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.