Transcript Document

Field evaluation of a rapid
immunochromatographic assay for
detection of Trypanosoma cruzi
infection by use of whole blood
1. Médecins Sans Frontières – Spain
2. Universitario de Medicina Tropical - Universidad Mayor de San Simon,
Bolivia
3. Instituto Oswaldo Cruz - Fiocruz, Rio de Janeiro, Brazil
Paul Roddy
June 2008
1. ELISA Conventional
2. IHA
3. ELISA Recombinant
Study Rationale
• Sensitivity and specificity when using whole
blood was unknown.
• Circumvent constraints associated with
conventional serological assays and bolster
Chagas disease control efforts.
Objectives
• Evaluate
• Measure
• Compare
Methods (1/4) - study location
and participants
April – June 2007
• Valle Hermoso district - Sucre municipality
(pop. = 46,000)
• 9-months to 17.9 years (pop. = 19,000)
Methods (2/4) – diagnostic test
procedures
• 10 microliters (µL) of whole blood.
• Independently interpreted at 15 minutes.
• 3.5 millilitres (mL) of whole blood.
• 2 out of the 3 conventional assays are
positive.
• Benznidazole treatment offered.
Methods (3/4) – laboratory quality
control
Quality control applied at
health clinic and laboratory
levels.
– Calibrations
– Positive and negative
serum controls
– ‘Good practices’ check list
– Internal and external
quality control
Methods (4/4) – measurements of
diagnostic test performance
• Sensitivity and specificity calculated.
• The inter-observer reliability measured =
kappa statistic.
• The ‘ease of use’ criterion were quantified
and compared.
Results – demographics
1,913 study participants
M:F ratio = 0.95
Mean age = 9.3 years (SD= 4.4)
Median age = 9.0 years (interquartile range = 6-13)
Results – sensitivity & specificity
93.4% sensitivity (95% CI: 87.4% - 97.1%)
99.0% specificity (95% CI: 98.4% - 99.4%)
StatPak
Positive
Negativ
e
Total
Conventional Serology
Positive Negative
Total
113
18
131
8
1774
1782
121
1792
1913
Results – inter-observer reliability
Trained, non-laboratory health workers
concurred on 1,911 of 1,913 test results
yielding:
99.9% overall agreement
kappa = .999 (p<.0001)
Results – ease of use criterion
Questionnaire items scored according to a
scheme outlined in the Methods section of the
article.
Total of 51 possible points:
Chagas Stat-Pak
= 39.25 points (77%)
IHA assay
= 34 points
(67%)
ELISA conventional = 30 points
(59%)
ELISA recombinant = 30 points
(59%)
Possible limitation
Cross-reactivity may occur between sera of
patients infected with T. cruzi and sera of
patients infected with Leishmania spp. when
using conventional assays for the diagnosis of
Chagasic infections.
8 / 121 = 6.6%
Conclusion and
Recommendations (1/2)
1. Effective laboratory-based solutions not widely
available or likely to be anytime in the near
future.
2. RDT cost remains ‘out of reach’ for many
impoverished people affected by the disease.
3. Compares less favorably with whole-blood RDT
diagnosis of other parasitological vector-borne
diseases.
Conclusion and
Recommendations (2/2)
4. Missed treatment opportunities and increased
likelihood of chronic-stage disease sequela.
5. Not an ideal screening or diagnostic test for the
population investigated in this study.
6. Hindered ability of the test to give dependable
results under the conditions outlined in the study.
7. MSF continues to use whole blood with the
Chagas Stat-PakTM RDT…