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Incorrect classification: consequences

Naturally, if a test result for airway obstruction is positive, a doctor will be tempted to make the diagnosis and institute treatment or order more tests. If it is negative, the doctor will be more inclined to reject the diagnosis. Each of these decisions comes at a cost and a benefit, in particular if performed with no or only a poor indication (i.e. with low a priori likelihood that the disease is present), such as:

• There is the cost of treating the true positive, but also of the false positive cases. If a disease is rare the costs of treating negative cases will far outweigh those of positive cases. The more prevalent a disease the better the balance between costs for treating the patient compared to treating negative cases. However, obviously the total cost increases.
• The rarer a disease, the more cases may be missed by the test. The cost of administering the test per person identified by the test as having the disease will therefore be high. Yet, much of the cost will be due to identifying false positives, and a relatively high proportion of patients may not be detected by the test. The more prevalent the disease, the lower the percentage of subjects in whom the diagnosis was missed. But unfortunately, due to the high prevalence many subjects will be subjected to the test, and this will lead to a significant absolute number of false positives being treated, and a significant number of false negatives not being treated.
• Treatment is associated with side effects, both in patients who have the disease and in the false positives. This will lead to discomfort to the patient, extra medical costs and absence from work, maybe even to mortality.
• One may reap benefits from effective treatment. Decreased morbidity might lead to fewer days lost from work, fewer days in costly hospitals, better quality of life, reduced mortality, beneficial effects on co-morbidity, etc.
• Even though the immediate costs of case finding and intervention might be high, such intervention may still be cost-effective. This would be the case if there would be an immediate cure, and this would prevent death or a more protracted course of the disease in a more advanced stage.

It is not difficult to add more relevant factors. It follows from the above that it is important to strike the optimal balance between true positive and false positive test results. In the case of COPD there is the well-documented benefit of stopping smoking, but little if any benefit from medical treatment except some in the more severe cases [1-2]. Therefore there is every reason to adjust the threshold value for medical intervention (but not for efforts to stop smoking) to a level where we accept fewer false positives and more false negative cases.