Nephrol Dial Transplant (2003) 18: 82–88
Socio-economic status and chronic renal failure: a population-based case-control study in Sweden
C. Michael Fored1,2, Elisabeth Ejerblad1, Jon P. Fryzek3,4, Mats Lambe1, Per Lindblad1, Olof Nyren1 and Carl-Gustaf Elinder1,2 ´
Department of Medical Epidemiology, Karolinska Institutet, Stockholm, 2Department of Renal Medicine, Huddinge University Hospital, Huddinge, Sweden, 3The International Epidemiology Institute, Rockville, MD and 4Department of Medicine, Vanderbilt University Medical Center, Nashville, TN, USA
Abstract Background. Low socio-economic status is associated with the occurrence of several different chronic diseases, but evidence regarding renal disease is scant. To explore whether the risk of chronic renal failure varies by socio-economic status, we performed a population-based case-control study in Sweden. Methods. All native residents from May 1996 to May 1998, aged 18–74 years, formed the source population. Cases (ns926) were incident patients with chronic renal failure in a pre-uraemic stage. Control subjects (ns998) were randomly selected within the source population. Exposures were assessed at personal interviews and relative risks were estimated by odds ratios (OR) in logistic regression models, with adjustment for age, sex, body mass index (BMI), smoking, alcohol consumption and regular analgesics use. Results. In families with unskilled workers only, the risk of chronic renal failure was increased by 110% [ORs2.1; 95% conﬁdence interval (CI), 1.1–4.0] and 60% (ORs1.6; 95% CI, 1.0–2.6) among women and men, respectively, relative to subjects living in families in which at least one member was a professional. Subjects with 9 years or less of schooling had a 30% (ORs1.3; 95% CI, 1.0–1.7) higher risk compared with those with a university education. The excess risk was of similar magnitude regardless of underlying renal disease. Conclusions. Low socio-economic status is associated with an increased risk of chronic renal failure. The moderate excess was not explained by age, sex, BMI, smoking, alcohol or analgesic intake. Thus, socioeconomic status appears to be an independent risk indicator for chronic renal failure in Sweden.
Correspondence and offprint requests to: Dr C. Michael Fored, Department of Medical Epidemiology, Karolinska Institutet, PO Box 281, SE-171 77 Stockholm, Sweden. Email: email@example.com
Keywords: case-control study; education; kidney failure, chronic; occupations; risk factors; socioeconomic factors
Chronic renal failure (CRF) is a severe condition that reduces life expectancy and typically progresses to end-stage renal disease (ESRD) and a need for renal replacement therapy. The prevalence of ESRD requiring treatment varies internationally from 600 to 1200 per million and is increasing steadily in most countries, with an estimated annual increase of 8%. In a large proportion of cases, CRF evolves from known renal or systemic diseases, but in some cases the pathogenesis remains unknown. Certain factors seem to promote CRF development irrespective of the underlying pathology: hypertension, proteinuria, hyperlipidaemia, high protein intake, smoking, heavy use of non-narcotic analgesics and certain occupational exposures [1–4]. A gradient by socio-economic status (SES) has been observed in a wide range of diseases . SES represents an important risk indicator for cardiovascular disease, but evidence linking low SES to renal disease is scant. Earlier studies have shown an inverse association between income and treated ESRD [6–9]. The income-earning capacity, however, is often reduced among ESRD patients and low income may be a consequence, rather than a cause, of ESRD. To shed light on the possible associations between SES and the risk of CRF, we analysed this exposure in relation to incident pre-uraemic disease in a nationwide, population-based case-control study in Sweden. An occupational-based socio-economic classiﬁcation scale and educational level were used as measures of SES.
2003 European Renal Association–European Dialysis and Transplant Association
Socio-economic status and chronic renal failure
Subjects and methods
Setting In Sweden, the county councils provide health care at hospitals and primary health care centres to all residents. Out-of-pocket charges are kept low enough to ensure equal health care access. The study base was well deﬁned through the continuously updated National Population Register and comprised 5.3 million native Swedes, aged 18–74 years, resident in the country during the ascertainment period 20 May 1996 to 31 May 1998. Subjects Monthly lists of all serum creatinine measurements were provided by medical laboratories covering essentially all inpatient and outpatient care in Sweden. Eligible as cases were patients in the source population whose serum creatinine level for the ﬁrst time, and permanently, exceeded 300 mmolul (3.4 mgudl) for men or 250 mmolul (2.8 mgudl) for women, the increase being due to renal causes. Case patient eligibility was determined in collaboration with local physicians. Patients with pre-renal causes (e.g. severe heart failure) or post-renal causes (e.g. outlet obstruction) of the serum creatinine elevation were not eligible. Diagnoses of underlying conditions were based on routine clinical work-up. Control subjects, frequency-matched to the case patients by age and sex, were randomly selected from the National Population Register on three occasions during the ascertainment period. All regional ethics committees and the Swedish Data Inspection Board approved the study protocol. Each study subject provided informed consent before inclusion. Data collection The study subjects received a mailed self-administered questionnaire about number of years of education, highest educational degree, marital status, anthropometrical measures, weekly alcohol intake and lifetime tobacco use. Information on every occupation held for more than a year was obtained during subsequent face-to-face interviews by professional interviewers from Statistics Sweden. In addition, occupations of spouses and parents were recorded to assess ‘household SES’. A detailed history of lifetime use of non-narcotic analgesics was also obtained during the interview, as described elsewhere . The interviewers could not be kept blinded to the case control status of the interviewees, but they were unaware of the study hypotheses, and they were trained to treat both categories in a strictly equal manner. The mailed questionnaire was checked during the interview and supplemented when needed. Statistical analysis Occupation and educational level were used independently to estimate SES. The SES associated with reported occupations was derived from the ofﬁcial Swedish socio-economic classiﬁcation scheme (SEI), and the scores were aggregated
into the following classes: (i) unskilled and semi-skilled manual workers; (ii) skilled manual workers; (iii) assistant non-manual employees; (iv) intermediate non-manual employees; (v) employed or self-employed professionals, higher civil servants and executives. A sixth group of self-employed (other than professionals) and farmers was analysed separately. Each subject was grouped according to the highest SEI score obtained from the occupational history. The spouse with the highest score determined the household SES. Students were classiﬁed according to the SES of their parents. Educational level was grouped into three categories based on the number of years of education (0–9, 10–12 and 13 years or more). Multivariate unconditional logistic regression models estimated relative risks [odds ratios (OR)] as measures of the association between SES and CRF, along with their 95% conﬁdence intervals (CI). Co-variates were considered if they were known or suspected a priori to be confounding factors, or if they were associated with both CRF and SES in the data. We initially considered height, body-mass index (BMI; the weight in kilograms divided by the square of the height in metres), number of siblings, cigarette smoking, weekly alcohol consumption and cumulative lifetime dose of analgesics during regular use. Smoking and alcohol consumption were grouped in quartiles according to the distribution among control subjects. The ﬁnal analysis model, assessed using the likelihood ratio test, contained terms for age, sex, BMI, cigarette smoking, alcohol consumption and ever vs never regular use of aspirin or paracetamol use. A simple indicator variable (ever vs never) of regular use was found to sufﬁciently control for possible confounding by aspirin or acetaminophen in relation to SES. Regular use of an analgesic was deﬁned as use at least twice a week for 2 months or longer. In our modelling, we excluded 37 case patients (4.0%) and 48 control subjects (4.8%) with missing information for one or more co-variates. Analysis of variance was used to investigate the relation between SES and glomerular ﬁltration rate.
During the study period, we identiﬁed 1189 eligible patients and 35 whose eligibility could not be established. Nine hundred and twenty-six patients (78%) participated, while 111 refused, 83 had severe diseases that precluded participation and 69 (6%) died shortly after diagnosis. Of the 1330 eligible control subjects sampled, 998 (75%) participated, 221 refused, 56 could not be reached and 55 had severe diseases that precluded participation. Characteristics of the participating case patients and control subjects are shown in Table 1. There were about twice as many men as women. As expected from the frequency-matched design, the mean age was identical among case patients and control subjects. Further, the mean age was similar among men (58 years) and women (57 years). Patients and control subjects did not differ materially
C. M. Fored et al.
with regard to number of reported occupations (medians3 in both groups). Thirty-one percent of the case patients were classiﬁed as having diabetic nephropathy, 24% had a glomerulonephritis diagnosis, 15% renal vascular disease, 11% a hereditary renal disease, 9% a systemic disease or vasculitis and 11% any other renal disease diagnosis. A majority of the patients were in the pre-uraemic stage, in no need of renal replacement therapy. The median value of the estimated glomerular ﬁltration rate (GFR) was 21 mlumin (interquartile range, 17–26 mlumin). Among patients in the unskilled manual workers group, 27% had a GFR in the lowest quartile, 24% in the second quartile, 27% in the third and 21% had a GFR in the highest quartile. The corresponding distribution of the GFR values among patients in the professionals group were 20% in the lowest quartile, 25% in the second, 23% in the third and 33% in the highest quartile. No statistically signiﬁcant differences in GFR between patients in the different SES groups were found (Ps0.07). Table 2 shows the distributions by SES of alcohol consumption, cigarette smoking and a diagnosis of hypertension or diabetes mellitus among the control subjects. The largest proportion of reported alcohol abstinence was found among the unskilled manual worker (30.1%) and self-employed subjects (31.8%), while weekly alcohol consumption within the highest
Table 1. Characteristics of the study subjects Variable Cases ns926 n 138 171 183 232 136 58 8 n 168 209 537 n 597 329 n 11 63 95 193 186 378 n 234 217 116 143 206 10 % 14.9 18.5 19.8 25.1 14.7 6.3 0.9 % 18.1 22.6 58.0 % 64.5 35.5 % 1.2 6.8 10.3 20.8 20.1 40.8 % 25.3 23.4 12.5 15.4 22.2 1.1 Controls ns998 n 210 215 175 206 103 88 1 n 220 246 525 n 653 345 n 20 58 97 186 204 433 n 202 258 135 190 209 4 % 21.0 21.5 17.5 20.6 10.3 8.8 0.1 % 22.0 24.7 52.6 % 65.4 34.6 % 2.0 5.8 9.7 18.6 20.4 43.4 % 20.2 25.8 13.5 19.0 20.9 0.4
Household SESa professionals intermediate non-manual assistant non-manual skilled manual unskilled manual self-employed missing data Years of education P13 10–12 O9 Sexb male female Age at interview (years)b 18–24 25–34 35–44 45–54 55–64 65–74 Weekly alcohol intake (g) none O17.6 17.7–33 33.1–66 )66 missing
quartile ()66 g alcohol (;75 cl wine) per week) was most common among the skilled manual subjects (23.3%) and the professionals (25.4%). The proportion of never smokers was largest among the professionals (50.9%), and in the heterogeneous group of selfemployed subjects and farmers (56.8%). Unskilled manual workers and skilled manual workers had the largest proportion of subjects (;18%) classiﬁed in the heaviest smoking category ()27.5 pack-years). The variation was small in the proportions of reported hypertension or diabetes diagnoses between the SES groups. Regular use of analgesics such as paracetamol or aspirin was most common among control subjects in the unskilled manual worker group (30.1%) and least common among the professionals (20.0%) (data not shown). More than 50% of the subjects categorized as skilled manual workers, assistant non-manual employees or self-employed had a BMI )25 (data not shown). The risk of CRF was inversely related to SES inferred from occupations of the individual study participants (Table 3). The relationship with household SES was even clearer. The gradients among women were at least as marked as those among men. Compared with women in families with the highest SES, female members of families with unskilled workers only had a 110% (OR 2.1; 95% CI 1.1–4.0) excess risk for CRF following adjustments for potential confounding factors. The corresponding excess among men was 60% (OR 1.6; 95% CI 1.0–2.6). Subjects with 9 years or less of schooling had a 30% (OR 1.3; 95% CI 1.0–1.7) higher risk compared with those who went to university, but this excess was mainly conﬁned to men. Compared with crude estimates of the associations between SES and CRF, the combined adjustments generally tended to move the point estimates moderately towards unity, in most instances less than 20% (data not shown). Table 4 shows the relationship by underlying pathology. Household SES was associated with a )2-fold risk gradient for all major types of CRF, although the dose–response curve varied somewhat in appearance between diagnostic categories. The relationship appeared less convincing for the miscellaneous group of underlying pathology. The trend with individual level of education was weaker for CRF classiﬁed as glomerulonephritis than for the other major types and it was absent for the miscellaneous group.
In this large case-control study, household SES emerged as a signiﬁcant risk indicator for CRF, independent of factors such as age, sex, BMI, cigarette smoking, alcohol intake and use of aspirin or paracetamol. The risk gradient from highest to lowest socio-economic stratum was similar for diseases as different as diabetic nephropathy, glomerulonephritis and renal vascular disease, but the detailed trend pattern varied with underlying pathology.
The highest occupational SES group within the subjects’ family. Study subjects were frequency matched by age and sex.
Socio-economic status and chronic renal failure
Table 2. Distribution of weekly alcohol intake, lifetime smoking and diagnosis of hypertension or diabetes mellitus among 997 control subjects by socio-economic groupa Variable Unskilled manual Skilled manual Assistant non-manual Intermediate non-manual Professionals Self-employed (ns103) (ns206) (ns175) (ns215) (ns210) (ns88) n 31 32 8 16 15 1 n 42 11 14 13 19 4 n 83 19 1 n 97 6 (%) (30.1) (31.1) (7.8) (15.5) (14.6) (1.0) (%) (40.8) (10.7) (13.6) (12.6) (18.4) (3.9) (%) (80.6) (18.4) (1.0) (%) (94.2) (5.8) n 49 46 39 23 48 1 n 75 30 26 37 36 2 n 167 36 3 n 188 18 (%) (23.8) (22.3) (18.9) (11.2) (23.3) (0.5) (%) (36.4) (14.6) (12.6) (18.0) (17.5) (1.0) (%) (81.1) (17.5) (1.5) (%) (91.3) (8.7) n 33 42 24 40 36 0 n 69 27 30 21 27 1 n 141 34 0 n 161 14 (%) (18.9) (24.0) (13.7) (22.9) (20.6) (%) (39.4) (15.4) (17.1) (12.0) (15.4) (0.6) (%) (80.6) (19.4) (%) (92.0) (8.0) n 42 62 29 37 44 1 n 97 34 28 28 27 1 n 184 31 0 n 200 15 (%) (19.5) (28.8) (13.5) (17.2) (20.5) (0.5) (%) (45.1) (15.8) (13.0) (13.0) (12.6) (0.5) (%) (85.6) (14.4) (%) (93.0) (7.0) n 19 55 31 55 50 0 n 107 25 27 35 15 1 n 180 30 0 n 199 11 (%) (9.0) (26.2) (14.8) (26.2) (23.8) (%) (50.9) (11.9) (12.9) (16.7) (7.1) (0.5) (%) (85.7) (14.3) (%) (94.8) (5.2) n 28 21 4 19 16 0 n 50 10 15 7 6 0 n 73 14 1 n 84 4 (%) (31.8) (23.9) (4.5) (21.6) (18.2) (%) (56.8) (11.4) (17.0) (7.9) (6.8) (%) (83.0) (15.9) (1.1) (%) (95.4) (4.6)
Weekly alcohol intakeb none O17.6 g 17.7–33 g 33.1–66 g )66 g missing Smokingc none O6.6 6.7–16 16.1–27.5 )27.5 missing Hypertension no yes missing Diabetes mellitus no yes
a c b
The highest occupational SES group within the subjects’ family. SES group missing for one control subject. Alcohol intake in grams per week. Sixty-six grams of alcohol are approximately the amount of alcohol in 75 cl of wine. Lifetime cumulative cigarette consumption. Pack years in quartiles.
The strengths of our study include the well-deﬁned study base, which permitted strict random sampling of control subjects, the unique and comprehensive organization for case ﬁnding that ensured identiﬁcation of all new case patients occurring in the study base, the objective diagnostic criteria that deﬁned
Table 3. The risk of CRF in relation to SES Variable Men Cases (ns597) Household SESa professionals intermediate non-manual assistant non-manual skilled manual unskilled manual self-employed Individual SESd professionals intermediate non-manual assistant non-manual skilled manual unskilled manual self-employed Years of education P13 years 10–12 years O9 years
CRF before its end-stage, minimizing selective loss of patients with rapid disease progression and early death, and the relatively large sample size. Weaknesses include an element of judgmental categorization inherent in occupation-based SES classiﬁcation, possible biases due to differential detection or
Women Controls (ns653) n 147 135 103 153 59 55 n 128 117 72 194 75 48 n 142 150 355 ORb 1.0 1.2 1.3 1.5 1.6 1.1 ORb 1.0 1.1 1.5 1.6 1.4 1.0 ORb 1.0 1.2 1.4 95% CIc Ref (0.8–1.7) (0.9–2.0) (1.0–2.1) (1.0–2.6) (0.6–1.8) 95% CIc Ref (0.8–1.7) (0.9–2.3) (1.1–2.3) (0.9–2.3) (0.6–1.8) 95% CIc Ref (0.8–1.7) (1.0–1.9) Cases (ns329) n 38 51 86 62 71 19 n 22 40 90 43 111 6 n 59 80 187 Controls (ns345) n 63 80 72 53 44 33 n 34 73 81 51 81 11 n 78 96 170 ORb 1.0 0.9 1.5 1.5 2.1 0.7 ORb 1.0 0.9 1.6 1.1 1.9 0.7 ORb 1.0 0.9 1.1 95% CIc Ref (0.5–1.6) (0.9–2.7) (0.8–2.7) (1.1–4.0) (0.3–1.5) 95% CIc Ref (0.4–1.8) (0.8–3.1) (0.5–2.3) (0.9–3.8) (0.2–2.3) 95% CIc Ref (0.6–1.5) (0.7–1.9)
n 100 120 97 170 65 39 n 86 95 72 227 73 30 n 109 129 350
ORb 1.0 1.1 1.4 1.5 1.9 0.9 ORb 1.0 1.0 1.5 1.5 1.7 1.0 ORb 1.0 1.1 1.3
95% CIc Ref (0.8–1.4) (1.0–1.9) (1.1–2.0) (1.3–2.7) (0.6–1.4) 95% CIc Ref (0.7–1.4) (1.1–2.2) (1.1–2.0) (1.2–2.5) (0.6–1.6) 95% CIc Ref (0.8–1.4) (1.0–1.7)
The highest occupational socio-economic group within the subjects’ family. OR adjusted for age, sex, BMI, cigarette smoking, alcohol intake, and regular aspirin anduor paracetamol use. c Ninety-ﬁve percent CI. d The highest individual occupational socio-economic group.
86 Table 4. The risk of type-speciﬁc CRF in relation to SES Variable Diabetic nephropathy (ns286) n 36 42 56 84 49 15 n 31 36 50 83 67 10 n 38 61 181 ORb 1.0 0.8 1.5 1.8 2.4 0.9 ORb 1.0 0.8 1.6 1.4 2.1 0.8 ORb 1.0 1.5 2.0 95% CIc ref (0.5–1.4) (0.9–2.5) (1.1–2.8) (1.4–4.2) (0.5–1.9) 95% CIc Ref (0.5–1.4) (0.9–2.7) (0.8–2.2) (1.2–3.5) (0.4–1.9) 95% CIc ref (0.9–2.4) (1.2–3.2) Glomerulonephritis (ns222) n 39 39 40 53 35 14 n 31 34 31 65 45 8 n 43 65 112 ORb 1.0 1.0 1.4 1.3 2.2 1.0 ORb 1.0 1.1 1.5 1.4 2.1 0.9 ORb 1.0 1.4 1.4 95% CIc ref (0.6–1.6) (0.8–2.3) (0.8–2.1) (1.3–3.8) (0.5–2.1) 95% CIc Ref (0.6–1.8) (0.8–2.6) (0.9–2.4) (1.2–3.7) (0.4–2.1) 95% CIc ref (0.9–2.2) (0.9–2.3) Renal vascular disease (ns139) n 13 31 30 35 18 12 n 10 20 26 52 22 8 n 12 24 101 ORb 1.0 2.2 2.5 2.4 2.5 2.0 ORb 1.0 1.9 2.6 3.1 2.2 2.3 ORb 1.0 1.6 2.4 95% CIc ref (1.1–4.5) (1.2–5.2) (1.1–4.8) (1.1–5.7) (0.8–4.9) 95% CIc Ref (0.8–4.4) (1.1–6.0) (1.4–6.6) (0.9–5.1) (0.8–6.6) 95% CIc ref (0.7–3.4) (1.2–4.7)
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Othera (ns279) n 50 59 57 60 34 17 n 36 45 55 70 50 10 n 75 59 143 ORb 1.0 1.1 1.2 1.2 1.4 0.8 ORb 1.0 1.0 1.5 1.3 1.4 0.9 ORb 1.0 0.7 0.8 95% CIc Ref (0.7–1.7) (0.8–2.0) (0.8–1.9) (0.8–2.4) (0.4–1.6) 95% CIc Ref (0.6–1.7) (0.9–2.4) (0.8–2.1) (0.8–2.3) (0.4–2.1) 95% CIc Ref (0.5–1.1) (0.5–1.2)
Household SESd professionals intermediate non-manual assistant non-manual skilled manual unskilled manual self-employed Individual SESe professionals intermediate non-manual assistant non-manual skilled manual unskilled manual self-employed Years of education P13 years 10–12 years O9 years
Each case-group was compared to all controls. a Diagnosis groups: hereditary renal disease, systemic diseaseuvasculitis and other renal disease. b OR adjusted for age, sex, BMI, cigarette smoking, alcohol intake, and regular aspirin anduor paracetamol use. c Ninety-ﬁve percent CI. d The highest occupational socio-economic group within the subjects’ family. e The highest individual occupational socio-economic group.
case ascertainment in different SES strata, possible differential willingness to participate among cases and controls across SES categories, and possible differential recall among case patients and control subjects. Detection bias, if any, would probably result in earlier diagnosis among socio-economically privileged people, thus counteracting the observed inverse relationship. We found no signiﬁcant relation between SES and estimated GFR at time of inclusion. Ascertainment bias is unlikely, given the equal access for everyone to Swedish health care, and the essentially complete coverage by our case-ﬁnding organization. It should be noted that serum creatinine testing was carried out for overt symptoms in routine clinical practice and not as a screening effort among symptomless people. This should allay concerns about possible selective recruitment from a pool of subjects with prevalent disease. Since non-participation, particularly that attributed to active refusal, may be unevenly distributed across SES strata, selection bias may have been introduced, since the refusal rate was higher among our control subjects than among our case patients. Although this may have inﬂated the observed association between SES and CRF, the difference in participation rates between case patients and control subjects is not large enough to entirely explain our ﬁnding. Likewise, we regard the possibility of misclassiﬁcation of SES level following incomplete recollection of previous occupations unlikely. Data from the USA indicate that the incidence of ESRD is 3–4-fold greater in African Americans and
Indians than in whites . However, this association may be the result of joint effects of both race and SES. Our study was conﬁned to ethnically homogeneous native Swedes, precluding confounding by race. Prior studies have been limited to the association between SES and renal replacement therapy among ESRD patients [6–9]. Rate of progression, therapeutic decisions and changes in SES due to reverse causation may have affected the results of these studies. To our knowledge, our study is the ﬁrst to examine the inﬂuence of SES on CRF before end-stage. Moreover, as opposed to some past studies, which used aggregated SES indices such as areabased measures of education and income or current income, which may have changed because of the renal or predisposing disease, we classiﬁed our subjects individually, according to the highest SES attained during a lifetime. Evaluations of the SEI scheme have shown that it performs well in comparison with other classiﬁcations . The reason why attained education was associated with a less marked risk gradient than the occupationalbased SES is not entirely clear. In an American case-control study, risk varied 7-fold with attained education . One explanation for this apparent discrepancy is that the lowest educational category in our study encompasses a considerable variation, which is partly concealed by the fact that 9 years of schooling became compulsory in the 1960s. Before then, only 6 years were required. The gradient might have been greater if the cut-off point for the reference
Socio-economic status and chronic renal failure
category had been set lower. But SES-related health gradients for several other diseases have also been generally less pronounced in countries such as Sweden, with its more egalitarian socio-economic policies . Notwithstanding, it was recently suggested that morbidity, assessed through national health interview surveys, according to occupational class is very similar among men from Sweden, Denmark, the UK, The Netherlands, Germany, Switzerland and France, implying that our results may be applicable to other European countries . SES per se does not plausibly affect renal function and cannot be regarded as a speciﬁc exposure but a marker for general material and cultural circumstances. Associated biologically meaningful exposures are likely to explain most or all of the relationship with CRF. Individuals with low SES may not receive the same medical attention to predisposing diseases as persons with higher SES. However, the gradient was similar for glomerulonephritis, which is not preceded by any known predisposing disease, as it was for diabetic nephropathy. Previous studies in the USA indicate that mechanisms other than limited access to health care are involved in the association of income with ESRD . Furthermore, in Sweden there is a strong commitment to equity of health care access and use. Control for several factors with known or suspected impact on renal function did not cancel the inverse relationship between SES and CRF. Other factors that may explain the association include other chronic diseases, diet and occupational exposures. Although the prevalence of hypertension and type 2 diabetes mellitus was higher among low SES individuals in some studies [13,14], these ailments were essentially evenly distributed across SES strata in our study, thus practically ruling out any important confounding role. A diet low in fruit and vegetables has been implicated in the aetiology of diseases of the cardiovascular system and may conceivably also affect the kidneys . Occupational exposure to nephrotoxins such as heavy metals and solvents occurs more often in occupations associated with lower SES . In a recent review, the meta-analysis shows an association between exposure to organic solvents and glomerulonephritis . The heterogeneous design among the 14 case-control studies included in the analysis and the methodological limitations of several of the included studies, however, preclude ﬁrm conclusions. Moreover, most evidence weighed against no important role of solvent exposure in the strongest study published so far . Accordingly, our ﬁndings cannot simply be explained by a possible socio-economic gradient in solvent exposure. Nonoccupational exposure to cadmium, a well-known risk factor for renal disease, has increased in modern times , thus possibly cancelling some of the excess noted previously among job-exposed workers. Further, the fact that household SES tended to be more strongly linked to CRF risk than was individual SES, might indicate that the association can be
attributed more to issues related to lifestyle and cultural factors than to occupational exposures. Socioeconomic disparities in fetal environmental factors and pregnancy outcome are other conceivable factors that warrant further studies . Moreover, accumulating evidence suggests that social status itself, regardless of associated material and economic advantages, may confer health beneﬁts possibly via psychosocial mechanisms . In conclusion, notwithstanding generally negligible inequalities with regard to access to health care in Sweden, and a tentatively limited range of exposure to SES-related factors with a potential impact on renal function, SES appears to represent an independent risk indicator for CRF in Sweden. Thus, the underlying mechanisms warrant further studies to identify possibly preventable risk factors of renal disease. SES should be considered in all aetiological research on renal failure.
Acknowledgements. We are indebted to Dr Gun Nise (Division of Occupational Medicine, Department of Public Health Sciences, Karolinska Institutet) and to Dr Scott M. Montgomery (Division of Medicine, Department of Clinical Epidemiology, Karolinska Hospital) for their valuable advice and guidance in this work. This study was ﬁnanced by the International Epidemiology Institute, Rockville, MD, USA.
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