By Mark Pownall
Spirometry cutoffs recommended by NICE to diagnose COPD misclassify up to one in four patients and should be replaced, according to the findings of an international study .
Researchers looked at the spirometry results of 11,413 patients from three respiratory testing centres, 5,702 patients from Birmingham and the rest from the US and New Zealand.
They found that using predicted and fixed thresholds for FEV1 and FVC, as recommended by both NICE and GOLD – the Global initiative for Obstructive Lung Disease – misclassified 24% of all patients, in comparison with using the fifth percentile of lung function as the lower limit of normal.
It also found that 10% of patients were wrongly classified as having COPD when they actually had no evidence of disease and 7% of patients were falsely said to have emphysema based on spirometry results.
Using an FEV1 of 80% as a cutoff meant airflow obstruction was missed in younger patients and overdiagnosed lung disease in men and older patients.
Similar patterns were reported from all three centres, so the researchers suggest their findings apply to results from all lung function laboratories.
Study lead Dr Martin Miller, consultant respiratory physician at University Hospital Trust Birmingham said there is a ‘strongly compelling’ case for ditching the use of a fixed ratio of FEV1/FEV in spirometry and using lower limits of normal results instead.
He said: ‘Lower limits of normal for lung function tests vary with age, height, sex and ethinc group and this may explain [the] falling back on a rule of thumb limit setting of normal as below or above 20% predicted.’
‘But this simplicity leads to a loss of accuracy and specificity.’
Wiltshire GP Dr Kevin Gruffyd-Jones said final draft NICE guidance on COPD due out acknowledged the limitations of spirometry based on predicted results.
He said: ‘Fixed ratios are much simpler, and until we get up to date post bronchodilator reference values, there is no advantage to using the 5% lower levels of normal.’ NICE had found a fixed FEV1/FVC ratio matched doctors’ diagnoses of COPD better than LLN FEV1/FVC, he said.
‘What this study emphasises, is that we should never rely on spirometry alone to diagnose COPD and other respiratory disease. You need to look for characteristic symptoms, take a good history and do an examination, and test whether any airway obstruction is reversible or not.’
‘The most important thing is that more people with probable respiratory disease should have spirometry so they can have a more accurate diagnosis.’
Chest, online 3 June
Spirometry cutoffs recommended by NICE to diagnose COPD misclassify up to one in four patients, the study found Spirometry cutoffs recommended by NICE to diagnose COPD misclassify up to one in four patients, the study found