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Blood Test Tracker

Widal, Salmonella Paratyphi B (BH) Titer

A Widal test titer for the flagellar (H) antigen of Salmonella Paratyphi B, a relative of the typhoid bacterium.

AI-drafted. Awaiting verificationAlso called BH, S. Paratyphi BHLOINC 21513-7

What it is

Salmonella Paratyphi B is a close relative of the typhoid bacterium. It causes paratyphoid fever, an illness much like typhoid. Typhoid and paratyphoid are together called enteric fever. The Widal test mixes your blood serum with killed bacteria and looks for clumping. The BH result looks at antibody to the whip-like tail (H antigen) of Paratyphi B. The lab dilutes your serum step by step and reports the highest dilution that still causes clumping, such as 1:80.

Why it matters

The BH titer is printed on some Widal reports to help judge whether a fever could be paratyphoid. It is hard to read. The CDC calls the Widal test unreliable, and a blood culture is the preferred way to confirm typhoid or paratyphoid. In a study of healthy adults in Jammu, India, only 3 of 302 reacted to BH, so a reaction was uncommon in healthy people there, but a positive still cannot confirm an infection.

If it’s high

A higher BH titer means your blood has antibody that reacts with Paratyphi B. It can be a sign of a current or recent paratyphoid infection. It can also be a reaction to antibody from typhoid or other Salmonella bacteria, or a leftover from an earlier infection. One result cannot tell these apart.

Common causes

  • Antibody from typhoid or other Salmonella bacteria that reacts with the test
  • Earlier exposure to Salmonella bacteria, which is common where enteric fever is widespread
  • A typhoid vaccine, because antibody tests cannot tell vaccination from infection
  • A current Paratyphi B infection (paratyphoid 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 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
  • Typhoid or paratyphoid exposure, past or recent, and typhoid vaccination
  • The lab's method and kit: slide and tube versions differ, and each lab sets its own cutoff
  • Cross-reaction with other Salmonella types
  • 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. Paratyphoid 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 typhoid, another Salmonella infection or a vaccine explain this titer?
  • Would a blood culture be more reliable for me?
  • Should we repeat the test later to see whether the titer is rising?

Questions people ask

Can the Widal test tell paratyphoid B from typhoid?

Not reliably. Typhoid and paratyphoid bacteria are close relatives, and the Widal test can pick up antibody made against other Salmonella bacteria. A blood culture is the usual way to tell which bacterium is causing a fever.

What titer counts as positive?

There is no single cutoff. Labs set their own, and no validated cutoff exists for single samples. Your lab prints its cutoff on the report, and your clinician reads your result against it.

Show the science and sources

For clinicians and researchers

BH is the flagellar antigen titer for S. Paratyphi B on a Widal slide or tube panel. Incubation is typically 1 to 10 days for paratyphoid fever, compared with 6 to 30 days for typhoid (CDC Yellow Book). There are no validated single-sample cutoffs for the Widal test in acute disease (WHO Bulletin 2026), and Widal results do not distinguish acute from past infection or vaccination. In the Jammu baseline study of 302 healthy adults, 3 reacted to BH and 3 to AH (versus 138 for Typhi O and 64 for Typhi H), using a 1:20 to 1:320 dilution series. Antigens shared among Salmonella serotypes and cross-reactivity are recognized causes of false positives (2025 review, which reported mean Widal sensitivity of about 63% and specificity of about 73% across 20 studies). Blood culture is the benchmark, with a sensitivity of roughly 50 to 65%.

LOINC 21513-7 (primary), 22522-7, 22521-9, 32213-1. 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. 21513-7), 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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