Reference	Objective	Design	Evidence
Allegra et al. [458]	To study the relationship between objective purulence and the presence of bacteria	PCS	3B+
Lieberman et al. [457]	To study the frequency of fever in exacerbations	PCS	3C+
Rohde et al. [168]	A case control study to investigate the role of different viruses on acute exacerbations of chronic obstructive pulmonary disease (AECOPD)	CCS	3C+
Lieberman et al. [463]	To study the role of Mycoplasma pneumoniae in hospitalized patients with AECOPD	PCS	2C+
Soler et al. [459]	To study the association between purulence and bacterial bronchoscopic samples	PCS	3A+
Brusse‐Keizer et al. [460]	To study the association between sputum colour and sputum bacterial load	PCS	3C−
Burley et al. [461]	To study the association between symptoms and Gram stain and sputum bacterial growth	PCS	3B+
Stolz et al. [462]	To study the value of procalcitonin to decrease the use of antibiotics in exacerbated COPD	RCT	2A+