interpretation of widal test
DESCRIPTION
widal testTRANSCRIPT
Widal test (A short presentation to stimulate a long
discussion!!)
Dr. Tshokey, MD Clinical Microbiologist
JDWNRH
Typhoid fever & the Widal test in
Bhutan
Background:
- Bhutan is a (hyper?)endemic country, but no laboratory proven data
- about 2000 cases reported annually
- 2009 - 1811 cases, with 640 admissions, 1 death
- 2010 Widal tests – JDW - 5365.
- ERRH-615
- CRRH – 942
- Almost all the district hospitals also do the test.
- Culture ?........
Diagnosis
• Isolation of organism
• Demonstration of the genome
• Antibody response
The definitive diagnosis of the disease requires the isolation of Salmonella typhi from the blood, faeces, urine or other body fluids.
Specimens for culture
• Blood
• Faeces
• Urine
• Intestinal aspirate/vomitus
• BMA
• Bile culture
Bone Marrow
Blood
Faeces Urine
90%
Isolation rates
Time in weeks after onset of symptoms
Clot culture
• Recommended for rural setting where no clinical microbiology facilities are available
• Blood → allowed to clot
• Clot and serum – sent to lab
• Clot → digestion by streptokinase or ground in Ox bile broth
• Incubated and subcultured on to routine media
• Serum is used for serology
• Very high rates of contamination
• Blood culture is superior but may be an option for our setting
Isolation rates 1/52 2/52 3/52 4/52 5/52
Blood Highest rate
Start to drop
Found only in 50% of 1/52 cases
Isolation from blood is infrequent
Feaces Found only 50% of cases
Rate ↑ more than blood
Highest at the end of 2/52 to 3/52
Bacteria does not disappear from faeces for long time, Chronic carriage > 1yr
Urine May present in urine for variable time after 3rd week
Bone Marrow aspirate
If properly performed highest isolation sensitivity, Even after 4/52 week possibility of having isolation rates up to 90%
The Widal test
• Since late 19th century, Widal test developed by Widal and Sicard.
• Principle:
Patients’ suffering from enteric fever would possess
antibodies in their sera which can react & agglutinate Salmonella antigens.
- S.typhi O and S.typhi H antigens.
- O antigens for S.paratyphi A and S.paratyphi B are not taken as they cross-react with S.typhi O antigen.
• Status –
Lost/losing importance in many developed countries
Serological tests • Has many disadvantages
• Culture Gold standard
• Useful in – non endemic setting and
– for retrospective diagnosis (confirm the clinical diagnosis)
– epidemiologically
– Specially useful for atypical presentations
• Both H and O are equally elevated and diagnostic or one of the two have been found to be more positive and diagnostic in different endemic areas and population.
Serological/Molecular tests for Typhoid
• Widal agglutination test
• Counter-immuno electrophoresis(CIEP)
• Haemagglutination
• Enzyme linked immunosorbent assay (ELISA)
• Bactericidal antibody test
• Adherence test for detection of IgM antibodies
• Radio immunoassay (RIA)
• Co-agglutination test
• Latex agglutination test
• Polymerase chain reaction
• Diazo test of Urine
General interpretation of Widal Test • Timing is important, antibodies begin to arise during
end of 1st wk.
• The titers increase during 2nd, 3rd and 4th wk after which it gradually declines.
• The test may be negative in early part of first week.
• Single test is usually of not much value.
• A rise in titer between two sera specimens is more meaningful than a single test.
• If the first sample is taken late in the disease, a rise in titer may not be demonstrable. Instead, there may be a fall in titer.
General interpretation…. • Baseline titer of the population must be known before
attaching significance to the titers.
• The antibody levels of healthy individuals in population of a given area give the baseline titer.
• A titer of 100 or more for O antigen is considered significant and a titer in excess of 200 for H antigens is considered significant generally.
• Patients already treated with antibiotics may not show any rise in titer, instead there may be fall in titer.
• Patients treated with antibiotics in the early stages may not give positive results.
General interpretation… • Patients who have received vaccines against Salmonella
may give false positive reactions.
• This can be differentiated from true infection by repeating the test after a week. True untreated infection results in rise in titer.
• Those individuals with past infections may develop anti-Salmonella antibodies during an unrelated or closely related infection…. “anamnestic response” …..differentiated from true infection by lack of any rise in titer on repetition after a week.
Problem with the test • Serological diagnosis relies classically on the
demonstration of a rising titer in paired samples at an interval of 10–14 days.
• However, a 4 fold rise is not always demonstrable, even in blood culture confirmed cases probably due to:
the acute phase sample was obtained late
high levels of background antibodies in endemic areas
antibody response blunted by the early administration of antibiotics
• So, treatment decision are mostly made on the basis of results obtained with a single acute phase sample
The baseline… • A study in Malaysia (endemic)showed, antibody titers
up to 1/160 for both H and 0 antigens normal population
• In Sri Lanka, another endemic area, normal population had titers of 1/80 for both
• Any interpretation as to the significance of a Widal test result must be made against this "baseline" information.
• For example, titers of 1/50 and 1/100 on a single specimen, which are considered significant in non-endemic areas, are of no diagnostic significance in areas where S. typhi is endemic.
• We need to set our diagnostic criteria on the basis of our baseline (already planned to work on this)
Why the Widal test still survives?
• Widely available & remains the only practical test available in most centers in developing countries
• Easy to perform and convenient
• Culture “the gold standard” not widely available and positivity rate is very low (<50% even in ideal situations)
• Good negative predictive value
• Still useful as a presumptive diagnosis in strong clinical settings (not confirmatory)
• Not other better options except slight modifications of the same test
Widal in our setting • In endemic areas, high titers could be
demonstrated at an early stage in the illness, often during the first week.
• This suggests that, in an endemic area with frequent exposure to S. typhi and antigenically related salmonellae, the immune response may often not be a primary one.
• About 70% of patients express high antibody titers in the first week and rarely express a 4 fold rise…thus a single acute-phase widal may be useful in endemic settings (with a strong baseline titer)
18 10/4/2011 Tshokey/CC 2011