Background The diagnosis of periprosthetic joint infection (PJI) in patients with failed metal-on-metal (MoM) bearings and corrosion reactions in hip arthroplasties can be particularly difficult, because the clinical presentation of adverse local tissue reactions may mimic that of PJI, because it can also occur concurrently with PJI, and because common laboratory tests used to diagnose PJI may be elevated in patients with MoM THAs. Questions/purposes We sought to determine the test properties of the serum erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), synovial fluid white blood cell (WBC) count, and synovial fluid differential (percent polymorphonuclear cells [PMNs]) in diagnosing One of the authors certifies that he (CJDV), or a member of his or her immediate family, has or may receive payments or benefits, during the study period, an amount of USD 10,000 to USD 100,000 from Biomet, Inc (Warsaw, IN, USA), an amount less than USD 10,000 from Convatec (Skillman, NJ, USA), an amount of USD 10,000 to USD 100,000 from DePuy (Warsaw, IN, USA), an amount of USD 10,000 to USD 100,000 from Smith & Nephew, Inc (Memphis, TN, USA), and an amount of USD 10,000 to USD 100,000 from CD Diagnostics (Wynnewood, PA, USA). One of the authors certifies that he (CJDV), or a member of his or her immediate family, has or may receive funding, during the study period, an amount USD 10,000 to USD 100,000 from Stryker (Kalamazoo, MI, USA). One of the authors certifies that he (BRL), or a member of his or her immediate family, has or may receive payments or benefits, during the study period, an amount less than USD 10,000 from CONMED Linvatec (San Mateo, CA, USA), an amount of USD 10,000 to USD 100,000 from DePuy, an amount of USD 10,000 to USD 100,000 from Biomet, Inc, an amount of USD 10,000 to 100,000, from Zimmer, Inc (Warsaw, IN, USA), an amount of less than USD 10,000 from Janssen Pharmaceuticals (Beerse, Belgium), and an amount of less than USD 10,000 from Orthoview (Jacksonville, FL, USA). One of the authors certifies that he (SMS), or a member of his or her immediate family, has or may receive payments or benefits, during the study period, an amount of USD 10,000 to 100,000, from Zimmer, Inc, an amount of USD 10,000 to 100,00 from Cadence Health (Winfield, IL, USA), an amount of USD 10,000 to USD 100,000 from Smith & Nephew, Inc, and an amount of less than USD 10,000 from SLACK Incorporated (Thorofare, NJ, USA). One of the authors (WGP) certifies that he has received or may receive payments or benefits, during the study period, an amount of USD 100,001-USD 1,000,000, from Zimmer, Inc, an amount of USD 100,001 to USD 1,000,000, from Stryker, an amount of USD 100,001 to USD 1,000,000, from DePuy, an amount of less than USD 10,000 from Medtronic (Minneapolis, MN, USA), and an amount of USD 10,000 to USD 1,000 from Lippincott (Philadelphia, PA, USA). One of the authors (JJJ) certifies that he has received or may receive payments or benefits, during the study period, an amount of USD 10,000 to USD 100,000 from ...