Typhoid Diagnosis in the Laboratory
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Direct answer
Widal serology, the aggregate test of Indian practice, measures antibodies against O (somatic) and H (flagellar) antigens, with TO and TH titres of 1:160 or more usually taken as significant in endemic areas, though a single titre is far weaker evidence than a four-fold rise on paired sera taken a week apart. Culture, not the Widal test, anchors the diagnosis: Salmonella Typhi is recovered from blood in the first week of fever (the classic yield, falling with each passing week as bacteraemia clears), from stool and urine in the second and third weeks, and from bone marrow at any week with the highest yield of all — including after antibiotics have started. Because prior vaccination, other salmonellae and past infection all raise titres, laboratories report titres and interpret them against the local baseline rather than declaring typhoid from a tube.
What you must remember
- Specimen-by-week rule: first week — blood culture; second to third weeks — stool and urine cultures; bone marrow culture at any stage, most sensitive and least affected by early antibiotics.
- Widal antigens: TO (somatic O antigen, somatic antibody, both IgM and IgG) reflects active infection better; TH (flagellar) antibody persists longer after vaccination and past infection.
- Significant titres: TO and TH of 1:160 or above is the common Indian working threshold (some centres require TH 1:320 in endemic zones); the stronger criterion is a four-fold rise between acute and convalescent sera seven to ten days apart.
- Culture media: blood into brain heart infusion or bile broth subcultured onto MacConkey (Salmonella Typhi grows as non-lactose-fermenting smooth colonies); biochemical screening (non-lactose fermenter, motile, citrate variable for Typhi) then polyvalent and factor antisera for slide agglutination.
- Rapid alternatives: rapid tests such as Typhidot trade speed for variable performance; culture remains definitive, and WHO guidance cautions against Widal-only treatment decisions.
- Chronic carriage: gallbladder carriage, classically in the elderly woman with gallstones, sheds Salmonella Typhi in stool for years — the public-health follow-up after an outbreak.
- Suspicion triggers: continuous fever with relative bradycardia, rose spots, and leucopenia with left-shifted count; culture before antibiotics whenever possible.
Why culture beats Widal
Consider two patients from the same household with week-long fever. Both get Widal tests. The first returns TO 1:320, TH 1:640 — reported significant, treated, recovers. The second returns TO 1:80 — negative, yet clinically typhoid; his blood culture grows non-lactose-fermenting colonies agglutinating with Typhi O and H antisera: typhoid despite a bland Widal, only seven days into the illness. The lesson examiners draw: Widal sensitivity in week one is poor, vaccination and past infection inflate titres, and baseline titres vary by geography — a healthy Indian adult may carry 1:80.
The bench pathway for a positive culture: bile broth grows in 24-72 hours; MacConkey yields smooth colourless colonies; triple sugar iron shows an alkaline slant and butt without gas or hydrogen sulphide for Typhi (Paratyphi B produces hydrogen sulphide), lysine decarboxylase positive; slide agglutination with O and H antisera finishes. Stool plated on Wilson-Blair bismuth sulphite agar shows jet-black colonies with metallic sheen — a practical-examination classic. Susceptibility testing follows, since multidrug-resistant Typhi shifted treatment from chloramphenicol-era drugs to fluoroquinolones, azithromycin and cefixime.
How the exam frames Widal
Practical papers ask the Widal procedure: doubling dilutions of patient serum from 1:20 (or 1:40) incubated with standardised O and H suspensions, agglutination read against a mirror box, the titre being the highest dilution with agglutination. Theory papers ask the traps: vaccinated individuals carry high H titres for years; early antibiotics blunt antibody responses; malaria and tuberculosis inflate results. The pairing question recurs: what a four-fold rise between acute and convalescent sera proves, and when the convalescent sample is drawn — a week or so later. And the modern note: typhoid conjugate vaccine in India's immunisation programme means rising titres may reflect immunisation, not infection.
Frequently asked questions
Which culture specimen is best in each week of typhoid fever?
Blood culture in the first week, stool and urine cultures in the second and third weeks; bone marrow culture is the most sensitive throughout, even after antibiotics.
What Widal titres are considered significant in endemic India?
TO and TH of 1:160 or above is the usual working threshold, with a four-fold rise on paired sera after seven to ten days being the more reliable criterion.
Why can the Widal test be negative in proven typhoid?
Antibodies may be absent in the first week, prior antibiotic use blunts the response, and some patients never mount detectable titres — culture is the definitive test.
What distinguishes Salmonella Typhi on TSI and MacConkey media?
Non-lactose-fermenting smooth colonies on MacConkey; on triple sugar iron an alkaline slant and butt without gas or hydrogen sulphide, with lysine decarboxylation positive.
What is the typhoid carrier state?
Persistent gallbladder (occasionally urinary) carriage, shedding Salmonella Typhi in stool for months to years, classically associated with gallstones, important in outbreak tracing.