COPD is classified based on the patient's FEV1 and its percentage of the predicted FEV1. In cases where FVC may be hard to measure, FEV6 (forced expiratory volume at 6 seconds) can be used. 
FEV1/FVC ratio <0.70; total absence of reversibility is neither required nor the most typical result
Checked as part of vital signs on acute presentation. A good pulse wave should be picked up by the device. In patients with chronic disease, an oxygen saturation of 88% to 90% may be acceptable.
If <92% arterial or capillary blood gases should be checked. 
low oxygen saturation
Checked in patients who are acutely unwell, especially if they have an abnormal pulse oximetry reading. Should also be performed in stable patients with FEV1 <35% predicted or with clinical signs suggestive of respiratory failure, or if peripheral arterial oxygen saturation is <92%.
Hypercapnia, hypoxia, and respiratory acidosis are signs of impending respiratory failure and possible need for intubation.
PaCO₂ >50 mmHg and/or PaO₂ of <60 mmHg suggests respiratory insufficiency
Seldom diagnostic, but useful in ruling out other pathologies.
Increased anteroposterior ratio, flattened diaphragm, increased intercostal spaces, and hyperlucent lungs may be seen.
May also demonstrate complications of COPD, such as pneumonia and pneumothorax.
This test may be considered to assess severity of an exacerbation and may show polycythaemia (haematocrit >55%), anaemia, and leucocytosis. 
raised haematocrit, possible increased WBC count
Risk factors for COPD are similar to those for ischaemic heart disease, so comorbidity is common.
signs of right ventricular hypertrophy, arrhythmia, ischaemia
In an acute exacerbation, empirical antibiotics should be given if the patient has three cardinal symptoms: increase in dyspnoea, sputum volume, and sputum purulence; or if the patient has two of the cardinal symptoms, if increased purulence of sputum is one of the two symptoms; or if the patient requires mechanical ventilation. In patients with frequent exacerbations, severe airflow limitation, and/or exacerbations requiring mechanical ventilation, sputum should be sent for culture. 
Useful for resolving diagnostic uncertainties and preoperative assessment.  Requires specialist laboratory facilities.
Decreased diffusing capacity of the lung for carbon monoxide (DLCO) is supportive of emphysema over chronic bronchitis.
obstructive pattern, decreased DLCO
Provides better visualisation of type and distribution of lung tissue damage and bulla formation than CXR.
In contrast to smoking-related COPD, alpha-1 antitrypsin deficiency mainly affects lower fields.
Useful in excluding other underlying pulmonary disease and for pre-operative assessment.
Low level in patients with alpha-1 antitrypsin deficiency. Test is done if there is high suspicion for alpha-1 antitrypsin deficiency, such as a positive family history and atypical COPD cases (young patients and non-smokers). The World Health Organization recommends that all patients with a diagnosis of COPD should be screened once, especially in areas with high prevalence of alpha-1 antitrypsin deficiency. 
should be normal in patients with COPD
Can be of value in patients with a disproportional degree of dyspnoea compared with spirometry.  It can be performed on a cycle or treadmill ergometer, or by a simple timed walking test (e.g., 6 minutes, or duration <6 minutes).  Exercise testing is of use in selecting patients for rehabilitation.
poor exercise performance or exertional hypoxaemia is suggestive of advanced disease
Obstructive sleep apnoea, a common finding in patients with COPD, is associated with increased risk of death and hospitalisation in patients with COPD. 
elevated apnoea-hypopnoea index and/or nocturnal hypoxaemia
Respiratory muscle function may be tested if dyspnoea or hypercapnia are disproportionately increased with respect to FEV1, as well as in patients with poor nutrition and those with corticosteroid myopathy. 
reduced maximal inspiratory pressure
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