C-reactive protein

Normal Values

• Post-spinal surgery, it peaks at 15 ± 25 mg/L a few days after OR
• Post-stroke, it can go up to 25 ± 10 mg/L
• Post-TKA, it can go up to 155 mg/L

Interpretation

• The higher the value, the more likely to be a bacterial infection
  • Non-infectious chronic inflammation, such as from smoking, uremia, or cardiac ischemia: 2 to 10 mg/L
  • Mild to moderate infection such as SSTI, cystitis or bronchitis: increases to 50 to 100 mg/L within 6 hours
• Typically begins to rise after 12 to 24 hours and has a half-life of about 19 hours

Condition	Cutoff	Sensitivity	Specificity	Notes
Diabetic foot osteomyelitis	>10	0.85	0.59	near-normal by day 7 to 21 of treatment1
	>14	0.84	0.83
	>17	0.77	0.89
Necrotizing SSTI	>150			included in LRINEC score
Prosthetic joint infection	>10	0.96	0.92	2
Spondylodiscitis				normalizes within 3 months of treatment
Septic arthritis	≥20	0.92		3

Comparison to ESR

ESR	CRP	Clinical Scenarios
high	low	rheumatoid arthritis, infection, low albumin, elevated creatinine
low	high	low albumin

• CRP less effected by older age than ESR

Hemodialysis

• Theoretically removed with hemodialysis, but CRP is often elevated in stable patients on hemodialysis and peritoneal dialysis

Further Reading

• Acute Phase Reactants in Infections: Evidence- Based Review and a Guide for Clinicians. Open Forum Infect Dis. 2015;2(3):ofv098. doi: 10.1093/ofid/ofv098.

Spine

• Serum C-reactive protein levels correlate with clinical response in patients treated with antibiotics for wound infections after spinal surgery. Spine J. 2006;6(3):311-315.
  • CRP better than ESR

Joints

• Serial measurement of the C-reactive protein is a poor predictor of treatment outcome in prosthetic joint infection. J Antimicrob Chemo. 2011;66(7);1590-1593.
  • Although the population of patients with failure had higher CRP on average, CRP measurement was not useful for predicting failure in individual patients.
• C-Reactive Protein, Erythrocyte Sedimentation Rate and Orthopedic Implant Infection. PLoS ONE. 2010:5(2):e9358.
  • ESR and CRP have poor Sn and Sp for diagnosing hardware infections
• C-reactive protein may misdiagnose prosthetic joint infections, particularly chronic and low-grade infections. International Orthopaedics. 2017.
  • 1/3 of culture-positive PJIs had normal CRP and 1/4 had normal ESR.
  • Most common associated organisms were coagulase-negative staphylococci and C. acnes
• Seronegative infections in hip and knee arthroplasty: periprosthetic infections with normal erythrocyte sedimentation rate and C-reactive protein level
  • 4% of PJI have normal CRP and ESR
  • CoNS was more common, Staph. aureus less common

Other

• Prognostic Value of Serial C-Reactive Protein Measurements in Left-Sided Native Valve Endocarditis. Arch Intern Med. 2008;168(3):302-307.
  • After 1 week following starting therapy, a CRP over 122 predicts failure (OR 10)
• Utility of C-reactive protein measurements for empyema diagnosis after pneumonectomy. Annals Thoracic Surg. 1994;57(4):933-936.
  • Post-operative rise or high-plateau of CRP predicted postoperative infection (empyema)
• Diagnostic Accuracy of C-reactive Protein for Intraabdominal Infections After Colorectal Resections. J Gastrointest Surg. 2009;13(9):1599-1606.
  • Post-resection day 3 (or 5 or 7) CRP predicts intraabdominal infection, with Sn 82% and Sp 73% using a cutoff of 190 mg/L (normal <10)