Background and Objectives: The advent of more sensitive diagnostic testing technologies and competition in public healthcare spending have resulted in a reevaluation of sexually transmitted disease (STD) screening practices in an attempt to target populations at greatest risk. Screening among populations with a < 2% prevalence of chlamydia and a < 1% prevalence of gonorrhea may not be cost-effective.
Goal: To identify subpopulations with a low prevalence of chlamydia or gonorrhea.
Study Design: The prevalence of genital chlamydia and gonorrhea among asymptomatic STD patients screened from 1997 to 1998 at San Francisco City Clinic was stratified by demographic and behavioral risk factors.
Results: The prevalence of chlamydia and gonorrhea was 3.4% and 1.1% among asymptomatic women and 4.0% and 1.0% among asymptomatic men, respectively. Two low-prevalence subpopulations identified among asymptomatic patients were women older than 29 years (chlamydia, 1.2%) and men who have sex with women (gonorrhea, 0.8%).
Conclusions: These data identified low-prevalence subpopulations among asymptomatic STD patients. As a result, the STD screening criteria at San Francisco City Clinic were changed accordingly.
PUBLIC SEXUALLY TRANSMITTED DISEASE (STD) clinics have historically been settings where indigent or low-income populations could receive diagnostic testing for symptomatic infections caused by STDs. Universal testing was, and still is, the standard protocol at most STD clinics in the United States, with the implicit understanding that any person frequenting an STD clinic is likely to be at risk, symptomatic, and therefore infected. Over time, because of the expansion of services provided at STD clinics, the recognition of the highly asymptomatic nature of many prevalent STDs, and the advent of new, more sensitive and patient-friendly testing technologies, more asymptomatic persons are seeking care at STD clinics. In addition, targeted screening for STDs has been introduced in a variety of settings. Clinic settings serving lower-prevalence populations, such as family planning clinics, have long ago adopted selective STD screening programs. In 1995, the California State Office of Family Planning established selective screening guidelines for infections caused by Chlamydia trachomatis based on age and type of family planning visit. Although the Centers for Disease Control and Prevention's 1998 STD Treatment Guidelines include age-based guidelines for screening for chlamydia, guidelines for gonorrhea are the "screening of all women at high risk for STDs (p. 60)."1 In both cases, the guidelines are broad and suggest a need for site-specific protocols based on local epidemiology.
Although funding for STD services has never been abundant, public programs continue to attempt to provide more services with static budgets. Improved technologies enable the testing of more patients and more accurate results, but at an increased cost. Selective screening is one potential solution. The San Francisco Department of Public Health STD Prevention and Control Services has recently revised chlamydia and gonorrhea screening protocols at the only municipal STD clinic, San Francisco City Clinic (SFCC), based on an evaluation of disease prevalence among asymptomatic patients and the identification of subpopulations at low risk for genital chlamydia and gonorrhea.
The prevalence of genital chlamydia and gonorrhea among asymptomatic STD clinic patients screened in 1997 and from January 1998 to June 1998 was stratified by demographic and behavioral risk factors including age, gender, sexual orientation, numbers of partners in the past 2 months, symptom status, and STD contact status. Patients were evaluated using urine-based ligase chain reaction (LCR; Abbott Laboratories, Abbott Park, IL) for chlamydia and a standard urethral and cervical culture technique for gonorrhea. Cost-saving estimates were based on a cost of $12 and $5 per test for LCR and culture, respectively.
Cost-effective percent-positive cutoffs for screening vary from 2% to 7% for genital chlamydia, depending on the population considered.2-6 Currently, there are no cost-effective percent-positive cutoffs for gonorrhea in the literature. Given the relatively small number of asymptomatic patients available for screening and with the expectation of low numbers of infections, we decided to use conservative percent-positive cutoffs as criteria for selective screening. We chose to identify subpopulations with a < 2% prevalence of chlamydia. Because the cost of gonorrhea culture is approximately half of the cost of LCR, we identified subpopulations with a < 1% prevalence of gonorrhea.
In the 1980s, the SFCC was a high-volume clinic that primarily served gay men with gonorrhea and syphilis. Although the clinic has since expanded its services to include family planning, 74% of current patients are male. Additionally, although public STD clinics traditionally serve low-income minority populations, 46% of SFCC patients are white, 51% earn > $10,000 annually, and 74% have more than a high school education (Table 1).
Between January 1997 and June 1998, 3,428 women and 7,164 men with no other ostensible risk factors (e.g., multiple partners, STD contacts) were tested for genital chlamydia or gonorrhea; 53% of women and 39% of men had no symptoms for either type of infection (i.e., discharge, dysuria, abnormal vaginal bleeding, pelvic pain). Overall, chlamydia and gonorrhea prevalence in women was 5.3% and 1.3%, respectively. Among men, the prevalence of chlamydia and gonorrhea was 11% and 21%, respectively. The prevalence of chlamydia and gonorrhea was 3.4% and 1.1% among asymptomatic women and 4.0% and 1.0% among asymptomatic men, respectively.
Based on cost-based cutoffs of 2% for chlamydia and 1% for gonorrhea, two low-prevalence subpopulations were identified among asymptomatic patients (Table 2): (1) women 30 years or older for chlamydia (1.2%); and (2) men who have sex with women for gonorrhea (0.8%). These subpopulations were therefore excluded from STD clinic chlamydia and gonorrhea screening guidelines established in 1998 (Table 3). The new screening guidelines for genital chlamydia include all men and all women younger than 30 years. Women older than 30 years will continue to be tested if symptoms are present, if they have had more than one sex partner in the past 2 months, if they are an STD contact, or if they are pregnant. The new guidelines for gonorrhea include screening all women, all men who have sex with men, and all men who have sex with both men and women. Symptomatic men who have sex with women will continue to be routinely tested. Based on 1997 data, these new screening guidelines translate to annual cost savings of more than $16,000, which is a significant amount for a single clinic with an annual visit load of 17,000 patients. As a result, 16% of the gonorrhea-testing budget is now available for increased screening among other high-risk individuals through community outreach.
Although the focus is often on the identification of high-risk populations in which to conduct targeted screening to treat as many infections as possible, it is equally important to identify populations in which screening is neither necessary nor cost-effective. Stratification by a single risk factor (e.g., symptom status) can reveal dramatic differences among such subpopulations. For example, data from SFCC show dramatically different urethral gonococcal infection rates between symptomatic and asymptomatic men (28% versus 1.0%). Without stratification, such differences may remain unnoticed and result in unnecessary testing.
Because of the small numbers of patients available-particularly among certain subgroups-and the paucity of conclusive evidence in the literature, conservative percent-positive cost-effective cutoffs of 2% for chlamydia and 1% for gonorrhea were applied. These cutoffs are therefore somewhat arbitrary, and may be lower than actual cost-effectiveness cutoffs. Furthermore, cost-effective prevalence cutoffs for chlamydia screening that have been put forth in the literature resulted from analyses using nonamplified testing methodologies (e.g., cell culture, direct fluorescent antibody, enzyme immunoassay, DNA probe). Although amplified tests are more expensive, they are also more sensitive; therefore, the cost-effective prevalence cutoffs associated with these tests that were used in this study are not definite.
Evidence from San Francisco emphasizes the importance of examining site-specific data stratified by subpopulation (e.g., symptom status, age, sexual orientation, other risk factors) on a regular basis, particularly when populations change and with the introduction of new diagnostic-testing technologies. The subsequent adjustment of screening criteria will potentially allow a reallocation of funds to enable increased screening among higher-risk populations in the broader community. For example, only half of persons currently entering jail-custody facilities in San Francisco receive STD screening. Pilot studies have revealed high rates of chlamydia and gonorrhea among detainees (6.2% and 2.0%, respectively). Hence, jail-custody facilities are potential sites to which funds could be reallocated to increase testing, ensure treatment, and decrease the spread of disease throughout the greater community.