Decreased Kidney Function Among Agricultural Workers
Decreased Kidney Function Among Agricultural Workers
The study was requested by the Salvadorian health authorities. Approval was obtained from the Bioethical Review Board of the Universidad Nacional in Costa Rica because there was no such entity in El Salvador at the time. All participants signed informed consent.
We selected 5 communities in El Salvador. Sugarcane production was the dominant economic activity in 2 sea-level communities on the Pacific Coast, 1 rural and 1 semirural. Previously, cotton production was important in these communities. ESRD previously had been observed as occurring in men in these communities. Three additional communities, for which there was no previous information about kidney problems, were selected to represent a range of economic activities and altitudes. The economy of one such community, found at an elevation of 500 m above sea level (masl), also was based on sugarcane production, with average temperatures during harvest seasons 1987–2000 that were ~4°C lower than at sea level. A community with heavy coffee production was chosen as an example of another agricultural activity at high altitude (1,650 masl). The final community was an urban neighborhood at 650 masl with a service-oriented economy, assumed to reflect the background distribution of SCr levels in El Salvador.
A house-to-house census of all adults was completed in each community, combined with an open informational meeting. One week later, written invitations were delivered to all adults aged 20–60 years. Between November 2006 and June 2007, two 3-day visits to the communities followed, 1 during weekdays and 1 including a weekend. The coffee and urban communities also were visited during 2 evenings. People were reminded at their homes and reasons for nonparticipation were noted. All examinations were conducted in health centers or schools.
Data Collection Methods Demographic and lifestyle data and occupational and medical histories were obtained through questionnaire. Blood pressure, weight, and height were measured with a calibrated digital sphyg sphygmomanometer and scale. Two certified laboratory technicians drew blood samples with 10-cc vacuum tubes containing a coagulation activator. Participants provided a nonfasting morning spot urine sample in a sterile 50-mL collector.
Occupational Exposure Current occupation was categorized into primary (agricultural workers), secondary (construction and textile factory workers), and tertiary (street vendors and workers in transport, public sector, commercial establishments, etc) sectors and economically inactive population (unemployed, students, and women working in the home). Home-based women who worked daily on their subsistence crops or were seasonal coffee or corn workers were classified as agricultural workers. Sugarcane, (past) cotton, and coffee work frequently overlapped with subsistence (corn and beans) activities. Years of work in sugarcane, cotton, coffee, subsistence crops, and their combinations were computed. Sugarcane work and subsistence farming were subdivided into coastal lowland or highland.
Medical Conditions and Lifestyle Hypertension was defined as self-reported medically treated hypertension or blood pressure ≥140/90 mm Hg at the examination; diabetes, as self-reported medically treated diabetes or glucosuria (glucose excretion ≥100 mg/dL) at examination; and obesity, as body mass index ≥30 kg/m. Self-reported intake of nonsteroidal anti-inflammatory drugs (NSAIDs) for treatment of chronic arthritis and self-reported history of nephrolithiasis were recorded. Lifetime tobacco smoking and alcohol consumption were summarized into current and ever variables.
Laboratory Analyses Blood samples were transported on ice to the clinical laboratory of the Rosales Hospital in San Salvador, where they were centrifuged and analyzed for SCr and serum urea nitrogen using the Jaffé compensated method. Aduplicate blood sample from each 10th participant was analyzed in the quality control laboratory of the Social Security in San Salvador with the same type of equipment and methods, resulting in 81% within 1 standard deviation and all within 2 standard deviations of the original measurement. Technicians measured glucosuria (positive [+] at glucose excretion ≥100 mg/dL) and proteinuria (at protein excretion ≥30-<300 mg/dL and ≥300 mg/dL) with reactive strips for chemical analyses at the examination. Of the 29 samples (4.4% of total) collected in the evening, none had detectable glucosuria.
SCr level >1.2 mg/dL for men and >0.9 mg/dL for women was used as an indicator of decreased kidney function. We estimated GFR (eGFR) based on the isotope-dilution mass spectrometry–traceable 4-variable Modification of Diet in Renal Disease (MDRD) Study equation (eGFRMDRD), categorized into ≥60 and <60 mL/min/1.73 m.We also calculated eGFR with the CKD Epidemiology Collaboration (CKD-EPI) creatinine-based equation (eGFRCKD-EPI). Proteinuria was categorized as low grade (protein excretion ≥30-<300 mg/dL) and high grade (≥300 mg/dL).
All analyses were performed separately for men and women using SPSS, version 16.0 (IBM, www-01.ibm.com/software/analytics/spss). Differences between communities and occupational groups were assessed using 2-sided χ tests (Fisher exact test at expected frequencies <5) and 1-way analysis of variance for continuous variables. Univariate logistic regressions examined associations between elevated SCr level and known risk factors. In multivariate models for elevated SCr level and eGFR<60 mL/min/1.72 m with coastal sugarcane/cotton plantation work as the exposure variable, we added covariates at change in the effect estimate >10%. Because hypertension and nephrolithiasis can be determinants as well as consequences of CKD, we added them to the adjusted models in a second step to examine changes in effect estimates. The effect of work history on sugarcane/cotton lowland plantations was investigated further in inhabitants of the coastal communities. Hypertension and nephrolithiasis were examined as secondary outcomes.
Methods
Study Design and Setting
The study was requested by the Salvadorian health authorities. Approval was obtained from the Bioethical Review Board of the Universidad Nacional in Costa Rica because there was no such entity in El Salvador at the time. All participants signed informed consent.
We selected 5 communities in El Salvador. Sugarcane production was the dominant economic activity in 2 sea-level communities on the Pacific Coast, 1 rural and 1 semirural. Previously, cotton production was important in these communities. ESRD previously had been observed as occurring in men in these communities. Three additional communities, for which there was no previous information about kidney problems, were selected to represent a range of economic activities and altitudes. The economy of one such community, found at an elevation of 500 m above sea level (masl), also was based on sugarcane production, with average temperatures during harvest seasons 1987–2000 that were ~4°C lower than at sea level. A community with heavy coffee production was chosen as an example of another agricultural activity at high altitude (1,650 masl). The final community was an urban neighborhood at 650 masl with a service-oriented economy, assumed to reflect the background distribution of SCr levels in El Salvador.
Study Population
A house-to-house census of all adults was completed in each community, combined with an open informational meeting. One week later, written invitations were delivered to all adults aged 20–60 years. Between November 2006 and June 2007, two 3-day visits to the communities followed, 1 during weekdays and 1 including a weekend. The coffee and urban communities also were visited during 2 evenings. People were reminded at their homes and reasons for nonparticipation were noted. All examinations were conducted in health centers or schools.
Data Generation
Data Collection Methods Demographic and lifestyle data and occupational and medical histories were obtained through questionnaire. Blood pressure, weight, and height were measured with a calibrated digital sphyg sphygmomanometer and scale. Two certified laboratory technicians drew blood samples with 10-cc vacuum tubes containing a coagulation activator. Participants provided a nonfasting morning spot urine sample in a sterile 50-mL collector.
Occupational Exposure Current occupation was categorized into primary (agricultural workers), secondary (construction and textile factory workers), and tertiary (street vendors and workers in transport, public sector, commercial establishments, etc) sectors and economically inactive population (unemployed, students, and women working in the home). Home-based women who worked daily on their subsistence crops or were seasonal coffee or corn workers were classified as agricultural workers. Sugarcane, (past) cotton, and coffee work frequently overlapped with subsistence (corn and beans) activities. Years of work in sugarcane, cotton, coffee, subsistence crops, and their combinations were computed. Sugarcane work and subsistence farming were subdivided into coastal lowland or highland.
Medical Conditions and Lifestyle Hypertension was defined as self-reported medically treated hypertension or blood pressure ≥140/90 mm Hg at the examination; diabetes, as self-reported medically treated diabetes or glucosuria (glucose excretion ≥100 mg/dL) at examination; and obesity, as body mass index ≥30 kg/m. Self-reported intake of nonsteroidal anti-inflammatory drugs (NSAIDs) for treatment of chronic arthritis and self-reported history of nephrolithiasis were recorded. Lifetime tobacco smoking and alcohol consumption were summarized into current and ever variables.
Laboratory Analyses Blood samples were transported on ice to the clinical laboratory of the Rosales Hospital in San Salvador, where they were centrifuged and analyzed for SCr and serum urea nitrogen using the Jaffé compensated method. Aduplicate blood sample from each 10th participant was analyzed in the quality control laboratory of the Social Security in San Salvador with the same type of equipment and methods, resulting in 81% within 1 standard deviation and all within 2 standard deviations of the original measurement. Technicians measured glucosuria (positive [+] at glucose excretion ≥100 mg/dL) and proteinuria (at protein excretion ≥30-<300 mg/dL and ≥300 mg/dL) with reactive strips for chemical analyses at the examination. Of the 29 samples (4.4% of total) collected in the evening, none had detectable glucosuria.
Main Outcomes
SCr level >1.2 mg/dL for men and >0.9 mg/dL for women was used as an indicator of decreased kidney function. We estimated GFR (eGFR) based on the isotope-dilution mass spectrometry–traceable 4-variable Modification of Diet in Renal Disease (MDRD) Study equation (eGFRMDRD), categorized into ≥60 and <60 mL/min/1.73 m.We also calculated eGFR with the CKD Epidemiology Collaboration (CKD-EPI) creatinine-based equation (eGFRCKD-EPI). Proteinuria was categorized as low grade (protein excretion ≥30-<300 mg/dL) and high grade (≥300 mg/dL).
Statistical Analysis
All analyses were performed separately for men and women using SPSS, version 16.0 (IBM, www-01.ibm.com/software/analytics/spss). Differences between communities and occupational groups were assessed using 2-sided χ tests (Fisher exact test at expected frequencies <5) and 1-way analysis of variance for continuous variables. Univariate logistic regressions examined associations between elevated SCr level and known risk factors. In multivariate models for elevated SCr level and eGFR<60 mL/min/1.72 m with coastal sugarcane/cotton plantation work as the exposure variable, we added covariates at change in the effect estimate >10%. Because hypertension and nephrolithiasis can be determinants as well as consequences of CKD, we added them to the adjusted models in a second step to examine changes in effect estimates. The effect of work history on sugarcane/cotton lowland plantations was investigated further in inhabitants of the coastal communities. Hypertension and nephrolithiasis were examined as secondary outcomes.