RÉSUMÉ
Introduction: Early Infant Diagnosis (EID) is the practice of testing infants for HIV within the first 46 weeks of life or at the earliest opportunity possible so as to promptly link HIV-infected infants to healthcare and treatment. In the absence of timely HIV testing and Antiretroviral Therapy (ART) initiation, one-third of infants living with HIV die before their first birthday and more than half die before 2 years. Worldwide, Mother To Child Transmission (MTCT) rates accounted for 8.9% of all HIV infections by the end of 2015, with Sub-Saharan Africa accounting for 12% of these infections. In Kenya, MTCT infection rates were at 14%, accounting for 7% of all new infant infections globally by the same period. Objection: The objectives of this study was to establish the determinants of HIV positive status at first PCR Test among infants seeking EID services in some selected hospitals in the Nairobi County, Kenya. Materials and Methodology: A longitudinal cohort study employing a mixed-method approach was used. Quantitative data was collected from pairs of 163 mother-infant using interviewer administered, pre-tested, and semi-structured questionnaires. While qualitative data was collected using Focus Group Discussion (FGDs) guides, it was coded, cleaned and analyzed using STATA Version 14. Quantitative data was analyzed using Fisher's exact test (p= 0.1) and Poisson Regression (p= 0.05) at bivariate and multivariable levels respectively. Thematic analysis was undertaken for qualitative data. Results: The findings from the adjusted parsimonious model revealed that, three variables influenced the infant HIV status at first PCR test. The study participants who had been administered with ART during pregnancy had a lower risk (RR= 0.06) of getting HIV positive infants relative to those who hadn't received ARVs during pregnancy (95% CI 0.014, 0.213 p= 0.000). Respondents who had been initiated on ART during the first trimester had lower risk (RR= 0.1) of getting an HIV positive infant than respondents who were administered with ARVs in the third trimester (95% CI 0.014, 0.021, p= 0.001). Respondents on first-line regimen had a lower risk (RR= 0.04) of getting HIV infected infants compared to those who were on the second line regimen (95% CI 0.012, 0.114, p= 0.000) confirming all the three significant variables as protective factors. Conclusion: The study corroborate that, first line ART regimen administered before pregnancy or during the first trimester of pregnancy was effective in lowering the risk of getting an HIV positive infant. Pediatric HIV infection, disease progression is quite rapid and without prompt treatment, most infants might not survive their second birthday