LLN spirometry inadequate to detect airflow obstruction

Research published in Thorax indicates that use of a lower limit of normal (LLN) spirometric threshold fails to identify a number of patients with airflow obstruction and chronic obstructive pulmonary disease (COPD).

The use of such a threshold has recently been proposed by the American Thoracic Society (ATS)/European Respiratory Society (ERS) but the findings show that a fixed criterion, as recommended in the Global Initiative for Chronic Lung Disease (GOLD) guidelines, was more effective.

Surya Bhatt (University of Alabama at Birmingham, USA) and colleagues report results from the COPDGene study, in which 7743 participants aged 45–80 years with at least 10 pack–years’ smoking history underwent both spirometry and computed tomography (CT).

The authors used two spirometric thresholds to define smoking-related airflow obstruction: a postbronchodilator forced expiratory volume in 1 s (FEV1) to forced vital capacity (FVC) ratio of less than 0.7 (GOLD criteria); and an FEV1/FVC ratio below the LLN, defined as the 5th percentile for age (ATS/ERS criteria).

On the whole, there was good agreement between the two thresholds with 92.7% of patients receiving the same diagnosis, regardless of which was used. However, concordance was slightly reduced in older patients, with a κ-value of 0.78 in those aged 61–80 years compared with 0.90 in both those aged 45–50 years and those aged 51–60 years.

The 548 patients who had been categorized as having airflow obstruction according to the fixed ratio only had significantly more emphysema (4.1 vs 1.2%) and gas trapping (19.8 vs 7.5%) on CT than the 18 patients who were categorized by the LLN criterion only.

“These subjects were misclassified as normal by LLN criteria,” comment Bhatt and colleagues, who note that these patients were more likely to be older, male, Caucasian and had a greater smoking burden than those misclassified as normal by the fixed ratio criteria. They were also more likely to be on home oxygen therapy at baseline and, during a mean 592 days of follow-up, were more likely to be started on home oxygen therapy and have exacerbations than the LLN-only group.

The authors explain that the use of a fixed ratio to define airflow obstruction has come in for criticism because several studies have shown that it underestimates airflow obstruction at younger ages and overestimates it at older ages.

However, they note that the LLN approach has not been validated against a gold standard, such as CT, or with respect to outcomes. By contrast, patients with spirometry values below the fixed threshold have been shown to have an increased risk for death over longitudinal follow-up in several studies.

“LLN has been proposed to be superior in that it would prevent unnecessary initiation of medication,” they write.

But they add: “While it may appear that the fixed ratio only identifies older patients with non-consequential or ‘senile’ emphysema, this is likely not the case as there is considerably more gas trapping in the Fixed discordant group, arguing for more concurrent airway disease.”

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