CALCA and chronic obstructive pulmonary disease: It has been found that, about 25% to 50% of COPD cases are induced by bacterial infection.10 Previously, clinical doctors determine bacterial infection according to experience or the changes of WBC and ESR; however, error diagnosis is easy to happen due to the low specificity and sensitivity, which can bring severe adverse impacts to the subsequent treatment of patients.11 PCT as a marker for bacterial infection has attracted more and more attentions in recent years.