The purpose of the work is to analyze the results of laboratory, microbiological and instrumental studies to search for screening criteria for the difference between aseptic and septic instability of endoprosthesis components after knee and hip joint arthroplasty. The materials contained 146 cases of revision arthroplasty of the knee and hip joints, performed under the conditions of Federal State Budgetary Institution Federal Center of Traumatology, Orthopedics and endoprosthesis replacement of Ministry of Health of the Russian Federation (Cheboksary) for a 3-year period, including cases of aseptic instability - 69, septic instability - 77. By methods for laboratory diagnosis were used to evaluate the blood levels of leukocytes, stab neutrophils, ESR, C - reactive protein (CRP), presepsin, procalcitonin, D-dimer, interleukin-6. Using ultrasound (ultrasonic) examination of the periprosthetic zone, the presence of fluid, granulation tissue in the joint cavity, altered lymph nodes was determined. Before the operation, the level of cytosis and cellular composition were determined in the synovial fluid; tissue bioptates, removed components of implants (after ULTRASONIC treatment) with sowing on the microflora were studied intraoperatively. Intraoperative tissue biopsies were used to make smears-prints on the glass, with gram staining, estimating the number of leukocytes and neutrophils in the field of vision. Results. Determination of indicators of ESR, presepsin and interleukin-6, the level of which goes beyond the generally recognized normal values in the presence of infection, can be recommended as a screening test in the diagnosis of the infectious nature of instability of the components of the endoprosthesis of large joints. The second stage of differential diagnosis of septic and aseptic instability can be the detection of lymphadenopathy. The third (final) stage of determining the infectious nature of instability is an invasive technique to determine the level of cytosis with the calculation of neutrophils in the synovial fluid.