Population-based cross-sectional descriptive study.

General adult population of 20 years or more living in Spain.

The participants in the study were men and women of 20 years or more, whose usual residence was one of the municipalities participating in the study, with capacity to comprehend and communicate with the interviewers, and who gave their consent to participate.

We designed a random sample of individuals of different ages that was stratified and was studied by groups. The sampling strategy was as follows:

• 1.

  Stratification in terms of the rural/urban nature of the municipalities, in accordance with the rural/urban ratio existing in Spain (75% of the population resides in urban municipalities). We randomly selected those Spanish autonomous communities in which we would study a rural municipality and in others an urban municipality. We considered urban municipalities to be those with at least a population of more than 10,000 inhabitants, in accordance with the data of the registry of 2015.20,21

• 2.

  Stratification according to autonomous communities. For this, we randomly selected a municipality in the communities with less than 4 million inhabitants and 2 municipalities in the communities of more than 4 million (Andalusia, Catalonia, Community of Madrid and the Valencian Community). The researchers in the study were rheumatologists of the referral hospital of the selected municipalities.

• 3.

  We selected 2 reserve municipalities in each autonomous community. We searched for municipalities with a referral hospital that was different from that initially selected, considering that the change could be that no rheumatologist of the referral hospital of the selected municipality agreed to participate in the study.

• 4.

  In each participating municipality, we realized a random selection of individuals stratified by age and by sex in accordance with the distribution of the Spanish population. The same numbers of individuals were recruited in each municipality.

To facilitate the recruitment of participants, we excluded municipalities with less than 1000 inhabitants. For this same reason, for rural areas, we decided to select 5 or 6 municipalities, rather than just one. To do this, first we randomly selected one municipality among all the rural towns in the corresponding autonomous community and, then, we randomly selected rural municipalities in the catchment area of the referral hospital first selected. The municipalities thus selected were considered to be only one for recruitment.

Assuming a Poisson distribution, a sample of 4000 individuals would permit a 95% confidence interval of 0.30–0.77 around a prevalence of 0.5% (expected prevalence in RA) and of 0.14–0.54 around a prevalence of 0.3% (expected prevalence in PsA). If we take into account a percentage of dropouts of 20%, it would be necessary to have of sample of 5000 individuals.

Before initiating the fieldwork, we issued information on the study in the selected municipalities, in the attempt to adapt the strategy of dissemination to each of them (by means of local announcements, informative posters in health centers and the rheumatology department of the referral hospital, etc.).

In each locality, we telephoned the participating individuals, to carry out the questionnaire to screen for the diseases to be studied (Annex 1). In the urban municipalities, the calls are focused on the populations of more than 10,000 inhabitants.

The calls are made to home telephones randomly selected in the municipalities included. To obtain the telephone numbers, we employ public sources. Initially, we interview any individual who lived at the contacted home and his or her consent to participate. As we began completing the quotas in terms of age and sex, in successive calls, we will request the participation of individuals in the strata that had not been completed, to finally reach the total sample of 5000 participants.

Both for the random selection of the telephone numbers in each locality and for the initial screening interviews, we will engage the participation of an external company dedicated to sociological studies with experience in the health area and with a call center service (Ipsos España). For this, they will employ a computer-assisted telephone interview (CATI), that enables them to randomly select the telephone numbers and make the calls through a software program. It controls the quotas (of sex and age) via an online system, which is continuously updated with the information of the interviews carried out. Each of the 4 interviewers assigned to the project works with a computer, the screen of which shows the exact text that should be read to the interviewee. This system of contacts and appointments makes it possible to program postponements, so that a registry can be recovered when the interviewee is not at home or requests that the questionnaire be carried out at some other time.

Prior to the initiation of the study, we compared the number of home telephones in each municipality with the number of homes, according to data from the latest population census. In those cases in which there were important differences, the change of municipality was assessed.

During recruitment, there is an intermediate analysis to determine the proportion of smokers in the sample. If in this analysis there is a significant difference with respect to the proportion of smokers in Spain (according to data available in the European Health Interview Survey conducted in Spain in 2014),2 this variable will be taken into account in the recruitment, so that said difference will not exist in the final sample. This same strategy will be followed with the, ratio between those born in Spain and those born in other countries, taking as a reference the statistic of the continuous registry available on the SSO website.20

If in the interview of the call center to an individual, the latter mentions having been diagnosed with any of the diseases in the study, he or she will be asked in which center and who was the attending specialist. Depending on the response, there could be 2 possibilities:

• –

  If the individual was diagnosed in the referral hospital of the municipality, the consent of the patient will be requested so that the rheumatologists who participate as researchers in said hospital can confirm the diagnosis in the clinical history.

• –

  If the individual was attended to in a center that is not the referral hospital of the municipality, the patient will be asked to give his or her permission so that the rheumatologist involved in the investigation in the referral hospital of the municipality contact with said physician to confirm the diagnosis. If this was not possible, the investigating rheumatologist would contact with the patient to ask for a copy of the clinical record in which the diagnosis is confirmed.

Individuals who have not been diagnosed, but have a positive result in screening for any of the diseases being studied on the basis of the associated symptoms, will be telephoned once again to assess the suspicion by means of a second more specific questionnaire (Annex 1). This second telephone interview will be carried out by the investigating rheumatologist from the referral hospital of the municipality.

Those individuals in which there remains the suspicion that they have a rheumatic disease after the second call, will be asked to visit the investigating rheumatologist of the referral hospital to complete the process of diagnostic confirmation (physical examination and ancillary tests), in accordance with the diagnostic classification criteria of the suspected disease (Table 1). In the case of osteoarthritis of the cervical and lumbar spine, due to the lack of validated criteria to be utilized in the study, they were established by the project's scientific committee, taking into account clinical and radiological aspects, and where they were performed. In order to enable the comparison with EPISER 2000, for the conditions included in said project, the criteria utilized in that study will again be employed.

The fieldwork will be coordinated in such a way that the time elapsed between the interview of the call center and the call from the rheumatologist was 45 days or less.

Being lost to follow-up will be considered in those cases in which the individual completes the interview of the call center and the rheumatologist cannot confirm or rule out the suspected diseases.

For the definition of the questionnaire, that employed in EPISER 2000 will be utilized, together with a nonsystematic review of the literature.22–29 To define its extension and the wording of the questions, we took into account the fact that it would be applied by telephone on the part of personnel that had no relation to health care. The questions were reached by consensus by the principal investigator (PI) and the scientific committee.

Prior to initiating the fieldwork, we held a session for training in the administration of the questionnaire for the interviewers, imparted by methodologists of the Spanish Society of Rheumatology (SER) and the project's PI.

The administration, comprehension and acceptance of the questionnaire was evaluated by means of a pilot study of 40 telephone interviews supervised by personnel of the SER, made to men and women of different age groups and educational levels. Thus, it was possible to make corrections and clarifications prior to initiating the sampling. The mean duration was 10–15min, which was considered acceptable for a telephone questionnaire.

Likewise, we did a study of the negative predictive value of the screening questionnaire. For this, we randomly selected 200 individuals (10 from each municipality) with a negative result in the screening. They will be telephoned by the investigating rheumatologist in their area for the purpose of confirming the negative findings. If it is impossible to rule out a positive result on the basis of the telephone call, the individual will be given an appointment for examination and the pertinent ancillary tests depending on the suspected disorder.

The main objective of this study is to estimate the prevalence of rheumatic diseases. For this, we will calculate estimators and their confidence intervals. Having proposed a complex design, adjustments will be calculated in each of the sampling stages in accordance with the probability of selection. We will take into account the distribution of the Spanish population according to data from the SSO, with the objective of achieving the greatest representativeness. In the analysis, we will utilize specific svy commands for complex sampling from the statistical package Stata v13.1 (StataCorp, College Station, Texas, United States).

While the call center is carrying out interviews, supervisors from that company can access the interviews taking place at that time (through audio and the monitor) to confirm that they are being administered correctly. Likewise, there will be a session in which the study methodologists will listen to the surveys in real-time without knowing who is the interviewer, to subsequently organize a meeting to deal with possible ways to improve the administration of the questionnaire; in this case, there will be another session to confirm that these points have been incorporated.

The research unit of the SER will monitor the database obtained by the call center with the information collected in the telephone calls. Special attention will be given to the search for relevant variables not complied with or inconsistencies in the responses.

The rheumatologists participating in the study have been instructed about the study procedure. They will contact the study coordinators and methodologists in the case of doubts or problems.

All of the information gathered by the rheumatologists will be included in an on-line data collection notebook designed for this study. It will be monitored on-line by the SER research unit to confirm that diagnostic criteria of the corresponding disease have been collected and that there are no inconsistencies or relevant missing values, especially in those cases in which the investigator rules out the suspicion and decides not to meet the individual, despite a positive result in accordance with the criteria defined by the scientific committee.

This study will be developed in accordance with the protocol and the standards of good clinical practice, as described in the harmonized tripartite guideline of the International Conference on Harmonisation (ICH).30

The investigator will insure that the study is performed in total conformity with the principles established in the Declaration of Helsinki referring to medical research in humans.

Verbal informed consent will be requested of all the individuals in the first telephone contact. In accordance with the constitutional law on the protection of personal data, the individuals will be informed about the reason for the telephone call and the study, what their participation would consist of and the management of their personal data; they will also be informed that, if they decide to participate, they will be free to interrupt their participation in the study at any time. The participants will need to give their approval and their approval will be taped. There will be no cuts in the recording, which will be complete, and the identity of the persons who give their consent must be made clear. In addition, they will previously be told of the recording before taping begins.

Moreover, we will request written informed consent from those who come to the research center to undergo a physical examination and ancillary tests.

The Spanish Foundation of Rheumatology (FER) will be responsible for the files that contain the general data acquired in the study. Thus, a contract for this purpose has been signed with Ipsos España. Prior to data collection, the FER sent the file to the Data Protection Agency. It will be incorporated and it has been integrated into a high level in the security document of that entity.

The study was approved by the research ethics committees of the participating centers.

The authors declare that the procedures followed were in accordance with the regulations of the relevant clinical research ethics committee and with those of the Code of Ethics of the World Medical Association (Declaration of Helsinki).

The authors declare that they have followed the protocols of their work center on the publication of patient data.

The authors have obtained the written informed consent of the patients or subjects mentioned in the article. The corresponding author is in possession of this document.