The Chagas Stat-Pak assay, a rapid diagnostic test (RDT) detecting antibodies against recombinant antigens of T. cruzi, showed good sensitivity (95.2 to 96%) and excellent specificity (99.8 to 99.9%) when tested with blood or serum samples from a large cohort of Latin-American adult migrants living in Geneva, Switzerland. The positive and negative predictive values were both above 98% in the population studied, which had a 12.5% prevalence of T. cruzi infection. The concordance of Stat-Pak results for blood and serum samples was near perfect (99.7%), and the reproducibility of the test was high.
Most previous studies evaluating the diagnostic accuracy of the Chagas Stat-Pak assay in South and Central America also revealed high sensitivity and specificity estimates (3
). Considering (i) the very high proportion (97.6%) of Bolivians among T. cruzi
-infected patients in our study and (ii) the probable influence of geography on the Chagas Stat-Pak test performance (20
), it is more appropriate to restrict the comparison of test sensitivity with studies conducted with Bolivian patients. Our findings are consistent with published sensitivity estimates of the Chagas Stat-Pak in Bolivian patients, ranging from 93.4% to 98.6% (3
). One study showed lower Stat-Pak sensitivity (87.5%) among pregnant Bolivian woman, but the precision of the estimate was limited by the small number (n
= 24) of T. cruzi
-infected patients tested (21
). Whereas the Chagas Stat-Pak assay appears less sensitive than some of the more sophisticated serological tests (e.g., ELISA, Western blotting [WB]) (14
), this relative deficit in sensitivity must be interpreted in light of its ease of use, low cost, and capacity to provide immediate results. These practical issues are crucial, as they allow more people at risk to have access to diagnosis both in countries where Chagas' disease is endemic and in countries where this disease is not endemic (e.g., utilization in primary health care centers). Nevertheless, the development of a RDT with improved sensitivity is a priority.
The very high specificity of the Chagas Stat-Pak assay found in this study is consistent with high specificity estimates (97.4 to 100%) found by others (3
). Very high positive predictive values are therefore found when the prevalence of T. cruzi
infection is moderate or high in the population tested. In these settings (e.g., Bolivians from areas where Chagas' disease is highly endemic) and provided that the test procedure is carefully followed, a positive Chagas Stat-Pak test result is diagnostic of T. cruzi
We found a very high reproducibility of the Chagas Stat-Pak assay. The performance of the test was not significantly altered (only three false-negative results) when the test was performed on blood samples by a trained nurse with no prior experience in laboratory work compared to the test performed on serum samples by an experienced laboratory technician. A mistake in reading the test result is unlikely, as the three false-negative RDT results were confirmed as negative by an independent reader after completion of the study. It is therefore possible that testing patients using blood samples instead of serum or plasma samples may result in a loss of sensitivity, as hypothesized by others (3
), but our data show that the resulting loss in sensitivity is small, if present at all.
Some limitations of our study must be emphasized. First, as already mentioned, the sensitivity estimate of the Chagas Stat-Pak assay is valid for Bolivian adults only, as children were not included in our study and as nearly all T. cruzi
-infected patients were originally from Bolivia. Second, the diagnostic accuracy of the Chagas Stat-Pak test may be overestimated, as the reference standard was also composed of serological tests. Unfortunately, this limitation cannot be avoided, as the sensitivity of other diagnostic approaches (i.e., parasitological and molecular techniques) is low during the chronic phase of Chagas' disease. Third, false-positive results of the Chagas Stat-Pak and ELISAs used as reference standards may have been caused by cross-reactivity with past or present leishmaniasis. Such cross-reactivity is seen mainly with visceral leishmaniasis, a rare and geographically restricted disease in Bolivia (5
). None of our patients had symptoms or signs of active leishmaniasis.
The study results have had a significant impact on the diagnostic management of Chagas' disease in our hospital (HUG). The Chagas Stat-Pak assay has been recently introduced as a point-of-care test (POCT) in the primary care center (UMSCO) that provides care to the vast majority of undocumented Latin-American migrants living in Geneva, Switzerland. The test is also used as a POCT in the maternity ward when pregnant Latin-American migrants with no prior Chagas testing are admitted for delivery. A positive result triggers investigations of the cord blood to diagnose congenital transmission (9
). Several nurses and midwives have been trained to perform the Stat-Pak assay procedure and quality surveillance has been implemented. In other departments (e.g., general outpatient department [OPD]) or settings (e.g., prenatal consultation at the maternity ward), the Chagas Stat-Pak assay is performed on serum samples in the parasitology lab in parallel with a commercial ELISA. In the case of discordant results, serum samples are sent to the Swiss Tropical and Public Health Institute where two other serological tests are available (immunofluorescence [IF] and in-house ELISA).
In conclusion, the Chagas Stat-Pak RDT proved to be an accurate diagnostic test for T. cruzi
infection in a cohort of adult Latin-American migrants living in Geneva, Switzerland, leading to improved access to diagnosis and care. The sensitivity of the test could be further improved, or another Chagas RDT with higher sensitivity could be used in parallel. A few other commercial serological RDTs have been evaluated, although less extensively than the Chagas Stat-Pak RDT, with promising results (4
). A multicentric evaluation of several Chagas RDTs is planned by the WHO. Accurate RDTs for Chagas' disease have a huge potential to improve access to diagnosis and care for this neglected disease both in countries where Chagas' disease is endemic and in countries where this disease is not endemic.