixes your blood serum with killed typhoid bacteria and looks for clumping. The O antigen is part of the bacterium's outer surface. The lab dilutes your serum step by step and reports the highest dilution that still causes clumping, such as 1:80. A larger second number means more antibody.

Why it matters

In some countries the Widal test is used to help decide whether a fever could be typhoid. The result is hard to read. Many healthy people in areas where typhoid is common have some O antibody, and other infections can raise it. The CDC calls the test unreliable, and a blood culture is the preferred way to confirm typhoid. Your clinician weighs your titer together with your symptoms, travel, vaccines and other tests.

If it’s high

A higher O titer means your blood has more antibody that reacts with typhoid bacteria. It can be a sign of a current or recent typhoid infection. It can also come from an old infection, a typhoid vaccine, or living where typhoid is common. One result cannot tell these apart.

Common causes

  • Past exposure to Salmonella bacteria, which is common where typhoid is widespread
  • A typhoid vaccine, because antibody tests cannot tell vaccination from infection
  • Another infection that cross-reacts with the test, such as other Salmonella types or some related bacteria
  • A current typhoid infection (enteric fever)

What can help

  • Talk with your clinician about what the result means alongside your symptoms, travel and vaccine history
  • Ask whether a blood culture or another typhoid test would be more reliable for you
  • Ask whether a second sample later in the illness would help, since a rising titer says more than one result

What can affect your result

  • How long you have been sick: antibodies take time to rise, so an early sample can be negative
  • Where you live or have lived: baseline titers are higher in areas where typhoid is common
  • Typhoid vaccination and past infection
  • The lab's method and kit: slide and tube versions differ, and each lab sets its own cutoff
  • Other fevers, which can sometimes raise the titer
  • Typhoid vaccination and past typhoid infection can raise titers, and antibody tests cannot tell them apart from a current infection (CDC Yellow Book).
  • Healthy people who live in, or come from, an area where typhoid is common often have a raised baseline titer. In a study of healthy adults in Jammu, India, 45.7% reacted to Typhi O and 21.2% to Typhi H.
  • Some people whose fever had another proven cause, such as malaria or dengue, still had raised titers in a large Vietnam study, so a raised result can be a false positive.
  • The slide version often has no firm threshold for a positive, and any visible clumping can be read as positive (WHO Bulletin 2026). The tube version is more informative when two samples are compared, but cutoffs still differ between labs.
  • A sample taken early in the illness can be negative because antibodies take time to rise.

Antibodies take time to rise, so an early sample can be negative. A second sample taken later can show whether the titer is rising (WHO Bulletin 2026).

When to talk to a clinician

See a clinician soon about a high fever that lasts more than a few days, especially with headache, belly pain, constipation or diarrhea, or a rash, or if the fever started after travel to an area where typhoid is common. Typhoid can be treated but can cause serious complications. Get urgent care if you feel very weak, are confused, have severe belly pain or cannot keep fluids down.

Questions you could ask

  • Is the Widal test the right test for my situation?
  • If this result is negative but I still have a fever, what are the next steps?
  • Could my symptoms, travel or vaccines explain this titer?
  • Would a blood culture or another typhoid test be more reliable for me?
  • Should we repeat the test later to see whether the titer is rising?

Questions people ask

Does a negative Widal test rule out typhoid?

No. Antibodies take time to rise, so an early sample can be negative, and the test misses some cases even later. In a large Vietnam study of people with culture-proven typhoid, 17% had an O titer below 100 when tested.

What titer counts as positive?

There is no single cutoff. Studies have suggested anything from about 1:80 to 1:200 for the O titer, depending on how common typhoid is where people live. Your lab prints its own cutoff on the report, and your clinician reads your result against it.

What is the difference between the O and H results?

They test antibodies against two parts of the same bacterium: the outer surface (O) and the whip-like tail (H). O antibodies tend to rise earlier in an illness. Labs usually report both, and your clinician looks at them together.

Show the science and sources

For clinicians and researchers

The Widal test is a serial-dilution agglutination assay (slide or tube) using killed S. Typhi suspensions for somatic (O) and flagellar (H) antigens. O antibodies rise early and are mainly IgM; H is a mixture of IgG and IgM (modified Widal study of 50 culture-proven cases and 50 healthy people). There are no validated single-sample cutoffs for acute disease (WHO Bulletin 2026), and cutoffs proposed in studies vary with local background titers: Vietnam suggested O of 200 or more, Jammu suggested 1:80 or more. In Vietnam, O was 100 or more in 83% of 1,400 culture-proven cases and in 18% of febrile controls; a cutoff of O 200 or H 100 detected 74% of culture-proven cases, and at 30.8% prevalence 14% of positive results were false and 10% of negative results were false. A 2025 review of 20 studies found mean sensitivity of about 63% and specificity of about 73%. Antigens shared with other Salmonella serotypes and cross-reactivity cause false positives. Blood culture is the benchmark, with a sensitivity of roughly 50 to 65%.

LOINC 34372-3 (primary), 42959-7, 23930-1, 17566-1. Specimen: serum.

Sources

  1. Typhoid and Paratyphoid Fever (CDC Yellow Book), Centers for Disease Control and Prevention
  2. Typhoid (fact sheet), World Health Organization
  3. A case for eliminating Widal testing for enteric fever, India, Bulletin of the World Health Organization (2026)
  4. Value of a single-tube Widal test in diagnosis of typhoid fever in Vietnam, Journal of Clinical Microbiology (1999)
  5. Estimation of Baseline Widal Antibody Titers among Apparently Healthy Urban Population of District Jammu, Indian Journal of Community Medicine (2021)
  6. Evaluating alternatives to the Widal test for typhoid fever diagnosis in developing countries: A targeted literature review, Journal of Public Health in Africa (2025)
  7. Role of modified Widal test in the diagnosis of enteric fever, Journal of the Association of Physicians of India (2003)
  8. LOINC codes verified with the NLM Clinical Tables LOINC search (e.g. 34372-3), U.S. National Library of Medicine

AI-drafted. Awaiting verification. Last reviewed 2026-10-03. How we build and check these pages.

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