title: Leukocyte esterase analysis in the diagnosis of joint infection: can we make a diagnosis using a simple urine dipstick?
abstract: OBJECTIVE
  Analysis of joint fluid remains a key factor in the diagnosis of periprosthetic infection. Recent reports have shown that neutrophils in infected joint fluid release esterase, an enzyme that is a reliable marker for infection. Testing for leukocyte esterase is routinely done in the analysis of urine for the presence of urinary tract infection, by a simple "dipstick" method. We report our experience with this technique in the evaluation of patients suspected of having septic arthritis or periprosthetic joint infection (PJI) by comparing results of leukocyte esterase positivity with confirmed joint infection as defined by the American Academy of Orthopaedic Surgeons (AAOS).
  
  
  METHODS
  We retrospectively reviewed leukocyte esterase test results performed on synovial fluid aspirated from 57 patients with prosthetic (52) and native (5) joints. Patients either presented with unexplained painful arthroplasties, routine testing of PROSTALAC (PROSthesis with Antibiotic-Loaded Acrylic Cement) orthopedic implants, or clinical suspicion of periprosthetic infection or septic arthritis. Synovial fluid was percutaneously aspirated using a standard technique. The patient age range was 31-91 years with a mean age of 69.1 years, consisting of 30 women (52.6 %) and 27 men (47.4 %). The "gold standard" for the presence or absence of infection at our institution and in the study group was based on the most recent recommendations of the AAOS. Positive culture remained the "gold standard" for native joint infection.
  
  
  RESULTS
  Of the total 57 joints aspirated and included in the study, 20 (35.1 %) were read as positive (2+) on the leukocyte test strip and 37 (64.9 %) were read as negative (negative, trace, or 1+). PJI was diagnosed in 19 patients and native joint septic arthritis was identified in one patient. Sensitivities were excellent at 100 % with no false negatives in the entire cohort. There was one false positive in the periprosthetic group yielding a specificity, positive predictive value and negative predictive value of 97, 95, and 100 %, respectively. The results for the native joints showed markedly less specificity and positive predictive value at 50 and 33 %; however, its negative predictive value remained at 100 %.
  
  
  CONCLUSIONS
  Our test results confirm that the leukocyte esterase test can accurately detect PJI and that it can be used as a part of the traditional PJI workup. In the assessment of native joints, its high negative predictive value suggests that it is a valuable tool in excluding native joint septic arthritis.
authors.affiliation: Department of Radiology, Mayo Clinic, 5777 E Mayo Boulevard, Phoenix, AZ, 85054, USA.
authors.forename: Otis C
authors.initials: OC
authors.lastname: Colvin
authors.forename: Mark J
authors.initials: MJ
authors.lastname: Kransdorf
authors.forename: Catherine C
authors.initials: CC
authors.lastname: Roberts
authors.forename: F Spencer
authors.initials: FS
authors.lastname: Chivers
authors.forename: Roxanne
authors.initials: R
authors.lastname: Lorans
authors.forename: Christopher P
authors.initials: CP
authors.lastname: Beauchamp
authors.forename: Adam J
authors.initials: AJ
authors.lastname: Schwartz
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001170:Arthritis, Infectious / Q000175:diagnosis* / Q000201:enzymology; D015415:Biomarkers / Q000032:analysis; D002265:Carboxylic Ester Hydrolases / Q000032:analysis*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D016459:Prosthesis-Related Infections / Q000175:diagnosis* / Q000201:enzymology; D011934:Reagent Strips; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013582:Synovial Fluid / Q000737:chemistry*; D016482:Urinalysis / Q000295:instrumentation*
chemical_list: D015415:Biomarkers; D011934:Reagent Strips; C021443:leukocyte esterase; D002265:Carboxylic Ester Hydrolases