Analytical performances of Exacto® HIV self-test in the Central African Republic

We herein evaluated the analytical performances of the CE-IVD capillary blood Exacto® HIV self-test (Biosynex, Strasbourg, France) in the Central African Republic (CAR). A cross-sectional study was conducted on a representative national panel of 200 sera positive for HIV and 200 negative for HIV, randomly selected thorough the CAR for HIV seroprevalence surveillance survey, according to reference test. The Exacto® HIV self-test showed 99.5% (95% CI: 98.2-99.9) sensitivity and 100.0% (95% CI: 99.0-100.0) specificity. The Youden´s J index and Cohen´s Kappa coefficient were 0.995. At HIV-1 seroprevalence of 3.5% in the general adult population of the CAR, the positive and negative predictive values were 100% (95% CI: 99.0-100) and 99.9% (95% CI: 98.9-100), respectively. The results are within the limits required by the WHO (i.e. sensitivity ≥ 99.0% and specificity ≥ 98.0%), making Exacto® HIV self-test suitable for routine use in the CAR.


Introduction
The Central African Republic (CAR) has an adult HIV prevalence rate of 3.5% according UNAIDS´ updated data, with approximately 130,000 people living with HIV-1 [1]. Of these, approximately twothird are unaware of their HIV status [1]. Opportunities for HIV testing could be enhanced by offering accepted and effective strategies such as HIV self-testing in populations that fear stigma and discrimination when accessing conventional HIV counselling and testing in health care facilities [2,3].
The CAR is continually carrying out new technologies for HIV testing in order to support the national HIV/AIDS policy. A recent study among lay users living in Bangui, the capital city of CAR, previously reported a high rate of the acceptability and the usability of the capillary blood-based Exacto ® HIV self-test (Biosynex, Strasbourg, France) [4]. Indeed, the Exacto ® HIV self-test is a third-generation immunochromatographic test, using a combination of a specific antibody binding protein that is conjugated to colloidal gold dye particles and synthetic antigens (gp41, gp36) able to detect antibodies (IgG and IgM) against HIV-1 or HIV-2 in whole-blood, serum or plasma, which are bound to the solid-phase membrane [3]. We herein investigated the analytical performances of the Exacto ® HIV self-test in the CAR, as recommended by the World Health Organization (WHO) before use [5]. Statistical analysis: data were entered into an Excel database (Redmond, WA, USA) and analysed using SPSS 20.0 (Chicago, IL, USA). Analytical performances of Exacto ® HIV self-test was defined using sensitivity, specificity, concordance, accuracy, positive predictive values (PPV) and negative predictive values (NPV) as previously described [7]. Briefly, the Cohen´s Kappa coefficient calculation was used to estimate the concordance, and interpreted according to the Landis and Koch scale; accuracy was estimated by Youden´s J index; the PPV and NPV were calculated by using Bayes´ formulae taking into account of the HIV prevalence of 3.5% in the CAR.

Results
The results of the analytical performances of Exacto ® HIV self-test are depicted in Table 1

Discussion
We herein evaluated the analytical performances of the Exacto ® HIV self-test (Biosynex, Strasbourg, France) in the CAR on a representative national panel. The Exacto ® HIV self-test showed high sensitivity (99.5%), specificity (100%), concordance, accuracy and positive and negative predictive values.

The virological performances of HIV self-test in
French-speaking countries in sub-Saharan Africa are yet poorly established. In practice, clinical validation of the analytical performances of HIV self-test in real-life should be always carried out in a sufficiently large number of target population subjects before introducing them into the routine diagnosis, as strongly recommended by the WHO [8].
In addition, Central Africa is characterized by broad genetic diversity of HIV-1 strains, which can be associated with false negativity of HIV immunochromatographic rapid diagnostic tests [9], and by a variety of factors which can be associated with false positivity or unspecific reactivities, including disturbances affecting the B cell-driven immunity during infectious diseases, such as marked immunological stimulation, strong non-specific polyclonal B-cell activation, hypergammaglobulinemia, and production of circulating immune complexes [10].
HIV self-tests are more and more developed and may be of variable quality [7]. In the present study, the analytical performances of Exacto ® HIV self-test in the CAR, were evaluated using a collection of 200 positive and 200 negative sera randomly selected thorough the CAR for HIV seroprevalence surveillance survey. The results showed excellent analytical performances of the Exacto ® HIV self-test, despite the risk of false-positive results with frequent inconclusive sera in this area of Africa [5,10]. Only one positive for HIV was found negative by the HIV self-test, but the final estimated sensitivity remained high. Finally, the analytical performances of Exacto ® HIV self-test were within the limits required by the WHO for HIV self-testing i.e. sensitivity ≥ 99.0% and specificity ≥ 98.0% [8], likely allowing it to detect HIV-1 strains circulating in the CAR. A previous study conducted in the Democratic Republic of the Congo showed similar high analytical performances of the Exacto ® HIV self-test [3].
This study has some limitations. This study did not consider the possibility of misinterpretation by lay users, as previously described. Indeed, under field conditions, misinterpretation of the HIV self-test could alter the proportion of falsely interpreted positive and negative results, thus reducing its analytical performance in the hands of lay users.

Conclusion
In conclusion, the Exacto ® HIV self-test showed high sensitivity and specificity, making the test within the analytical limits required by the WHO to be used in routine in the general population for HIV testing in CAR. Taken together, the high analytical performances in addition to the previously reported high practicability and usability of the Exacto ® HIV self-test make the test suitable for routine use in the CAR.

What is known about this topic
 Field validation of capillary HIV Self-Test constitutes a mandatory prerequisite before use;  Broad spectrum of HIV-1 strains are circulating in Central Africa;  HIV rapid test may lack specificity in the Central African context.

What this study adds
 Exacto® HIV Self-Test is highly sensitive and specific in Central Africa;  Exacto® HIV Self-Test can be used safely by adult profane public with high confidence in Central Africa. Table   Table 1: analytical performances of Exacto ® HIV selftest  The results were presented as a 95% confidence interval (CI) using the Wilson score bounds; $ The Cohen's K coefficient calculation was used to estimate the concordance, and interpreted according the Landis and Koch scale as follows: < 0 as indicating no agreement, 0-0.20 as slight, 0.21-0.40 as fair, 0.41-0.60 as moderate, 0.61-0.80 as substantial, and 0.81-1 as almost perfect agreement; £ Accuracy was estimated by Youden's J index = sensitivity + specificity -1; accuracy is expressed in percentage; β The positive predictive values (PPV) and negative predictive values (NPV) were calculated by using Bayes' formulae taking account of the HIV prevalence of 3.5% in the Central Africa Republic