Variations in spirometry interpretation between labs in Australia is leading to inconsistencies in diagnosis and management  of obstructive lung disease, a study suggests.

Findings from a survey of 36 TSANZ-accredited lung function laboratories in Australia and New Zealand reveal ‘substantial’ variation between institutions in how laboratories define the lower limit of normal range, airflow obstruction and significant bronchodilator reversibility, as well as their choice of spirometry reference equations.

All but one of the laboratories were public facilities and most tested adult or both adult and paediatric populations, according to the results published by Victorian respiratory specialists in Internal Medicine Journal.

Most laboratories (75%) defined the lower limit of the normal range using the 5^th percentile, while almost one in five (19.4%) used a fixed cut-off definition that risked ‘under classification of obstruction in the young and over classification in older adults’.

The study also found 80.5% of labs used FEV₁/FVC as the sole parameter to identify airflow obstruction with a minority of labs using other combinations of parameters.

And most labs (69.4%) defined airflow obstruction as FEV₁/FVC below the 5^th percentile versus FEV₁/FVC < 0.70 or other fixed cut-offs.

In line with American Thoracic Society (ATS) guidelines, most laboratories used ≥ 200mL and ≥12% increase in FEV1 and/or FVC as the definition of significant bronchodilator response however others used the GOLD or COPD-X definition of ≥ 200mL and ≥ 12% increase in FEV₁ only.

Laboratories used a variety of reference equations with fewer than half (44.4%) using the Global Lung Initiative equations (GLI 2012) as recommended by the Australia and New Zealand Society of Respiratory Science (ANZSRS).

The study authors said the discordance in interpretation practice was substantial especially given the fact that all labs were TSANZ accredited.

“The observed variation is likely to be amplified when non-accredited testing facilities throughout Australia and New Zealand are taken into account. The variation in practice may translate into clinically appreciable differences for the diagnosis and management of common obstructive respiratory conditions,” they wrote.

Co-author Dr Nicolette Holt from the Alfred Hospital’s department of respiratory medicine told the limbic increased education, training and awareness regarding current ATS/ERS definitions and choice of reference sets was required.

“Professional bodies ANZSRS and TSANZ need to continue to work towards standardisation of interpretation practices in Australia and New Zealand,” she said.

At the moment, two patients of the same gender, age, height, race, and with the same absolute lung function result performed at different centres may be managed differently due to the different definitions.

“Potentially, referring clinicians can re-interpret results based on current ATS/ERS spirometry interpretation definitions rather than accept a lab report. But really it is the responsibility of labs to ensure that they conform to latest recommendations regarding interpretation,” she said.

The TSANZ accreditation manual recommends use of the ATS/ERS standards for interpretation of spirometry.

However the study noted that the differences between clinical practice guidelines contributed to the variation in spirometry interpretation.