C2.3 Spirometry

The diagnosis of COPD rests on the demonstration of airflow limitation which is not fully reversible (Global Initiative for Chronic Obstructive Lung Disease 2017) [evidence level II].
It is important in general practice settings to obtain accurate spirometric assessment (Walters 2011b) [evidence level III-3].

Because COPD is defined by demonstration of airflow limitation which is not fully reversible, spirometry is essential for its diagnosis (see Figure 4) and this may be performed in the community or prior to discharge from hospital (Rea 2011). Most spirometers provide predicted (“normal”) values obtained from healthy population studies, and derived from formulas based on height, age, sex and ethnicity.

Airflow limitation is not fully-reversible when, after adminis­tration of bronchodilator medication, the ratio of FEV1 to forced vital capacity (FVC) is <70% and the FEV1 is <80% of the predicted value. The ratio of FEV1 to vital capacity (VC) is a sensitive indicator for mild COPD. FEV1/FEV6 has a high level of agreement with FEV1/FVC on both the fixed ratio and Lower Limit of Normal (LLN) criteria for the diagnosis of COPD (Bhatt 2014a). There is controversy regarding the optimal cut-off to define airflow limitation (FEV1/FVC less than 0.7 vs lower limit of normal). There is evidence that the fixed ratio can lead to over diagnosis of COPD in older populations, under diagnosis in younger people (Cerveri 2008, Vollmer 2009, Swanney 2008) and may lead to gender imbalances as women have higher FEV1/FVC than their male counterparts (Guerra 2009). A systematic review of 11 studies which examined the relationship of each criterion with clinical outcomes found both were related to clinical outcomes and concluded that on current evidence one could not be preferred over the other. The LLN appeared to be a better criterion in older patients with less severe airflow limitation (van Dijk 2014). There is conflicting data comparing the two cut-offs regarding mortality and healthcare utilisation, however a study (Bhatt 2014b) shows that the fixed cut-off of 0.7 identified more people with CT diagnosed emphysema.

Figure 4: Maximal expiratory flow-volume curves in severe chronic obstructive pulmonary disease (COPD) and chronic asthma

Maximal expiratory flow-volume curves in severe chronic obstructive pulmonary disease (COPD) and chronic asthma

The patient with COPD has reduced peak expiratory flow, and severely decreased flows at 25%, 50% and 75% of vital capacity compared with the normal range (vertical bars), and shows minimal response to bronchodilator (BD). By comparison, the patient with chronic asthma shows incomplete, but substantial, reversibility of expiratory flow limitation across the range of vital capacity. After BD the forced expiratory volume in one second (FEV1) was within the normal range (82% predicted). Absolute and per cent predicted values for FEV1 and forced vital capacity (FVC) before and after BD are shown for each patient.

A detailed systematic review states that spirometry, in addition to clinical examination, improves the diagnostic accuracy of COPD compared to clinical examination alone reinforcing the importance of spirometry (Wilt 2005) [evidence level I]. The current inaccuracy of diagnosis in community settings and the importance of using spirometry was demonstrated by an Australian study where only 58% of general practice patients being treated for COPD were confirmed to have the diagnosis on post-bronchodilator spirometry (Zwar 2011). The unreliability of clinical assessment for the diagnosis of COPD has also been shown in a study in Dutch primary care (Lucas 2012). More studies are required to define any benefit from the use of spirometry for case finding in COPD, and to evaluate the effects of spirometric results on smoking cessation.

The spirometric tests require high levels of patient effort and cooperation, and there are important quality criteria that should be met in conducting spirometry (Miller 2005).

Indications for spirometry include:

  • breathlessness that seems inappropriate;
  • chronic (daily for two months) or intermittent, unusual cough;
  • frequent or unusual sputum production;
  • relapsing acute infective bronchitis; and
  • risk factors such as exposure to tobacco smoke, occupa­tional dusts and chemicals, and a strong family history of COPD.

There is evidence of both underdiagnosis (Toelle 2013) and misdiagnosis of COPD in the community (Zwar 2011). In a general practice setting, patients with comorbidities may be more commonly mis-diagnosed with COPD.  Spirometric assessment is important in these patients to minimise this risk (Zwar 2011). Two pulmonologists reviewed 333 patients with physician-diagnosed COPD and/or asthma. The patients had two or more emergency room visits or admissions over the preceding 12 months, with prospective evaluation over the next 10 months. The study found that a third of these patients had neither asthma nor COPD, and a quarter may not even have any form of airflow limitation. The study highlighted the importance of spirometry in making the correct diagnosis, which had been performed in less than a third of the patients studied (Jain 2015).