Background: Age mixing may explain differences in HIV prevalence across populations in sub-Saharan countries, but the validity of survey data on age mixing is unknown.
Methods: Age differences between partners are frequently estimated indirectly by asking respondents to report their partner's age. Partner's age can also be assessed directly by tracing partners and asking them to report their own age. We use data from 519 relationships, collected in Likoma (Malawi), in which both the partners were interviewed and tested for HIV. In these relationships, age differences were assessed both indirectly and directly, and estimates could thus be compared. We calculate the specificity and sensitivity of the indirect method in identifying age-homogenous/age-disparate relationships in which the male partner is less/more than 5 or 10 years older than the respondent.
Results: Women were accurate in identifying age-homogenous relationships, but not in identifying age-disparate relationships (specificity ≈90%, sensitivity = 24.3%). The sensitivity of the indirect method was even lower in detecting partners older than the respondent by 10+ years (9.6%). Among 43 relationships with an HIV-infected partner included in this study, there were about 3 times more age-disparate relationships according to direct measures of partner's age than according to women's reports of their partner's age (17% vs. 46%).
Conclusions: Women's survey reports of their partner's age significantly underestimate the extent of and the HIV risk associated with age mixing in this population. Future studies of the effect of sexual mixing patterns on HIV risk in sub-Saharan countries should take reporting biases into account.