Using lower limit of normal for FEV1/FVC ratio may reduce misclassification of airway obstruction but despite years of discussion and arguments it remains unclear whether it is a better approach. The simplicity of the fixed ratio approach to me remains a very important consideration and it is the ap...

FEV1/FVC of 70% is simple to understand. That may lead to misclassification at the ends of the age spectrum.

I continue to use both in relation to my pre-test probability of disease. If I have a patient with symptoms and risk factors for COPD, but has a ratio greater than the LLN and less than 70%, will use the 70% and consider COPD as a diagnosis. If a patient has no risk factors and no symptoms, again ha...

While clinical context remains the most important consideration, use of LLN rather than a fixed ratio prevents over/under recognition of obstructive lung disease. 

I have used the ratio mostly but am careful how it might over diagnose obstruction to airflow in older patients.