What are the typical laboratory findings in appendicitis?

Laboratory Findings in Appendicitis

The most valuable laboratory findings in appendicitis are elevated white blood cell count (WBC) and C-reactive protein (CRP), particularly when used in combination, which provides a positive likelihood ratio of 23.32 for acute appendicitis. 1

Key Laboratory Findings

White Blood Cell Count (WBC)

• Elevated WBC (>10,000/mm³) has a positive likelihood ratio of 2.47 for appendicitis 1, 2
• Higher WBC (>15,000/mm³) increases positive likelihood ratio to 3.47 1
• WBC alone has limited diagnostic value with sensitivity of 76% and specificity of 52% 3
• Normal WBC does not exclude appendicitis, as laboratory results may be normal in early disease 1

C-Reactive Protein (CRP)

• CRP level ≥10 mg/L has a positive likelihood ratio of 4.24 for acute appendicitis 1, 2
• CRP is particularly valuable in pediatric patients, with levels ≥10 mg/L being a strong predictive factor 1
• Like WBC, CRP alone has limited diagnostic accuracy 4

Combined Laboratory Markers

• The combination of WBC >10,000/mm³ and CRP >8 mg/L is highly predictive:
  • Positive likelihood ratio: 23.32 (95% CI 6.87 to 84.79)
  • Negative likelihood ratio: 0.03 (95% CI 0.0 to 0.14) 1, 2
• When all inflammatory markers (WBC, neutrophil count, and CRP) are within normal range, acute appendicitis is highly unlikely 5

Neutrophil Count

• Elevated absolute neutrophil count is a useful marker 2
• Neutrophilia (>75%) improves diagnostic accuracy 2
• In pediatric patients, absolute neutrophil count >6,750/mm³ is significantly associated with acute appendicitis 1

Special Considerations

Pediatric Patients

• WBC ≥16,000/mL is a strong predictive factor for appendicitis in children 1
• CRP level ≥10 mg/L has high predictive value in children 1
• Absolute neutrophil count is particularly valuable in pediatric diagnosis 1

Laboratory Tests in Clinical Scoring Systems

• Laboratory values are incorporated into clinical scoring systems like:
  • Alvarado Score: includes WBC >10,000/mm³ and neutrophilia >75% 1, 2
  • Appendicitis Inflammatory Response (AIR) Score: incorporates CRP values 2
  • Pediatric Appendicitis Score (PAS): includes laboratory parameters 2

Diagnostic Pitfalls

• Relying solely on a single laboratory test (either WBC or CRP) has poor sensitivity and specificity 3, 4
• Laboratory tests may be normal in early appendicitis 1
• Body temperature >99°F has minimal statistical association with appendicitis (sensitivity 47%, specificity 64%) 3
• Laboratory values should always be interpreted in conjunction with clinical findings 1

Diagnostic Algorithm

1. Obtain both WBC with differential and CRP in all patients with suspected appendicitis
2. If both WBC >10,000/mm³ and CRP >8 mg/L: high probability of appendicitis
3. If all inflammatory markers are normal: appendicitis is unlikely
4. For intermediate results: incorporate findings into appropriate clinical scoring system (AIR or Alvarado for adults, PAS for children)
5. Consider imaging studies for patients with intermediate-risk classification based on laboratory and clinical findings 1

In summary, while individual laboratory tests have limited diagnostic value, the combination of elevated WBC and CRP significantly increases diagnostic accuracy for appendicitis. These laboratory findings should be integrated with clinical assessment and appropriate scoring systems to guide further management decisions.