The Centers for Disease Control and Prevention (CDC) and the Office of Population Affairs (OPA) initiated a Chlamydia demonstration project in the northwestern region of the United States, HHS Region X. In 1993, this demonstration project was expanded to include HHS Regions III, VII and VIII, and in 1995 services were implemented in the remaining regions (I, II, IV, V, VI, IX). The projects are funded through the Regional Infertility Prevention Projects legislation, which aims to reduce the costly and destructive sequelae of Chlamydia and gonorrhea on the reproductive health of women.
Sources of Data 
All regional projects, in collaboration with state STD control and family planning programs, report their Chlamydia positivity data to the regional database (housed at JSI Research & Training Institute, Inc., in Denver, for Region VIII) which is then forwarded to CDC. For some of the programs in Region VIII, federally-funded programs supplement existing local and state-funded testing programs. These publicly-funded programs support Chlamydia screening primarily in family planning clinics, but also in some STD clinics, jails and juvenile detention centers, Indian Health Service sites, and other sites. Region VIII comprises the states of Colorado, Montana, North Dakota, South Dakota, Utah and Wyoming.
The interpretation of Chlamydia data is compounded by several factors.
- First, case reports and prevalence data result from the use of several different types of diagnostic tests for Chlamydia infection (e.g., DNA probe assay, DNA amplification); these tests vary in their sensitivity and specificity.
- Second, Chlamydia positivity among women attending clinics is an estimate of prevalence; it is not true prevalence. Crude positivity may include those women who are tested two or more times during a single year. Comparisons of positivity with prevalence have shown that in family planning clinics, positivity is generally similar to or slightly higher than prevalence, and in STD clinics, positivity is somewhat lower than prevalence; however, these differences are usually small, with the relative difference <10%. 
- Third, while nearly all family planning clinics perform universal screening of sexually active women <20 years of age, and most clinics do so among women <25 years of age, some selective screening is performed among women 20-24 years of age and some level of screening is frequently performed among women >25 years of age.
- Fourth, while monitoring prevalence among persons seeking care at clinics provides important information on certain segments of the population, these data cannot be generalized to the population as a whole. 
- Fifth, the data that are reported into the regional database do not reflect all the screening and testing being performed across Region VIII.
- Last, the enhanced data elements (clinical signs and risk history) and gonorrhea data only reflect a certain portion of records reported to the regional database.
 Centers for Disease Control and Prevention. Sexually Transmitted Disease Surveillance 2001 Supplement, Chlamydia Prevalence Monitoring Project. Atlanta, GA: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, October 2002.
 Dicker LW, Mosure, DJ, Levine WC. Chlamydia positivity versus prevalence: What's the difference? Sexually Transmitted Diseases 1998; 25:251-3.