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sociated with CP (5, 17–19), and some of these are
more frequent in widespread pain than in local pain
(14). Moreover, some studies provide prediction mo-
dels after adjustment for baseline pain (4, 5, 7), while
others do not (6). Thus, it is not known whether the
reported pattern of pain predictors would be the same
if such adjustments were performed.
The impact of socioeconomic factors and comorbi-
dities on characteristics of pain (intensity, spreading
and sensitivity) might differ from the longitudinal
perspective. More knowledge of predictors of inten-
sity, spread and sensitivity of CP is needed, and it is
reasonable to assume that several factors could serve
as predictors for the development and persistence of
different pain characteristics. There is agreement that
both the pain experience and the CP condition must
be bio-psychosocially assessed and managed in the
clinical situation (20, 21).
The rationale for this study is that longitudinal associ-
ations between pain characteristics and sociodemograp-
hic and physical and psychological comorbidities have
been incompletely examined in multivariate models.
The aim of this study was to elucidate the multiva-
riate longitudinal associations, using 2-year follow-up
epidemiological data (collected in 2013 and 2015)
from a general population in south-eastern Sweden
(the SWEPAIN cohort) to examine whether characte-
ristics of pain are predicted by demographic features,
socioeconomic conditions and certain comorbidities.
It was hypothesized that:
• sociodemographic features and certain comorbidities
would predict pain intensity, spread and sensitivity
at a 2-year follow-up survey of a general population;
• baseline adjustments of pain intensity, spread of pain
on the body, and sensitivity would markedly affect
the pattern of important predictors.
METHODS
Design, subjects and procedures
The present study used data from the SWEPAIN cohort (14,
22), which has been approved by the local ethics committee
of Linköping University, Sweden (Dnr: 2011 72/31). Baseline
data (T0) were collected using a stratified random sample of
34,000 individuals from a sampling frame based on the Swe-
dish Total Population Register. The sample frame consisted of
404,661 individuals who were 16–85 years old and living in
south-eastern Sweden.
The random sampling was stratified by sex and municipality
to reach individuals living in urban and rural areas (14). Data
were collected by Statistics Sweden. The selected individuals
received a postal questionnaire in March 2013, which could be
returned either by post or electronically. A reminder was sent
to non-responders after 2 weeks and, if necessary, another re-
minder was sent 2 weeks later. The collection of questionnaires
ended in May 2013. Follow-up data (T1) were collected 2 years
www.medicaljournals.se/jrm
later. Only individuals who completed and returned the first
questionnaire were eligible to participate in the follow-up as-
sessment. Eligible individuals received a postal survey in March
2015, which could be returned by post or electronically. Two
reminders were sent. Collection of follow-up data ended in May
2015. The surveys at T0 and T1 included the same questions.
The Strengthening the Reporting of Observational Studies in
Epidemiology (STROBE) (23) statement was followed.
Predictor variables
Sociodemographic features. The survey questions about basic de-
mographic data (age and sex), country of birth (Sweden vs abroad
(i.e. being an immigrant)), citizenship (Swedish vs other), marital
status (single, married, divorced, or widowed), educational level
(elementary school, secondary upper school/vocational training,
or university education), employment status (employment vs
unemployment), and household annual income in 2010.
Physical and psychological comorbidities. Assessment of co-
morbidities was based on a self-reported questionnaire published
elsewhere (10, 24, 25). A copy of the questionnaire is available
from the corresponding author on request. Briefly, it covers 12
disorders and diseases: traumatic injuries; rheumatoid arthritis
and osteoarthritis (RA/OA); cardiovascular disorders (CVD, in-
cluding high blood pressure, angina pectoris, and heart attacks);
pulmonary disorders; gastrointestinal (GI) disorders; disorders
of the central nervous system (CNS) (including ophthalmolo-
gical and ear–nose–throat disorders); urogenital disorders; skin
disorders; tumours and cancer; metabolic diseases (including
diabetes, obesity, anorexia, bulimia, and goitre); depression; and
anxiety. These comorbidities were reported on a 5-point scale: 1:
no; 2: yes, according to both my own and my doctor’s opinions;
3: yes, according to my own opinion; 4: yes, according to my
doctor’s opinion; and 5: I do not know. The answers for 2, 3,
and 4 were combined into category “yes” in order to obtain a
robust measurement of the presence of the specific comorbidity
vs the answer “no” (10, 24, 25). The answer option “I do not
know” was also recorded as “no”. Self-reported assessments of
comorbidities are widely used in the literature and have been
reported to be reliable (26).
Selection of predictor variables
The selection of these predictor variables (e.g. socio-demo-
graphic factors and comorbidities) was based on recognized
associations with pain intensity, spread and sensitivity of pain
(2–8, 27–31) and on disease states common worldwide.
Outcome variables
Definition of chronic pain. All respondents were asked to report
if they had CP, defined by a single question “Do you frequently
(usually) have pain lasting more than 3 months?” (yes/no).
Subjects who responded “no” were assigned to the no pain (NP)
cohort, while those who responded “yes” were assigned to the
chronic pain (CP) cohort.
Pain intensity. Only those respondents who were assigned to
the CP cohort were additionally asked to complete their mean
pain intensity during the previous 7 days on a numeric rating
scale (NRS7d) (0 = not at all to 10 = worst imaginable pain) (32).
Pain spreading categories based on the number and location
of pain sites. The participants with pain marked the site of their
pain during the previous 7 days on a body chart divided into 45
sections (22 on the front and 23 on the back) (14). One marked