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A grading system of airway obstruction

As discussed on the previous page a comprehensive system for grading the severity of airway obstruction using lung function tests should be applicable in asthma and COPD, take into account complicating restrictive lung disease, and be applicable to any ethnic group. The dimensionless FEV1/VC ratio meets this criterion. Hence, whether there is airway obstruction is decided from either

In adults the above ratio declines with age. Therefore age is taken into account, and the z-score = standard deviation score (SDS) used. In children and adolescents the according to some authors.

An FEV1/VC ratio below the age and height adjusted 5th percentile of healthy subjects is regarded as evidence of airway obstruction. It is classified as:

Mild FEV1/VC ratio below 5th percentile, FEV1%pred > 60%
Moderate FEV1/VC ratio below 5th percentile, 40% < FEV1%pred > 60%
Severe FEV1/VC ratio below 5th percentile, FEV1%pred < 40%

There are several grading systems recommended by various organizations. The above is in agreement with that of the BTS.

NB: The classification into mild, moderate or severe airway obstruction is based solely on spirometric data; it is not based on clinical criteria. The correlation between spirometric and clinical findings is generally poor.

Reference:
BTS Guidelines for the Management of Chronic Obstructive Pulmonary Disease. Thorax 1997; 52; S1-S28.

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