Here are two pieces on chronic obstructive pulmonary disase (COPD). COPD is important because it drives a lot of emergency admissions to hospital, especially in the winter.
In the USA a study of patients over 65 and in the Medicare system showed that some places diagnose a lot of COPD but that does not translate into a lot more COPD deaths. (And likewise for eight other chronic conditions.) More precisely the case fatality ratio drops as the frequency of diganosis increases. This suggests that milder cases are being identified. Tha authors point out that identifying more cases has various economic and other effects - the more diagnoses you record the more you get paid. I guess this unworthy thought arose because the Medicare data analysed were claims for item-of-service fees.
I called this 'overdiagnosis' in the blog title but actually the diagnosis may be appropriate and accurate. At a minimum it means that a diagnosis of COPD carries a different prognostic import in different places. Which in turns means that if you are adjusting outcome measures for severity (e.g. co-morbidity) you are not comparing like with like.
The study in England used a different strategy:
Step1: A few years ago, the Health Survey for England included a component which measured lung function - FVC and FEV1. Hence we have an objective measure of lung impairment, which can be used to make an objective diagnosis of COPD (e.g. FEV1 less than 70% of FVC).
Step 2: Analyse for predictors of COPD - age, sex, smoking, ethnicity etc.
Step 3: This allows you, for any given population, and knowing its composition by age / sex/ ethnicity etc, to estimate how many people should have COPD.
Stpe 4 - compare this with the number of GP-diagnosed cases, using data returns in the Quality Outcomes Framework (which incidentally like the Medicare data is linked to payment of doctors, albeit less directly).
Net conclusion, after some unfortunately very complicated spatial statistics: underdiagnosis in London.
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