Objective: To estimate the prevalence of undiagnosed HIV infection in men and women aged 16-80 years, having blood tests within primary care in Madrid, Spain.
Methods: A serosurvey to monitor vaccine-preventable diseases in the general population aged 16-80 years was conducted in 2008-2009. Eligible individuals were those having blood tests. The blood extraction centres, the primary sampling units, were chosen in proportion to the size and socio-economic characteristics of the target population, aiming for a sample size of 5355 subjects with equal sex distribution within five age bands. Migrants aged 16-40 years were oversampled. Previous HIV diagnoses were excluded. Prevalence rates of HIV infection with 95% CIs were estimated allocating weights inverse to their probability of selection.
Results: Overall, 3695 subjects agreed to participate, yielding a response rate of 69%, similar for men (66%) and women (73%); individuals recruited at healthcare centres or by telephone; and for all age groups except those aged ≥60 (57%) years. HIV infection was diagnosed in 12 subjects (0.35%; 95% CI 0.13 to 0.57); prevalence, higher in men (0.51%; 95% CI 0.12 to 0.89) than in women (0.20%; 95% CI 0.00 to 0.44); participants from other countries (0.61%; 95% CI 0.03 to 1.18) as compared with Spanish born (0.30%; 95% CI 0.06 to 0.53) and aged 21-30 years (0.65%; 95% CI 0.01 to 1.29), or 31-40 years (0.71%; 95% CI 0.02 to 1.41). None of the differences were statistically significant. Most of the 12 subjects were under follow-up for medical conditions; 11 had visited the primary care clinic in the preceding month.
Conclusion: The prevalence of undiagnosed HIV infection is very high and calls for strategies to unveil occult HIV infection.