Ok Doc, So, What Exactly Is CRP?

• C-reactive protein (CRP) is an acute-phase reactant produced by the liver in response to inflammation.

So, What Is CRP Telling Me?

• CRP reflects inflammation—especially bacterial.
• It rises quickly (within 6–8 hours) and can peak within 48 hours of an inflammatory stimulus.
• It falls with treatment, so it’s also helpful for monitoring trends in admitted patients.
• It won’t tell you where the problem is—but it tells you there is a problem, often before other tests do.

When Does CRP Help in the ED?

• When everything looks “okay”
  • Normal vitals, reassuring WBC, and no fever don’t rule out bacterial infection—especially in the elderly, immunocompromised, or infants.
  • CRP >100 mg/L should trigger a deeper search for source.
• When imaging is negative or borderline
  • Negative CXR in early pneumonia?
  • Benign abdomen with vague pain?
  •  Limping child with normal x-ray?
  •  CRP doesn’t diagnose, but it keeps you from missing the forest for the trees.
• When you’re deciding about discharge
  • A CRP <20 mg/L in a well-appearing, afebrile patient can support discharge, especially when your gut says “probably viral.”
  • A CRP >100 mg/L with no clear source? You need to dig—or keep them.

Ok Doc, but how do I Actually Use CRP in Practice

• Undifferentiated Fever
  • CRP <20 mg/L: Likely viral, often no antibiotics needed.
  • CRP >100 mg/L: Likely bacterial—consider empiric antibiotics and search for source.
• Sepsis Evaluation
  • Use CRP alongside lactate, vitals, and WBC to assess severity.
  • CRP >200–300 mg/L often signals severe infection.
  • Serial CRPs help track response after source control.
• Respiratory Infections
  • CRP <20 mg/L: Likely viral—hold antibiotics.
  • CRP >100 mg/L: Likely bacterial—even if CXR is borderline.
  • CRP 20–100 mg/L: Use clinical judgment and have follow up
• Abdominal Pain & Possible Surgical Abdomen
  • Appendicitis: CRP >50 mg/L supports the diagnosis.
  • Diverticulitis: High CRP with localized tenderness is suggestive.
  • Ischemia or perforation: CRP >150–200 mg/L raises suspicion.
• Pediatric Fever Without Source (≥ 3 months)
  • CRP >40–60 mg/L increases concern for serious bacterial infection, especially with abnormal WBC or PCT.
• Back Pain
  • Elevated CRP points to an infectious cause

Take Home Points

• CRP >100 but normal exam? Don’t discharge without knowing why it’s high.
• CRP <20? If everything else looks viral, it probably is.
• In vague cases—especially elderly or pediatric—CRP can be the only early red flag.
• Always interpret CRP in clinical context. It’s not the answer—but it nudges you toward it.

References

Aabenhus, Rune, et al. “Biomarkers as Point-of-Care Tests to Guide Prescribing of Antibiotics in Patients with Acute Respiratory Infections in Primary Care.” Cochrane Database of Systematic Reviews, no. 11, 2024, CD010130. 

Froom, Paul, and Margalit Shimoni. “C-Reactive Protein.” Clinical Biochemistry, vol. 49, no. 4–5, Mar. 2016, pp. 291–292. 

Pepys, Mark B., and Gideon M. Hirschfield. “C-Reactive Protein: A Critical Update.” The Journal of Clinical Investigation, vol. 111, no. 12, June 2003, pp. 1805–1812. 

Sproston, Nicola R., and Jeremy J. Ashworth. “Role of C-Reactive Protein at Sites of Inflammation and Infection.” Frontiers in Immunology, vol. 9, 18 May 2018, p. 754. 

Uzzan, Boris, et al. “Procalcitonin and C-Reactive Protein as Prognostic Markers of Sepsis.” Critical Care Medicine, vol. 51, no. 3, 2023, pp. 356–365. 

Virk, Ajit, and Jonathan M. W. Chan. “C-Reactive Protein as a Biomarker of Bacterial Infection.” British Journal of Hospital Medicine, vol. 78, no. 7, July 2017, pp. 399–402.

1. Transient Tenosynovitis
2. Guillain-Bare Syndrome
3. Muscular Dystrophy
4. Benign Acute Childhood Myositis

Who does Benign Acute Childhood Myositis (BACM) tend to affect?

BACM usually affects school-aged children, typically around 6-8 years old and usually more in BOYS, though it can occur in both genders.

How does it present? How is it diagnosed?

• BACM is usually a clinical diagnoses and is based on the clinical presentation of
  • History of recent viral infection – Fever, sore throat
  • Symmetric bilateral leg / calf pain,
    • Often a “stiff legged gait” or hesitant to walk
    • Pain is WORSE after rest
  • Normal or slightly elevated temperature

Are there any tests that are useful in making this diagnosis?

• Creatinine Kinase (CK) is very useful because it is usually elevated in BACM
• Typical tests to order:
  • CBC, Cr, CK, CRP, Urea, Urine analysis
    • Note: WBC and platelet counts are typical normal or decreased
  • Pro Tip: Check Cr and Urine to make sure the rhabdomyolysis (a complication of BACM) is not also present
    • Cr – would be elevated
    • Urine – would show signs of myoglobinuria (tea coloured)

Hey Doc, Transient Synovitis also causes a limp after a febrile illness. What are the main differences between transient synovitis and Benign Acute Childhood Myositis?

• Transient Synovitis is
  • UNILATERAL, and
  • NO CK involvement

Are there any complications of BACM?

• Complications are quite RARE.
• The most common complication, if there is one, is rhabdomyolysis, and rarely acute kidney injury if the rhabdomyolysis is severe

What is the prognosis or outcomes for BACM?

• Prognosis for BACM is Excellent,
•  Most children have a full recovery with fluids and pain control
• No long-term effects and recurrence is extremely rare

Take Home Points

• Benign Acute Childhood Myositis (BACM) also known as Viral Myositis is COMMON
• It typically follows a viral infection and tends to occur in BOYS 6 – 8 yrs
• It is SELF RESOLVING majority of the time and just needs Fluids and Pain control
• CK tends to be elevated,
• Complications such as rhabdomyolysis is RARE
• Hospitalization is rarely required and most children are back to normal in a couple of days

References

Brisca, G., Mariani, M., Pirlo, D. et al. Management and outcome of benign acute childhood myositis in pediatric emergency department. Ital J Pediatr 47, 57 (2021).

Magee H, Goldman RD. Viral myositis in children. Can Fam Physician. 2017 May;63(5):365-368.

Skrzypczyk P, Przychodzień J, Pańczyk-Tomaszewska M. Benign acute childhood myositis complicating influenza B infection in a boy with idiopathic nephrotic syndrome. Cent Eur J Immunol. 2016;41(3):328-331.

Terlizzi V, Improta F, Raia V. Simple diagnosis of benign acute childhood myositis: Lessons from a case report. J Pediatr Neurosci.