Prostate-Specific Antigen to Ascertain Reliability of Self-Reported Coital Exposure to Semen.

RESEARCH THAT EVALUATES BEHAVIORAL INTERVENTIONS or barrier methods for the prevention of pregnancy or sexually transmitted infection (STI)/HIV often relies on self-reports of sexual activity. A work group sponsored by the U.S. National Institutes of Health to examine the validity of these measures concluded that “most people will provide what they believe are truthful responses” if specific data collection conditions are met. However, the group did not provide empirical evidence or references to support their conclusion. In contrast, other research has questioned the validity of self-reported sex and condom use data.

A biologic marker of unprotected sex, detectable in vaginal fluids, could serve to support or supplant the use of self-reported data. Prostate-specific antigen (PSA), a protein produced by the prostate gland and secreted into the urethra during ejaculation, can be detected in vaginal fluid samples with high sensitivity using solid-phase enzyme-linked immunosorbent assay (ELISA). PSA clears from vaginal fluid within 48 hours with a mean clearance time of approximately 20 to 27 hours. We evaluated the validity of self-reports of recent unprotected sex among female sex workers (SWs) participating in a condom promotion study by comparing their reported sexual activity and condom use with PSA detection in vaginal samples.

Discussion

Biologic and behavioral studies related to sex depend extensively on self-reports. However, the validity of these data is difficult to establish. For example, Zenilman et al failed to find a correlation between self-reported condom use and STI risk reduction. Some have interpreted this as evidence for the fallibility of self-reports, whereas others argued that STI is a poor surrogate for unprotected sex. Choosing an appropriate surrogate is vital to avoid incorrect research conclusions. Both PSA and Y chromosome appear to be good biomarkers of recent exposure to semen through unprotected coitus. We tested for PSA, which is found in high concentration in semen and can be reliably detected in the vaginal vault after unprotected intercourse.

We detected PSA in the vaginal vault of 21% of women who reported no sex and 39% of those who reported protected sex only within the past 48 hours. These results likely underestimate misreporting because PSA concentrations begin to decline immediately after exposure. Macaluso et al found that 71% of specimens tested negative for PSA 24 hours after clinical insemination of 1 mL of their partner’s semen. This rapid clearance of PSA from vaginal fluid after exposure to semen prevents us from measuring discordance in the other direction (i.e., testing negative for PSA despite reporting recent unprotected sex). Furthermore, menstruation, vaginal washing (an apparently common practice in the study population), or other behaviors hypothetically could have interfered with PSA detection or hastened its clearance.

In contrast, a positive test for PSA is unlikely to occur in the absence of recent semen exposure given the high test specificity. Incorrect condom use or condom malfunctions could have resulted in female exposure to semen. If this were the case, however, among those testing positive for PSA, PSA concentrations should be lower for those who reported protected sex only compared with those who admitted to at least one unprotected act. Despite the careful phrasing of questions, women could have misunderstood the questionnaire. Also, participants might have had difficulty considering the required time periods. However, when we compared participant responses for our and the larger study’s questionnaires (both of which were completed at the same visit), we found high internal consistency. Because achieving consistency between the 2 questionnaires often required both an affirmative and negative answer, women could not have achieved consistency simply by giving the same responses. Given the high internal consistency, poor comprehension does not appear to explain the apparently substantial misreporting.

We studied female SWs participating in an 18-month condom promotion study in Madagascar; the findings might not be generalizable to other populations. For example, social desirability bias could have caused the participants to overreport condom use during the face-to-face interviews. Also, the choice of study population could have resulted in selection bias; the subset of participants in the larger study who adhered to the study protocol by returning for their last scheduled visit might have been more likely to underreport unprotected sex. Alternatively, participants might have been less likely to misreport condom use than non-SW populations because the interviewers were aware of their occupation and the resistance to condom use that they often encounter from their clients.

Self-reported data are used for informing policy, research, and funding decisions regarding STI/HIV and pregnancy prevention efforts. Participants might give inaccurate responses as a result of self-presentation or courtesy bias, imperfect recall, poor question comprehension, limited topical vocabulary, exaggeration resulting from social norms or to comply with study eligibility criteria, personal salience of the sexual event, or emotional responses to sensitive questions. The high level of misreported recent exposure to semen that we demonstrated substantiates that self-reports of unprotected sex cannot be assumed to be valid measures. Future STI/HIV and pregnancy prevention studies should establish the veracity of self-reported measures of sex and condom use or should use end points that do not rely on self-reported data.

Conclusions:

The substantial disagreement between self-reports and measurement of a biologic marker of semen exposure in vaginal specimens substantiates that self-reports of sexual behavior cannot be assumed to be valid measures. Future sexually transmitted infection/HIV and pregnancy prevention studies should confirm the validity of self-reports or use end points that do not rely on self-reported data.