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

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 < .0001). Some less-virulent species such as yeast and 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 < .0001). Bacterial gram type had no influence on the median CRP result. The rate of false-negative CRP values was 50.9% for yeast, 29.4% for 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.