Chagas disease affects thousands in America, and diagnosis of chronic Chagas diseases requires two different tests. Four tests that are currently available include the Hemagen, Ortho, Wiener, and InBios tests. In this paper, the authors test the accuracy of these tests. From the results they obtained, they also suggest which sorts of tests to use as the two required diagnostic tests.
Chagas disease is a tropical disease common to North, Central, and South America. It is caused by the parasite, Trypanosoma cruzi, which comes from insect feces. The effects of the disease can last a lifetime, and it can significantly decrease one’s lifespan. The FDA recently approved a drug that can treat the disease. This makes accurate testing of the disease more necessary.
In order to diagnose someone for chronic Chagas disease, that person must test positive for two serological tests for antibodies against T. cruzi. There are currently four available serological tests: Hemagen, Ortho, and Wiener ELISAs, as well as InBios. All four seem to show high sensitivity / specificity. However, there may be differences in how well these tests perform based on the location of origin of the Chagas infection. The authors of this paper explored these differences with the hopes of helping to improve the accuracy of Chagas disease testing.
Materials and Methods
This study was approved by various review boards.
Sample Selection and Preparation
A total of 800 blood samples were obtained from the American Red Cross. 500 of these samples were from patients who were confirmed positive for Chagas disease. The authors tried to get as many positive samples from patients with a known country of origin. The other 300 samples were from patients who were confirmed negative for Chagas disease. The authors then tested the samples using the four tests described earlier.
The authors used three different analyses to test the accuracy of the four tests. One analysis compared the tests’ results to the results of previous tests performed on the blood samples. Another analysis compared each of the tests’ results to the results of the other tests performed in this study. The third analysis examines the results of the tests with what is called “latent class analysis,” a set of math modeling techniques.
The authors found that blood samples that were confirmed positive for Chagas disease were more likely to come from people of Hispanic origin than samples that were confirmed negative. In the three analyses described earlier, the InBios test had the highest sensitivity and the lowest specificity. The Hemagen test had the lowest sensitivity, but high specificity. The measurements for the other two tests were generally between these two sets of extremes.
The authors also compared the sensitivity of the tests on blood samples from positive patients originating from different regions. The sensitivity of the tests were lowest among those from Mexico, higher among those from Central America, and higher still among those from South America.
This study has revealed variations in sensitivity and specificity in the four tests studied, as well as variations in sensitivity across different regions of origin.
Using two tests is standard for screening for Chagas disease. While this may be cost-effective in areas where the disease is rampant, it could be costly (relative to the benefit) for areas where the disease isn’t as common. Many will use one test to do an initial screening of the disease, and use a second test only to confirm positive results from the first test. In this case, it is important for the first test to have the highest sensitivity possible. From the results gathered, the authors suggest using a third test if the first and second test disagree with each other.
No single test studied showed optimal performance. Understanding why these tests have lower sensitivities for blood samples from Mexican patients is important for improving them. It is possible that these low sensitivities may be explained by the fact that different strains of T. cruzi inhabit different regions.
Some limits of this study include the fact that the blood samples used came from donors, who may be healthier than the average person. Also, while the Hemagen test generally takes blood serum, the authors gave the test blood plasma.
The InBios test had the highest sensitivity but lowest specificity. The Hemagen test had the lowest sensitivity. The Hemagen, Ortho, and Wiener tests all had high and similar levels of specificity. Sensitivities for these tests were lowest in patients who came from Mexico, higher in patients from Central America, and highest in patients from South America. The authors conclude that the best diagnostic results can be obtained from using a high-sensitivity test first, followed by a high-specificity test.