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Blood Test Tracker

Widal, Salmonella Typhi H Titer

A Widal test titer for the typhoid bacterium's H (flagellar) antigen. It can stay raised after an old infection or a vaccine.

AI-drafted. Awaiting verificationAlso called TH, S. Typhi HLOINC 34371-5

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 H antigen is part of the bacterium's whip-like tail. The lab dilutes your serum step by step and reports the highest dilution that still causes clumping, such as 1:160. A larger second number means more antibody.

Why it matters

The H titer is reported next to the O titer to help judge whether a fever could be typhoid. It is hard to read on its own. H antibody can stay raised after an old infection or a typhoid vaccine, and antibody tests cannot tell that apart from a current illness. The CDC calls the Widal test unreliable, and a blood culture is the preferred way to confirm typhoid.

If it’s high

A higher H titer means your blood has more antibody that reacts with the typhoid bacterium's tail. It can be a sign of a current or recent typhoid infection. It can also be left over from an old infection or a typhoid vaccine, or reflect living where typhoid is common. One result cannot tell these apart.

Common causes

  • A past typhoid infection or earlier exposure to Salmonella, 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
  • 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
  • Past typhoid infection or vaccination, which can keep the H titer raised
  • Where you live or have lived: baseline titers are higher in areas where typhoid is common
  • The lab's method and kit: slide and tube versions differ, and each lab sets its own cutoff
  • 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?
  • Could an old infection or a vaccine explain this H 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

Can the H titer be raised even if I am not sick now?

Yes. H antibody includes IgG, which generally lasts longer than the early IgM, and an earlier infection or a typhoid vaccine can leave it raised. Antibody tests cannot tell this apart from a current illness.

What titer counts as positive?

There is no single cutoff. Studies have suggested anything from about 1:80 to above 1:160 for the H titer, depending on how common typhoid is where people live. Your lab prints its own cutoff, and your clinician reads your result against it.

Show the science and sources

For clinicians and researchers

H (flagellar) agglutinins are a mixture of IgG and IgM, whereas the early-rising O agglutinins are mainly IgM (modified Widal study of 50 culture-proven cases and 50 healthy people). IgG generally lasts longer than IgM, so a raised H titer can reflect an earlier infection or a vaccine, and antibody tests cannot separate that from a current illness (CDC Yellow Book). In the Vietnam study, an H titer of 100 or more was found in 67% of 1,400 culture-proven cases, 8% of febrile controls and 1% of tetanus controls, and at 30.8% disease prevalence H gave better positive predictive value than O. A cutoff of O 200 or H 100 detected 74% of culture-proven cases, and 14% of positive results were false and 10% of negative results were false. Jammu baseline data in healthy adults showed 21.2% reactive for H, mostly at 1:40, and proposed 1:80 or more as diagnostic locally. There are no validated single-sample cutoffs (WHO Bulletin 2026). A 2025 review of 20 studies found mean sensitivity of about 63% and specificity of about 73% for the Widal test as a whole. Blood culture is the benchmark.

LOINC 34371-5 (primary), 42960-5, 47388-4, 14227-3, 22529-2. 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. 34371-5), 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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