Hip and Knee Section, Diagnosis, Definitions: Proceedings of International Consensus on Orthopedic Infections Derek Amanatullah 1 , Douglas Dennis 2 , Ester Garcia Oltra 1 , Luiz S. Marcelino Gomes 3 , Stuart B. Goodman 1 , Brian Hamlin 4 , Erik Hansen 5 , Aresh Hashemi-Nejad 6 , Georgios Komnos 7 , Antonios Koutalos 7 , Konstantinos Malizos 7 , Juan C. Martinez Pastor 1 , Ed McPherson 4 , Geert Meermans 4 , Jake A. Mooney 1 , Javad Mortazavi 5 , Ali Parsa 2 , Jos  e Ricardo P  ecora 2 , Gilberto A. Pereira 3 , Marta Sabater Martos 1 , Noam Shohat 3 , Alexander J. Shope 6 , Sergio S. Zullo 3 article info Article history: Available online xxx Keywords: leukocyte esterase (LE) c-reactive protein (CRP) erythrocyte sedimentation rate (ESR) synovial white cell count synovial polymorphonuclear (PMN) percentage histology positive cultures septic arthritis septic arthritis diagnosis septic arthritis definition septic arthritis criteria surgical site infection (SSI) superficial surgical site infection (SSI) deep surgical site infection (SSI) toxic synovitis periprosthetic joint infection (PJI) clinical findings periprosthetic joint infection (PJI) clinical features painful prosthetic joint sinus tract purulence intraoperative purulence abscess necrosis fever erythema aseptic loosening culture-negative periprosthetic joint infection (PJI) undiagnosed periprosthetic joint infection (PJI) infection grade McPherson schema Question 1: What is the definition of septic arthritis in a native knee? Recommendation: Native septic arthritis of the knee is a clinical diagnosis supplemented by relevant laboratory data. Signs of septic arthritis include painful effusion, limited range of motion, and warmth. Elevated serum inflammatory markers, particularly C- reactive protein (CRP), synovial white cell counts (50,000 cells/ mm 3 ), polymorphonuclear (PMN) cell count percentages (>90%), and purulent appearance of the synovial fluid, indicate a high likelihood of septic arthritis. Level of Evidence: Moderate Delegate Vote: Agree: 92%, Disagree: 7%, Abstain: 1% (Super Majority, Strong Consensus) Rationale: Native septic arthritis of the knee classically presents with a painful effusion and limited range of motion. Diagnosis of this clinical entity cannot be made on the basis of laboratory data alone, with infections occurring in the presence of negative cultures, and absent in the presence of markedly elevated intra-articular cell counts [1]. The frequency of native knee septic arthritis appears to be increasing, and major concerns for serious medical compli- cations and mortality persist [2]. The most robust information on laboratory data diagnostic for septic arthritis is available for the One or more of the authors of this paper have disclosed potential or pertinent conflicts of interest, which may include receipt of payment, either direct or indirect, institutional support, or association with an entity in the biomedical field which may be perceived to have potential conflict of interest with this work. For full disclosure statements refer to https://doi.org/10.1016/j.arth.2018.09.044. 1 Question 6. 2 Question 1. 3 Question 4. 4 Question 7. 5 Question 5. 6 Question 3. 7 Question 2. Contents lists available at ScienceDirect The Journal of Arthroplasty journal homepage: www.arthroplastyjournal.org https://doi.org/10.1016/j.arth.2018.09.044 0883-5403/© 2018 Published by Elsevier Inc. The Journal of Arthroplasty xxx (2018) 1e9