Jonny Myers

Prevalence odds ratio or prevalence ratio in the analysis of cross sectional data : what is to be done?

OBJECTIVES: To review the appropriateness of the prevalence odds ratio (POR) and the prevalence ratio (PR) as effect measures in the analysis of cross sectional data and to evaluate different models for the multivariate estimation of the PR. METHODS: A system of linear differential equations corresponding to a dynamic model of a cohort with a chronic disease was developed. At any point in time, a cross sectional analysis of the people then in the cohort provided a prevalence based measure of the effect of exposure on disease. This formed the basis for exploring the relations between the POR, the PR, and the incidence rate ratio (IRR). Examples illustrate relations for various IRRs, prevalences, and differential exodus rates. Multivariate point and interval estimation of the PR by logistic regression is illustrated and compared with the results from proportional hazards regression (PH) and generalised linear modelling (GLM). RESULTS: The POR is difficult to interpret without making restrictive assumptions and the POR and PR may lead to different conclusions with regard to confounding and effect modification. The PR is always conservative relative to the IRR and, if PR > 1, the POR is always > PR. In a fixed cohort and with an adverse exposure, the POR is always > or = IRR, but in a dynamic cohort with sufficient underlying follow up the POR may overestimate or underestimate the IRR, depending on the duration of follow up. Logistic regression models provide point and interval estimates of the PR (and POR) but may be intractable in the presence of many covariates. Proportional hazards and generalised linear models provide statistical methods directed specifically at the PR, but the interval estimation in the case of PH is conservative and the GLM procedure may require constrained estimation. CONCLUSIONS: The PR is conservative, consistent, and interpretable relative to the IRR and should be used in preference to the POR. Multivariate estimation of the PR should be executed by means of generalised linear models or, conservatively, by proportional hazards regression.

Seven-year experience of a primary care antiretroviral treatment programme in Khayelitsha, South Africa

Objectives:We report on outcomes after 7 years of a community-based antiretroviral therapy (ART) programme in Khayelitsha, South Africa, with death registry linkages to correct for mortality under-ascertainment. Design:This is an observational cohort study. Methods:Since inception, patient-level clinical data have been prospectively captured on-site into an electronic patient information system. Patients with available civil identification numbers who were lost to follow-up were matched with the national death registry to ascertain their vital status. Corrected mortality estimates weighted these patients to represent all patients lost to follow-up. CD4 cell count outcomes were reported conditioned on continuous virological suppression. Results:Seven thousand, three hundred and twenty-three treatment-naive adults (68% women) started ART between 2001 and 2007, with annual enrolment increasing from 80 in 2001 to 2087 in 2006. Of 9.8% of patients lost to follow-up for at least 6 months, 32.8% had died. Corrected mortality was 20.9% at 5 years (95% confidence interval 17.9–24.3). Mortality fell over time as patients accessed care earlier (median CD4 cell count at enrolment increased from 43 cells/μl in 2001 to 131 cells/μl in 2006). Patients who remained virologically suppressed continued to gain CD4 cells at 5 years (median 22 cells/μl per 6 months). By 5 years, 14.0% of patients had failed virologically and 12.2% had been switched to second-line therapy. Conclusion:At a time of considerable debate about future global funding of ART programmes in resource-poor settings, this study has demonstrated substantial and durable clinical benefits for those able to access ART throughout this period, in spite of increasing loss to follow-up.

Silicosis prevalence and exposure-response relations in South African goldminers

Aims: To measure the prevalence of silicosis among black migrant contract workers on a South African goldmine and to investigate exposure-response relations with silica dust. Methods: In a cross sectional study, 520 black goldminers (aged >37 years) were interviewed and had chest radiographs taken. Silicosis was defined as International Labour Organisation Classification radiological profusion of 1/1 or greater. Results: Mean length of service was 21.8 years (range 6.3–34.5). The mean intensity of respirable dust exposure was 0.37 mg/m3 (range 0–0.70) and of quartz 0.053 mg/m3 (range 0–0.095). The prevalence of silicosis was 18.3–19.9% depending on reader. Significant trends were found between the prevalence of silicosis and length of service, mean intensity of exposure, and cumulative exposure. Conclusion: Results confirm a large burden of silicosis among older black workers in the South African goldmining industry, which is likely to worsen as such miners spend longer periods in continuous employment in dusty jobs. An urgent need for improved dust control in the industry is indicated. If the assumption of stability of average dust concentrations on this mine over the working life of this group of workers is correct, these workers developed silicosis while exposed to a quartz concentration below the recommended occupational exposure limit (OEL) of 0.1 mg/m3. This accords with a mounting body of evidence that an OEL of 0.1 mg/m3 is not protective against silicosis.

Lung function loss in relation to silica dust exposure in South African gold miners

Objective To estimate exposure-response relationships between respirable dust, respirable quartz and lung function loss in black South African gold miners. Methods 520 mineworkers aged >37 years were enrolled in a cross-sectional study. Gravimetric dust measurements were used to calculate cumulative respirable dust and quartz exposures. Excess lung function loss was defined as predicted minus observed forced expiratory volume in one second (FEV1) and forced vital capacity (FVC). The association between excess loss and exposure was estimated, adjusting for smoking, tuberculosis and silicosis. Results Mean service length was 21.8 years, mean respirable dust 0.37 mg/m3 and mean respirable quartz 0.053 mg/m3. After adjustment, 1 mg-yr/m3 increase in cumulative respirable dust exposure was associated with 18.7 ml mean excess loss in FVC [95% confidence interval (CI) 0.3, 37.1] and 16.2 ml in FEV1 (95% CI -0.3, 32.6). Mean excess loss with silicosis was 224.1 ml in FEV1 and 123.6 ml in FVC; with tuberculosis 347.4 ml in FEV1 and 264.3 ml in FVC. Conclusion Despite a healthy worker effect, lung function loss was demonstrable whether due to silicosis, tuberculosis or an independent effect of dust. A miner working at a respirable dust intensity of 0.37 mg/m3 for 30 years would lose on average an additional 208 ml in FVC (95% CI 3, 412) in the absence of other disease, an impact greater than that of silicosis and comparable to that of tuberculosis. Improved dust control on the South African gold mines would reduce the risk of silicosis, tuberculosis and lung function impairment.

Case-control study of mesothelioma in South Africa

BACKGROUND South Africa has, uniquely, mined, transported, and used crocidolite, amosite, and chrysotile. A multicenter case-control study was done in South Africa to examine the details of asbestos exposure in cases and controls, and to calculate relative risks for level of certainty of asbestos exposure, nature of exposure (e.g., environmental, occupational) and fiber type. METHODS Cases and controls (one cancer and one medical per case) were collected by six study centers from referral hospitals, and exposure information was collected by interviewing cases and controls in life. RESULTS One hundred and twenty-three cases were accepted into the study. None had purely chrysotile exposure. Twenty-three cases had mined Cape crocidolite; three had mined amosite; and three Transvaal crocidolite plus amosite. A minimum of 22 of the cases had exclusively environmental exposure, 20 were from the NW Cape crocidolite mining area. The relative risks associated with environmental exposure in the NW Cape (crocidolite) were larger than for environmental exposure in the NE Transvaal (amosite and crocidolite): 21.9 vs. 7.1 and 50.9 vs. 12.0 for the cancer control and medical control datasets, respectively. CONCLUSIONS The results confirm the importance of environmental exposure in the Cape crocidolite mining area, the relative paucity of cases linked to amosite, the rarity of chrysotile cases and are consistent with a fiber gradient in mesotheliomagenic potential for South African asbestos with crocidolite > amosite > chrysotile.

Repeatability and validity of a field kit for estimation of cholinesterase in whole blood.

OBJECTIVES--To evaluate a spectrophotometric field kit (Test-Mate-OP) for repeatability and validity in comparison with reference laboratory methods and to model its anticipated sensitivity and specificity based on these findings. METHODS--76 farm workers between the age of 20 and 55, of whom 30 were pesticide applicators exposed to a range of organophosphates in the preceding 10 days, had blood taken for plasma cholinesterase (PCE) and erythrocyte cholinesterase (ECE) measurement by field kit or laboratory methods. Paired blinded duplicate samples were taken from subgroups in the sample to assess repeatability of laboratory and field kit methods. Field kits were also used to test venous blood in one subgroup. The variance obtained for the field kit tests was then applied to two hypothetical scenarios that used published action guidelines to model the kits sensitivity and specificity. RESULTS--Repeatability for PCE was much poorer and for ECE slightly poorer than that of laboratory measures. A substantial upward bias for field kit ECE relative to laboratory measurements was found. Sensitivity of the kit to a 40% drop in PCE was 67%, whereas that for ECE was 89%. Specificity of the kit with no change in mean of the population was 100% for ECE and 91% for PCE. CONCLUSION--Field kit ECE estimation seems to be sufficiently repeatable for surveillance activities, whereas PCE does not. Repeatability of both tests seems to be too low for use in epidemiological dose-response investigations. Further research is indicated to characterise the upward bias in ECE estimation on the kit.

Applying upstream interventions for interpersonal violence prevention: An uphill struggle in low- to middle-income contexts

In South Africas Western Cape province, interpersonal violence was identified among the key prevention priorities in the provincial governments Burden of Disease (BoD) Reduction project. To date, there are no adequate systematic reviews of the full range of potential intervention strategies. In response, available data and the literature on risk factors and prevention strategies for interpersonal violence were reviewed with a view to providing policy makers with an inventory of interventions for application. Given the predominance of upstream factors in driving the provinces rates of interpersonal violence, efforts to address its burden require an intersectoral approach. Achievable short-term targets are also required to offset the long-term nature of the strategies most likely to affect fundamental shifts. Documentation and evaluation will be important to drive long-term investment, ensure effectiveness and enable replication of successful programmes and should be considered imperative by interpersonal violence prevention policymakers in other low- to middle-income contexts.

Reducing the burden of injury: an intersectoral preventive approach is needed.

The BoD project is one of some 32 cluster-based multi-departmental cabinet committee work streams typically led by a single provincial government department. Linkages across clusters are difficult to establish, but ostensibly occur at higher levels in sector co-ordinating and top management committees, which report to Cabinet.

Diet and Esophageal Cancer Risk in the Eastern Cape Province of South Africa

A multicenter hospital-based case-control study comprising 670 incident cases of esophageal cancer (EC) and 1188 controls, frequency-matched for age and sex, was conducted to evaluate the role of diet on EC development in the Eastern Cape Province, South Africa. A locally relevant lifestyle and dietary questionnaire was used. Adjusted odds ratios (OR) and 95% confidence intervals (CI) were computed using unconditional multivariable logistic regression. Individually, maize or sorghum consumption vs. never or rare consumption were not associated with EC (P > 0.1). Males and females consuming green leafy vegetables 5–7 days/wk had 38% (P = 0.04) and 50% (P = 0.007) reduced odds of developing EC, respectively, compared with consumption ≤1 day/wk. A similar reduction in odds was observed with fruit consumption. Principal component factor analysis revealed 3 distinct dietary patterns. In females, high vs. low consumption of Pattern 1 (sorghum, green leafy vegetables, green legumes, fruits, meat) was inversely associated with EC development (OR = 0.54; 95% CI: 0.34–0.89), whereas for Pattern 2 (maize, wild greens-imifino, dry beans) the odds were elevated (OR = 1.67; 95% CI: 1.04–2.67). Compared with low adherence, high adherence to Pattern 3 (wheat-based products) reduced the odds by 35% for both sexes. This study provides further evidence on the role of diet in minimizing EC risk in this population.

Assessing Quality of Existing Data Sources on Road Traffic Injuries (RTIs) and Their Utility in Informing Injury Prevention in the Western Cape Province, South Africa

Objectives: This study assessed whether the quality of the available road traffic injury (RTI) data was sufficient for determining the burden of RTIs in the Western Cape Province and for implementing and monitoring road safety interventions. Methodology: Underreporting was assessed by comparing data reported by the South African Police Services (SAPS) in 2008 with data from 18 provincial mortuaries. Completeness of the driver death subset of all RTIs was assessed using the capture–recapture method. Results: The mortuary and police data sets comprised 1696 and 860 fatalities respectively for the year 2008. The corresponding provincial road traffic mortality rates were as follows: 32.2 deaths/100,000 population per year (95% confidence interval [CI]: 30.7–33.8) and 16.3 deaths/100,000 population per year (95% CI: 15.3–17.5). The police data set contained 820,960 crashes, involving 196,889 persons, indicating substantial duplication of crash events. There were varying proportions of missing data for demographic and other identifying variables, with age missing in nearly half of the cases in the police data set. The estimated total number of driver deaths/year was 588.6 (95% CI: 544.4–632.8), yielding estimated completeness of the mortuary and police data sets of 57.6 and 46.4 percent separately and 77.3 percent combined. Conclusion: This study found extensive data quality problems, including missing data, duplication, and significant underreporting of traffic injury deaths in the police data. Not all assumptions underlying the use of capture–recapture method were met in this study; hence, the estimates provided by this analysis should be interpreted with caution. There is a need to address the problems highlighted by this study in order to improve data utility for informing road safety policies. Supplemental materials are available for this article. Go to the publishers online edition of Traffic Injury Prevention to view the supplemental file.