Analysis of periprosthetic aspirate in the diagnosis of prosthetic joint infection

Abstract

Background: Complications during the life span of an artificial implant in prosthesis joint implant surgery (arthroplasty) are aspetic biomechanical loosening (ABL) and prosthesis joint infection (PJI). Aseptic loosening is the most common cause of prosthetic joint failure, followed by infection. It is important to differentiate between these two entities because their management differs. Combination of pre- and peri-operative diagnostic methods is usefull. It is ideal to diagnose or exclude PJI preoperatively to facilitate an appropriate surgical management and initial choice of systemic antimicrobial agents. At this point preoperative examination of a joint aspirate for cell count and differential as well as for microbiological culture is helpfull for differentiating a septic from an aseptic process. Th e cut-off values for leukocyte count and differential for diagnosing PJI are established and diff er from those applied for native joints. Methods: The pre- and peri-operative cytological joint aspirate examination comprises leukocyte count and differential with PMN % (polimorphonuclear leucocyte). After synovial aspiration the sample is drawn into a lavander blood test tube containing K2EDTA solid anticoagulant. Th e examination in the medical laboratory is completed in 1-2 hours. Th e gross appearance and a microscopic examination of the sample under 400 xmagnifi cation is sought to assess the cells present in the sample. All samples for the cytological examination are first treated with hyaluronidase solution. Th e leukocytes are counted in the Neubauer’s counting chamber using a leukocyte counting solution (2–3% aq.solgentiana violet in acetic acid). Th e result is expressed in SI units (in 109/L). Th e differential count is made from a stained smear according to Pappenheim and minimum 200 cells are differentiated. Th e result is exspressed as a percentage of PMN and other types of white blood cells. Conclusion: Pre- and peri-operative joint aspirate cytological examination with leukocyte count and differential is a valuable diagnostic tool for differentiating a prosthesis joint infection from an aseptic loosening. We presume that a reported joint aspirate leukocyte count of > 1.7 x 109/L and differential of > 65% PMN have adequate specificity (88% and 98%) and sensitivity (94% and 97%) for diagnosing PJI regardless the type of implant. Th e exact values for these diagnostic modalities, defining also NPV (negative predictive value) and PPV (positive predictive value), will be possible after the completion of our study.