Clinical usefulness of automated cellular analysis of synovial fluids: a paradigmatic case report for diagnosing peri-prosthetic infections

We describe here the case of an 18 years old male patient who underwent osteosynthesis surgery with a plate and screws for multi-fragment fracture of his right femur, in December 2000. The patient developed a series of complications up to May 2017, when he was readmitted with severe functional impairment and inability to walk without crutches. Physical examination revealed erythema, swelling at surgical site and leakage of pus from sinus-tract. An arthrocentesis was hence performed, followed by white blood cell count and differential in synovial fluid with Sysmex XN 2000 body fluid mode. The final leukocyte count was 38×10⁹/L, with 95% polymorphonuclear leukocytes (PMN), thus compatible with peri-prosthetic infection. The patient underwent additional surgery to remove the knee replacement, accurate debridement and antibiotic impregnated static cement spacer implantation. Three months after surgery, the patient was readmitted for removing the spacer, replaced by a new static device. In May 2018, surgical debridement and removal of the spacer was scheduled with resection arthroplasty. In November 2018 the patient presented again a sinus tract, and underwent another arthrocentesis for physical and microbiological examination of synovial fluid. The leukocyte count in the synovial fluid was again performed with Sysmex XN 2000 body fluid mode, and revealed the presence of 44.5×10⁹/L cells, with 90% PMN. The synovial fluid was positive for Van-A Enterococcus faecalis, so that right hip disarticulation was urgently planned. In conclusions, this case provides clear evidence that automated leukocyte count and differential in synovial fluid shall now be considered an unavoidable part of the diagnostic approach to patients with suspected peri-prosthetic infections.