2.1 Study population
The study included breastfeeding women with acute mastitis who were treated in Ningbo Medical Center Lihuili Hospital between January 2016 and August 2018. The inclusion criteria were the followings: (1) every patient was diagnosed with acute mastitis according to the widely accepted criteria of mastitis (see below); (2) blood sample for SLCs was routinely obtained in all patients directly or shortly after presentation to the ED, and if there was any suspicion of breast abscess, diagnostic breast ultrasound was performed; (3) conservative treatments including breast massage for emptying the breast, ibuprofen for analgesia, a hot compress applied prior to breastfeeding for stimulating milk flow, and a cool compress applied after breastfeeding for reducing swelling and pain, were performed on every patient; (4) common breast signs, including inflammatory characteristics of redness, swelling, heat and pain in the breast and systemic symptoms, including fever, chill, nausea and vomiting, were documented in medical records; (5) follow-up data were necessary, including the name of the antibiotic, the duration of the antibiotic, and the treatment outcome. If women attended the hospital for more than one episode of mastitis, we included only the first admission. The exclusion criteria included the followings: (1) nonpostpartum mastitis; (2) special breast inflammation, including plasma cell mastitis (PCM), granulomatous lobular mastitis (GLM) and breast tuberculosis; (3) the presence of breast abscess before treatment; (4) inflammatory breast carcinoma (IBC) with similar breast signs to acute mastitis; and (5) having received medicinal management for current mastitis before visiting the emergency department. The principles outlined in the Declaration of Helsinki were followed,and written informed consent was obtained from every patient.