For research and education purposes, departments of family medicine of most Dutch universities have primary care networks consisting of GPs from different practices in the region. There are 11 GP networks in the Netherlands which do not overlap. The networks are independent, there is no routine dataflow to a national database. These GP networks collect computer based information about patient care using uniform data collection and registration methods. At regular intervals the information from local registration systems is fed into a central database. On the national level, two GP networks exist that collect data on morbidity, prescriptions and referrals. The aim of regional and national networks is to collect data about primary care in a standardized way, suitable for scientific evaluation. For our study we extracted information about chlamydia diagnoses and treatment from four electronic networks. We selected two nationwide networks as to be able to compare earlier findings in  with results form another national network. Since the CSI project was conducted in Amsterdam, Rotterdam and the southern part of Limburg we selected also the two GP networks in Amsterdam. In Rotterdam there was not a regional GP network available; the database of the GP network in Limburg has a focus on a limited number of diagnoses, largely chronic diseases. Therefore, only two regional networks from Amsterdam were included. Ethical approval for this study was not necessary since all patient information in the databases is anonymous, no intervention was done and all patients were informed that their GP participates in scientific research. The four networks: Amsterdam Medical Centre (AMC) network, Academic General Practice Network VU University medical centre (VUmc), Integrated Primary Care Information (IPCI) network and Landelijk Informatie Netwerk Huisartsen (LINH, Netherlands Information Network of General Practice) provided approval for the use of the GP data for this specific study.
Definition of Chlamydia trachomatis diagnoses using ICPC codes
To define chlamydia diagnoses in GP networks, we used a selection of codes within the International Classification of Primary Care (ICPC-1), a worldwide system for coding patient symptoms and diagnoses in primary care . Depending on clinical features chlamydia infections in men and women are recorded under general main-codes that include a broader range of diagnoses (for vaginitis, cervicitis, PID and epididymitis) or specific sub-codes (diagnoses with chlamydia). Since some GPs only register main-codes (ending with .00) and no sub-codes, these main-codes were included only if the treatment (prescribed within two months after the code date) fitted a chlamydia diagnosis. The following antibiotics were included: Azythromycin, Doxycyclin, Amoxicillin, Erythromycin, Ciprofloxacin, and Ofloxacin; they were coded using the Anatomical Therapeutic Chemical (ATC) classification scheme applied by the WHO. We did not use doses of antibiotics.
Electronic networks and data extraction
Amsterdam Medical Centre (AMC) network
Five primary healthcare centres in the south-east of Amsterdam participate in the continuous morbidity registration network of the Department of General Practice, AMC, University of Amsterdam. For our study, we selected all patient records in the year 2007 with a chlamydia-related ICPC code or with the string 'chlam' in the free text. We verified these probable chlamydia diagnoses, based on information of laboratory results in the free text and the prescription of chlamydia-specific antibiotics. Patients' sex, age, date of consultation and duration of registration were used as well as the total number of patients and their time registered in the participating practices in 2007 (patient years).
Academic General Practice Network VU University medical center (VUmc)
The VUmc general practitioner network covers 21 practices in the cities Haarlem (13), Amstelveen (1) and Amsterdam (7). Similar to the data extraction of the AMC database, a researcher examined all medical records of 2007 containing the string 'chlam' or a related ICPC code to determine chlamydia episodes. The same patients' characteristics as in the AMC network were available with exception of the time period that patients had been registered in practice.
Integrated Primary Care Information (IPCI) network
The IPCI database is a longitudinal observational database which contains data from electronic medical records of 81 practices throughout the Netherlands. Medical records of 2007 were not available; therefore we used all medical records of 2005 to identify chlamydia cases. Like in the networks stated above, records were searched for the string 'chlam' or specific ICPC codes. The identified records were manually validated using all information available in the database to determine the presence of a chlamydia diagnosis. The IPCI database contains demographic information of the patient (date of birth, sex, degree of urbanization of home address, duration of registration), medical notes per consultation (ICPC codes, symptoms, physical examination, assessments and diagnoses), prescriptions, referrals, hospitalizations and laboratory results.
Landelijk Informatie Netwerk Huisartsen (LINH) network
LINH is based on electronic medical records from 81 general practices, spread throughout the Netherlands. Data include longitudinal information on patient's characteristics such as age, sex, degree of urbanization of home address, duration of registration in practice, as well as medical information on consultations, prescriptions, referrals, and diagnoses. Based on ICPC codes and ATC codes probable chlamydia infections in 2007 were identified (similar to previous study) . More than in the three other networks, GPs participating in the LINH network are instructed to use ICPC codes for every patient contact. Free text from medical records is not available in this database.
Chlamydia infections were counted as 'episodes': one or more patient consultations for the same medical diagnosis. In all four networks we included a second episode for the same patient only after an interval of at least two months after the first diagnosis. We assessed diagnoses by ICPC codes, free text and ATC codes in the different networks and calculated incidence of chlamydia diagnoses per age group. The incidence of chlamydia diagnoses was defined as the number of new chlamydia episodes divided by the number of patient years in participating practices in the AMC, IPCI and LINH database. For the denominators of the VUmc network we used the numbers of patients at the end of the year. We also described the incidence of diagnosed cases by level of population density in the LINH and IPCI network.
In accordance with Statistics Netherlands the following definitions for population density were used:
Level 1: highly urban: 2,500 addresses or more per square km
Level 2: 1,500 to 2,500 addresses per square km
Level 3: 1,000 to 1,500 addresses per square km
Level 4: 500 to 1,000 addresses per square km
Level 5: rural: fewer than 500 addresses per square km