EBM Consult

Lab Test: C-Reactive Protein, CRP (Serum) Level

    Lab Test
    • C-Reactive Protein (CRP)
    • Serum C reactive protein level
    • Measures serum C-reactive protein (CRP), an acute phase reactant that is a biomarker of inflammation.  It is believed to be of value in predicting coronary events.
    Reference Range
    • Adults:  0.08-3.1 mg/L
    • Adults (high sensitivity):  0.02-8 mg/L
    • Cardiac risk: 
      • Low:  <1.0 mg/L 
      • Average:  1.0 to 3.0 mg/L 
      • High:  >3.0 mg/L
    Indications & Uses
    • Suspected meningococcemia in children with associated septic shock and purpura - have moderate diagnostic but not prognostic value for meningococcal disease
    • Bacterial meningitis - CPR levels may be increased up to 100-fold or more.  Most useful for monitoring patient response to therapy after primary diagnosis of invasive infectious or inflammatory disease
    • Community-acquired pneumonia - CRP is elevated in about 25% of patients with pneumonia, but it is not helpful in discriminating between viral and bacterial pneumonia in children.  May be a useful adjunct for diagnosing Legionella pneumonia.  The odds of having a CRP level  > 25 mg/dL almost 7 times higher in patients with Legionella infection than in those with pneumonia due to other causes.
    • Coronary artery disease - CRP levels > 10 mg/L increase risk of death 4-fold compared with CRP levels < 3 mg/L.  A serum CRP concentration above 3 mg/L correlates with a poorer prognosis in patients with unstable angina.
    • Rheumatoid arthritis - CRP > 1 mg/dL indicates an inflammatory process but it is nonspecific.
    • Serum marker to assess the severity of acute pancreatitis - CRP levels > 150 mg/dL at 48 hours predict disease severity.
    • Suspected and known Kawasaki disease - CRP level of 3 mg/dL or greater is found in nearly all cases of Kawasaki disease during the cute febrile stage.  CRP > 10 mg/dl have been associated with a poor response to intravenous immunoglobulin.
    • Suspected and known malaria - CRP level correlates well with the severity of P falciparium  malaria and is useful in following the response to therapy.
    • Suspected and known osteomyelitis - CRP elevated in 90% of children with osteomyelitis overall and in almost all patients with vertebral osteomyelitis.
    • Suspected appendicitis - CRP elevated in relation to severity of appendiceal inflammation
    • Suspected pelvic inflammatory disease (PID) - an elevated CRP level supports the diagnosis of PID Suspected sepsis - CRP level > 2 standard deviations above normal is an indicator of inflammation.   CRP is elevated at the onset of infection (greater than 20 mg/L) and will increase during the initial course, then decrease as the patient recovers.
    Clinical Application

    CRP levels do not consistently rise with viral infections.  CRP is a protein produced primarily by the liver during an acute inflammatory process and other diseases.  A positive test result indicates the presence, but not the cause, of the disease.  The synthesis of CRP is initiated by antigen-immune complexes, bacteria, fungi, and trauma.  CRP is functionally analogous to immunoglobulin G, except that it is not antigen specific.  CRP interacts with the complement system. 

    The CRP test is a more sensitive and rapidly responding indicator than the erythrocyte sedimentation rate (ESR). 

    Increased levels indicate:

    • Acute, noninfectious inflammatory reaction (e.g., arthritis, acute rheumatic fever, Reiter syndrome, Crohn disease)
    • Collagen-vascular diseases (e.g., vasculitis syndrome, lupus erythematosus)
    • Tissue infarction or damage (e.g., acute myocardial infarction {AMI}, pulmonary infarction, kidney or bone marrow transplant rejection, soft-tissue trauma)
    • Bacterial infections such as postoperative wound infection, urinary tract infection, or tuberculosis
    • Malignant disease
    • Bacterial infection (e.g., tuberculosis, meningitis)
    • Increased risk for cardiovascular ischemic events
    Related Tests
    • Erythrocyte sedimentation rate - nonspecific test used to detect inflammatory, infectious, and necrotic processes.
    • Complement assay
    • Feibrinogen
    • Lipoproteins - important risk factor for heart disease
    • Homocysteine - important risk factor for heart disease
    Drug-Lab Interactions
    • Elevated test results can occur in patients with hypertension, elevated body mass index, metabolic syndrome/diabetes mellitus, chronic infection (gingivitis, bronchitis), chronic inflammation (rheumatoid arthritis), and low high-density lipoprotein (HDL)/high triglycerides. 
    • Cigarette smoking can cause increased levels.
    • Decreased test levels can result from moderate alcohol consumption, weight loss, and increased activity or endurance exercise. 
    • Medications that may increase test results include estrogens and progesterones.
    • Medications that may decrease test results include fibrates, niacin, and statins.
    Test Tube Needed
    • Red top tube
    • Collect serum sample
    • Avoid hemolytic and lipemic specimens
    • Apply pressure or a pressure dressing to the venipuncture site and assess the site for bleeding.
    Storage and Handling
    • Analyze fresh specimen or store at 4°C or less than 72 hours
    • Specimen is stable frozen at -20°C for 6 months or indefinitely at -70°C.
    What To Tell Patient Before & After
    • Explain the procedure to the patient.
    • Tell the patient that fasting usually is not required; however some laboratories require a 4- to 12-hour fast.  Water is permitted.
    • LaGow B et al., eds. PDR Lab Advisor. A Comprehensive Point-of-Care Guide for Over 600 Lab Tests.  First ed. Montvale, NJ: Thomson PDR; 2007.
    • Pagana K, Pagana TJ eds. Mosby's Manual of Diagnostic and Laboratory Tests. 5th Ed.  St. Louis, Missouri. 2014.

MESH Terms & Keywords

  • C-Reactive, Protein, CRP