CRP and chronic obstructive pulmonary disease: COPD: chronic obstructive pulmonary disease; APACHE IV: Acute Physiology and Chronic Health Evaluation IV; SOFA: Sequential Organ Failure Assessment; ICU LOS: length of stay at the intensive care; WBC: white blood cell; CRP: C-reactive protein; PCT: procalcitonin; MR-proADM: mid-regional proadrenomedullin; MR-proANP: mid-regional proatrial natriuretic peptide