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



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.



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.



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.
The Rothman Institute, Thomas Jefferson University, The Lankenau Institute for Medical Research, Wynnewood, Pennsylvania; CD Diagnostics, Claymont, Delaware.
Carl A
CA
Deirmengian
CD Diagnostics, Claymont, Delaware.
Patrick A
PA
Citrano
CD Diagnostics, Claymont, Delaware.
Simmi
S
Gulati
University of Michigan Medical School, Ann Arbor, Michigan.
Erick R
ER
Kazarian
CD Diagnostics, Claymont, Delaware.
James W
JW
Stave
CD Diagnostics, Claymont, Delaware.
Keith W
KW
Kardos
D001170:Arthritis, Infectious / Q000175:diagnosis* / Q000382:microbiology; D002097:C-Reactive Protein / Q000032:analysis*; D054884:Host-Pathogen Interactions; D006801:Humans; D016459:Prosthesis-Related Infections / Q000175:diagnosis* / Q000382:microbiology; D012189:Retrospective Studies; D013211:Staphylococcus aureus / Q000502:physiology; D013212:Staphylococcus epidermidis / Q000502:physiology; D013582:Synovial Fluid / Q000737:chemistry* / Q000382:microbiology
D002097:C-Reactive Protein
C-reactive protein; bacterial infection; diagnostic testing; periprosthetic joint infection; revision arthroplasty