title: Results of laboratory evaluation of acute knee effusion after anterior cruciate ligament reconstruction: what is found in patients with a noninfected, painful postoperative knee?
abstract: BACKGROUND
  Infection after anterior cruciate ligament reconstruction is a rare and potentially devastating complication. No normative data have been reported for knee aspiration after anterior cruciate ligament reconstruction in the early postoperative period.
  
  
  OBJECTIVE
  Determining normative laboratory data from a retrospective review of noninfected early postoperative anterior cruciate ligament reconstruction knee effusions will allow for the calculation of an aspirate white blood cell (WBC) threshold value indicative of infection.
  
  
  METHODS
  Case series (diagnosis); Level of evidence, 4.
  
  
  METHODS
  A 2-year retrospective chart review of 151 anterior cruciate ligament reconstruction patients was performed. Thirty-one noninfected patients meeting the inclusion and exclusion criteria and 1 infected patient had laboratory data collected, including peripheral blood and knee effusion aspirate analyses. Laboratory data from pertinent published studies of infected knees after anterior cruciate ligament reconstruction were combined with the data of our 1 infected patient, establishing a historical control group. Data were analyzed and results were then compared. Infected aspirate WBC threshold value statistics were then calculated.
  
  
  RESULTS
  Analysis of noninfected knee effusion aspirates revealed a mean WBC count of 9600/uL (standard deviation [SD], 15 200), and a mean of 66% polymorphonuclear (PMN) cells (SD, 34). Aspirate WBC 98% confidence interval (CI) was 2800/uL to 16 200/uL, and the 98% CI for PMN cells was 58% to 84%. Aspirate WBC count >16 200/uL is 86% sensitive, 92% specific, and has a positive likelihood ratio of 10.4 as an indicator of infection.
  
  
  CONCLUSIONS
  Benign effusion after anterior cruciate ligament reconstruction is common and is associated with elevated inflammatory markers. When concerned, knee aspiration after anterior cruciate ligament surgery gives the highest yield to differentiate between a painful effusion and a septic knee in the early postoperative period while awaiting definitive culture results. The authors report confidence intervals defining the range of cell count variables for noninfected patients requiring aspiration, specifically WBC and PMN, and suggest a WBC threshold value of >16 200/uL be used as an indicator of infection. On the basis of comparison with historical control data, the authors believe these data are significant and will be reliable for clinical use.
authors.affiliation: Department of Orthopaedics, Stony Brook University, Health Sciences Center, Level 18, Room 020, Stony Brook, NY 11794-8181, USA. james_paci@yahoo.com
authors.forename: James M
authors.initials: JM
authors.lastname: Paci
authors.forename: Scott K
authors.initials: SK
authors.lastname: Schweizer
authors.forename: Danielle M
authors.initials: DM
authors.lastname: Wilbur
authors.forename: Levi G
authors.initials: LG
authors.lastname: Sutton
authors.forename: Frederick W
authors.initials: FW
authors.lastname: Werner
authors.forename: Matthew G
authors.initials: MG
authors.lastname: Scuderi
authors.forename: John P
authors.initials: JP
authors.lastname: Cannizzaro
mesh_terms: D000208:Acute Disease; D000070598:Anterior Cruciate Ligament Injuries; D018771:Arthralgia / Q000209:etiology / Q000382:microbiology; D001265:Athletic Injuries / Q000382:microbiology / Q000601:surgery*; D044963:Biopsy, Fine-Needle; D016001:Confidence Intervals; D005260:Female; D006801:Humans; D007239:Infections / Q000209:etiology* / Q000382:microbiology; D007718:Knee Injuries / Q000209:etiology* / Q000382:microbiology; D007962:Leukocytes; D008297:Male; D011183:Postoperative Complications / Q000209:etiology* / Q000382:microbiology; D019651:Plastic Surgery Procedures / Q000009:adverse effects*; D012189:Retrospective Studies; D012307:Risk Factors; D012680:Sensitivity and Specificity; D013997:Time Factors; D055815:Young Adult