Objectives
Persistent CD4:CD8 ratio inversion (< 1) is associated with mortality in older people. We investigated the interaction of the effects of baseline CD8 count and age at HIV diagnosis on CD4:CD8 ratio recovery with antiretroviral therapy (ART).

Methods
An observational study (1 January 2007 to 31 December 2016) was carried out using routinely collected data from the HIV outpatient services at Imperial College Healthcare NHS Trust, London, UK. CD4 and CD8 counts, prior to and during ART, treatment during primary HIV infection (PHI) and HIV‐1 viral load were included in univariate and multivariate analyses using Cox proportional hazard regression.

Results
Data were included for 876 patients starting ART, where HIV suppression was achieved. Of these patients, 741 of 876 (84.6%) were male and 507 of 876 (57.9%) were Caucasian. The median time on ART was 38 [interquartile range (IQR) 17–66] months. CD8 count change on ART was bidirectional; low CD8 counts (≤ 600 cells/μL) increased and high CD8 counts (> 900 cells/μL) decreased. The median pre‐ART CD4:CD8 ratio was 0.41 (IQR 0.24–0.63), and recovery (≥ 1) occurred in 274 of 876 patients (31.3%). Pre‐ and post‐ART CD4:CD8 ratios were lower in those aged > 50 years compared with young adults aged 18–30 years (P < 0.001 and P = 0.002, respectively). After adjustment, younger age at HIV diagnosis (P < 0.001) and treatment during PHI (P < 0.001) were favourable for CD4:CD8 ratio normalization.

Conclusions
Older age (> 50 years) at HIV diagnosis was associated with persistent CD4:CD8 ratio inversion, whereas treatment of PHI was protective. These findings confirm the need for testing and early treatment of people aged > 50 years, and could be used in a risk management algorithm for enhanced surveillance.