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On the definition of relevant disease
  1. B J Marais
  1. Department of Paediatrics and Child Health, Centre for TB Research and Education (CENTRE), Faculty of Health Sciences, University of Stellenbosch, PO Box 19063, Tygerberg, 7505, Cape Town, South Africa;

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    Historically the art of medicine was defined by the ability to develop a personal reference for normality. This internal reference allowed the subjective identification of abnormality by pure clinical acumen. In contrast, modern medicine provides an objective scientific definition of abnormality, based on accurate measurement and statistical analysis. However, the medical profession still struggles to find the optimal balance between the art (subjective) and the science (objective), in providing holistic health care.

    Statistical differentiation into normal and abnormal serves us well when dealing with continuous physiological variables like blood pressure, cholesterol, or weight. The differentiation becomes problematic when dealing with the complex biological balance that exists between infecting organisms and host responses. The initial simplistic differentiation into harmless colonising organisms and dangerous invasive pathogens has been replaced by a more complete appreciation of the complex, dynamic relation that exists between the host and the organism. The challenge facing modern medicine is to translate improved scientific understanding into clear, pragmatic guidelines applicable in diverse settings.

    Once abnormality is identified, whether subjectively or objectively, the crucial question that remains is: Does this abnormality constitute disease? Disease is defined by present reduced quality of life (morbidity) as well as the increased risk for future morbidity or mortality. Knowledge of a condition’s natural history becomes invaluable when initial morbidity is minimal. Accurate description of the natural history of disease allows scientific risk:benefit analyses of a proposed intervention. The scientific quest does not end here; the true quest is to quantify the relative benefit of the suggested intervention within a particular setting.

    The idea of relative benefit or risk, to an individual within a particular setting, differs from the classical public health approach. With this approach the relevance of disease is determined by the total burden placed on a specific society, completely ignoring the individual patient. Although it assists with focusing scarce resources, it undermines the moral basis of the medical profession, which promises sympathetic care to every individual who seeks help. In contrast the concept of relative risk maintains the primary focus on the individual patient, but takes the baseline risk determined by his/her particular setting into account.

    Relevant disease defines the point where relative risk increases significantly beyond the baseline risk within a particular community. This implies that contrary to the classic public health approach, any severe disease represents relevant disease, irrespective of its contribution to the total burden of disease. This definition also challenges the classic pharmaceutical approach, which promotes any statistically significant benefit measured against a minimal baseline risk, as the accepted “standard of care”, irrespective of the particular setting.

    Identifying the appropriate intervention once relevant disease is diagnosed requires careful analysis, weighing the relative risk posed to the individual and society against the possible benefit, risk and cost of available treatment. The challenge posed by the concept of relevant disease, is to identify the sub-population at highest risk in order to focus cost effective interventions appropriately.

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