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Open Access Research article

The role of 'confounding by indication' in assessing the effect of quality of care on disease outcomes in general practice: results of a case-control study

Johan S de Koning1*, Niek S Klazinga2, Peter J Koudstaal3, Ad Prins4, Gerard JJM Borsboom1 and Johan P Mackenbach1

Author Affiliations

1 Department of Public Health, Erasmus MC, University Medical Centre Rotterdam, the Netherlands

2 Department of Social Medicine, Academic Medical Centre, University of Amsterdam, the Netherlands

3 Department of Neurology, Erasmus MC, University Medical Centre Rotterdam, the Netherlands

4 Department of General Practice, Erasmus MC, University Medical Centre Rotterdam, the Netherlands

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BMC Health Services Research 2005, 5:10  doi:10.1186/1472-6963-5-10

Published: 27 January 2005

Abstract

Background

In quality of care research, limited information is found on the relationship between quality of care and disease outcomes. This case-control study was conducted with the aim to assess the effect of guideline adherence for stroke prevention on the occurrence of stroke in general practice. We report on the problems related to a variant of confounding by indication, that may be common in quality of care studies.

Methods

Stroke patients (cases) and controls were recruited from the general practitioner's (GP) patient register, and an expert panel assessed the quality of care of cases and controls using guideline-based review criteria.

Results

A total of 86 patients was assessed. Compared to patients without shortcomings in preventive care, patients who received sub-optimal care appeared to have a lower risk of experiencing a stroke (OR 0.60; 95% CI 0.24 to 1.53). This result was partly explained by the presence of risk factors (6.1 per cases, 4.4 per control), as reflected by the finding that the OR came much closer to 1.00 after adjustment for the number of risk factors (OR 0.82; 95% CI 0.29 to 2.30). Patients with more risk factors for stroke had a lower risk of sub-optimal care (OR for the number of risk factors present 0.76; 95% CI 0.61 to 0.94). This finding represents a variant of 'confounding by indication', which could not be fully adjusted for due to incomplete information on risk factors for stroke.

Conclusions

At present, inaccurate recording of patient and risk factor information by GPs seriously limits the potential use of a case-control method to assess the effect of guideline adherence on disease outcome in general practice. We conclude that studies on the effect of quality of care on disease outcomes, like other observational studies of intended treatment effect, should be designed and performed such that confounding by indication is minimized.