Indian J Med Res 123, April 2006, pp 513-516
Prospective evaluation of a rapid diagnostic test Typhidot ® for
Mary V. Jesudason & S. Sivakumar
Department of Clinical Microbiology, Christian Medical College, Vellore, India
Received October 21, 2004
Background & objectives: Typhoid fever still continues to be a major public health problem around
the world. A simple, reliable and affordable rapid diagnostic test has been a long felt need of the
clinicians. We therefore prospectively evaluated the sensitivity and specificity of Typhidot® test.
Methods: The study was carried out between January 2002 and December 2003, on a total of 563
samples from patients clinically suspected to have typhoid fever; blood culture as well as serum for
Typhidot® test were received.
Results: Of the 563 samples, Typhidot® test and blood culture were positive in 36 patients, both the
tests were negative for 503 patients. Typhidot® test was positive for 9 patients with S. Paratyphi A
infection. The sensitivity and specificity of the test using blood culture as gold standard were 92.3
and 98.8 per cent respectively for the typhoid fever.
Interpretation & conclusion: Typhidot® test is rapid, easy to perform and reliable test for diagnosing
typhoid fever, useful for small less equipped laboratories as well as for those with better facilities.
Key words S. Typhi - Typhidot ® test - typhoid fever
Typhoid fever continues to be prevalent in several from each other. Therefore, reliable laboratory tests
countries around the world. These are mostly low- are essential to establish aetiologic diagnosis so that
or middle-income countries with inadequate appropriate treatment can be given. Delay or error
sanitation and hygiene, particularly regarding food, in diagnosis prolongs the misery of the patient and
water and disposal of human excreta. In such places increases the cost of treatment and other expenses
there are also other causes of febrile illnesses, such for the family. The current standard diagnostic test
as vector-borne malaria, dengue fever and for typhoid fever is blood culture, which may not be
rickettsiosis as well as environmentally transmitted available or done properly in many clinics or small
leptospirosis and melioidosis 1-5. During the first week hospitals. Among the many thousands of laboratories
of fever these illnesses are not easily distinguished in India, about 200 have joined the available
514 INDIAN J MED RES, APRIL 2006
microbiology external quality assessment scheme run acetate disc each for IgM and IgG was incubated in
under the auspices of Indian Association of Medical it, for 20 min at room temperature, with continuous
Microbiologists 6. The availability of a simple and slow rotation. The two discs were then washed with
reliable rapid diagnostic test has been a long-felt need the supplied buffer solution and again incubated as
of clinicians in many parts of the world. Recently before, one with anti-human IgM and the other with
such a test has become commercially available under anti-IgG. After 15 min, they were washed again and
the proprietary name Typhidot ® (Malaysian treated with freshly prepared substrate solution for
Biodiagnostics Research Sdn. Bhd, Selangor Darul color development. The positive control formed
Ehsan, Malaysia). It is an enzyme linked duplicate dark spots of 2 mm diameter each. Any
immunosorbant assay (ELISA) in the dot test format, test sample showing similar or darker spots was
which detects IgM and IgG antibodies against defined positive. The absence of visible spot
Salmonella Typhi. One or another version of this indicated a negative test result. If the spots were
test has been evaluated in Malaysia, Singapore, fainter than the control, that sample was also
Pakistan and India and the results have been quite considered negative, according to the directions given
satisfactory 7-10 . In Singapore and Pakistan, it was in the kit. In case of discrepant appearance of the
evaluated by using a variety of case definitions; the duplicate spots, the test was repeated and only if both
sensitivity ranged from 84 to 93 per cent and dots were darker than control, the sample was taken
specificity from 77 to 89 per cent8,9. Our own previous as positive.
retrospective assessment of Typhidot test on stored
samples showed specificity of 80 per cent, but Results
sensitivity of 100 per cent10.
Of the 563 febrile patients concurrently tested by
blood culture and Typhidot test, 39 were positive for
In the light of such encouraging results, we
S. Typhi and 18 for S. Paratyphi A. The results were
undertook this study to conduct a large scale
analysed after excluding the 18 subjects with
prospective evaluation of Typhidot test.
paratyphoid fever. Among the remaining 545
patients, Typhidot test was positive in 42 (Table).
Material & Methods The test sensitivity was 92.3 per cent and specificity
98.8 per cent. The positive predictive value was 85.7
This study was carried out in the Department of per cent and negative predictive value was 99.4 per
Clinical Microbiology, Christian Medical College cent. Among the 18 subjects with S. Paratyphi A in
& Hospital, Vellore, south India during two years blood culture, 9 were positive in Typhidot test.
(from January 2002 to December 2003). Blood
samples from patients clinically suspected to have The case histories of the six patients with
typhoid fever were collected for culture and often, apparent false positive Typhidot test result were
a parallel clotted blood sample as well. Blood re-evaluated. Five of them had fever more than
culture was done according to standard procedures 7 days without evidence of other febrile illnesses,
using the BacT\ALERT automation system and were presumptively diagnosed to have typhoid
(Biomerieux, Lyon, France) 11. Any isolated bacteria fever and treated with ciprofloxacin. They made
were identified according to the recommended full recovery. However, their blood cultures had
standard protocol12. grown α- Streptococci (2 subjects), obvious
contaminants (2) and Proteus mirabilis (1). The
The Typhidot test was done on serum samples, two patients with α- Streptococci were also treated
according to the procedure stipulated by the with doxycycline covering for bacteraemia from
manufacturer. Each time the test was done, positive cryptic source. The sixth patient was diagnosed
and negative controls supplied in the kit were also with tuberculosis lymphadenitis and showed
included. Each serum sample was diluted to 1:100 marked clinical improvement on anti-tuberculosis
with the supplied diluent and one nitrocellulose treatment.
JESUDASON & SIVAKUMAR: PROSPECTIVE EVALUATION OF TYPHIDOT® TEST 515
In three patients the Typhidot test result was There were six false positive serum samples in
falsely negative. On review of case records it was the rapid test, by definition. Five were indeed
found that one subject was tested on day 2 of fever diagnosed clinically as typhoid fever, and treated
as her son had just been diagnosed to have typhoid with ciprofloxacin and all had recovered
fever. Another patient had typhoid fever two months uneventfully. In one subject, the final diagnosis was
earlier and this time, suspecting relapsing typhoid tuberculosis (lymphadenitis, laboratory proven). This
fever, blood was taken for culture on day 3 of fever. subject indeed had false positive result, but it is
The third patient had one month of fever and grew possible that an anamnestic response could have
S. Typhi in blood culture. been the real reason.
Discussion There were three false negative results. In two
The Typhidot® test was easy and rapid to perform subjects blood had been collected as early as two or
three days from the onset of fever, clearly too early
and to read. The turn around time for the test was
to detect antibody in the test. One of them had relapse
about one hour, whereas blood culture results were
of illness which could probably be due to antibody
available one day later in most instances, but
deficiency. Ordinarily typhoid fever would be
occasionally 2-4 days later.
suspected and blood collected for serology only after
one week of continuous fever. The third subject had
Since S. Paratyphi A is closely related to S. Typhi fever for a month, the blood culture grew S. Typhi
18 subjects with the former were excluded. and yet the rapid test was negative. On account of
However, in spite of the fact that the test is meant the high accuracy indices of the test, Typhidot
to diagnose only typhoid fever, we found it useful requests were frequent and the department received
in paratyphoid A fever also, with 50 per cent request for Typidot test (without blood culture) in
sensitivity and 98.8 per cent specificity. If we were 1324 patients and among them 280 (21%) were
to use Typhidot test to test for “enteric fever” positive. During the study period (January 2002 to
(typhoid and paratyphoid A fevers), there were 51 December 2003) the actual proportion positive was
positive as against 57 positive by culture. Thus, if 7.5 per cent. One drawback of this rapid test is that
the entire study population was included (563), the one does not have the organism to check
sensitivity and specificity of the test for enteric antimicrobial sensitivity.
fever diagnosis were 78.9 and 98.9 per cent,
respectively. The corresponding positive and In our earlier study 10 , using a retrospective
negative predictive values were 88.2 and 97.6 per approach by testing stored samples, the sensitivity
cent, respectively. and specificity were 100 and 80 per cent,
Table. Accuracy indices of Typhidot test against blood culture as gold standard
Typhidot test Blood culture for S. Typhi
No. positive No. negative Total
No. positive 36 6 42
(True positive) (False positive)
No. negative 3 500
(False negative) (True negative) 503
Total 39 506 545*
*18 subjects with S. Paratyphi A in blood culture were excluded in this analysis
Sensitivity = 92.3%, Specificity = 98.8%
Positive predictive value = 85.7%, Negative predictive value = 99.4%
516 INDIAN J MED RES, APRIL 2006
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Reprint requests: Dr Mary V. Jesudason, Professor, Department of Clinical Microbiology
Christian Medical College & Hospital, Vellore 632004, India