The C-Reactive Protein May Not Detect Infections Caused by Less-Virulent Organisms

Abstract Background The aim of the present study was to evaluate the influence of organism type on the performance of the synovial fluid C-reactive protein (CRP) test. Methods We retrospectively reviewed the results of 21,422 synovial fluid samples sent to one common laboratory for the purpose of diagnostic testing for periprosthetic joint infection. Both a synovial fluid CRP result and a positive culture were present for 1789 submitted samples. The cultured organisms were grouped by species, virulence, and gram type; and the median CRP level was determined for each group. Results The median synovial fluid CRP level was significantly lower for less-virulent organisms, when compared to those organisms classified as virulent (15.10 mg/L vs 32.70 mg/L; P Staphylococcus epidermidis were associated with a 4-10 times lower CRP response than those of virulent organisms such as Streptococcus agalactiae and Staphylococcus aureus ( P S. epidermidis , 28.5% for all less-virulent organisms, and 11.6% for all virulent organisms. Conclusion The CRP response appears to be highly dependent on the infecting organism and is more likely to provide false-negative results in the setting of less-virulent organisms. Although the use of a CRP level is an important part of the workup for periprosthetic joint infection, surgeons must be aware that this protein may yield a false-negative result in the setting of less-virulent organisms.