What lab tests indicate appendicitis?

Laboratory Tests Indicating Appendicitis

The most useful laboratory tests for appendicitis are white blood cell (WBC) count with differential (specifically absolute neutrophil count) and C-reactive protein (CRP), but these should never be used alone to diagnose or exclude appendicitis—they must be combined with clinical assessment and imaging. 1

Key Laboratory Markers

White Blood Cell Count and Differential

• Neutrophil percentage >75% or absolute neutrophil count (ANC) >7,500/mL is the single most sensitive laboratory marker (82-97.8% sensitivity), making it superior to total WBC count alone 2, 3
• Total WBC count >10,000-10,500/mm³ has moderate sensitivity (81-86%) but poor specificity (32%) for appendicitis 4, 5
• The WBC differential with calculation of absolute neutrophil count should be routinely ordered in all suspected cases 1, 3

C-Reactive Protein (CRP)

• CRP ≥10 mg/L is a strong predictive factor for appendicitis in pediatric patients 1
• CRP >3 mg/dL significantly increases likelihood of appendicitis in children (adjusted odds ratio 3.44) 6
• CRP has lower sensitivity (76.5%) than WBC markers but provides complementary information 5
• Important caveat: CRP may be lower in uncomplicated appendicitis compared to normal appendix, but rises significantly with perforation (median 96.8 mg/L vs 39.6 mg/L) 5

Critical Clinical Pitfalls

Normal Labs Do Not Exclude Appendicitis

• Appendicitis can occur with completely normal laboratory values—the American College of Emergency Physicians emphasizes this critical point 2
• A normal WBC count has a negative likelihood ratio of only 0.25, meaning it does not reliably exclude appendicitis 2
• Only 6 out of 740 patients (0.8%) with confirmed appendicitis had all three tests (WBC, neutrophil %, CRP) simultaneously normal 5
• Early appendicitis may not yet demonstrate laboratory abnormalities 2

When Combined Normal Labs Are Reassuring

• The combination of normal WBC count AND normal CRP together has high negative predictive value (84-100% in some populations), making appendicitis very unlikely 2, 5
• In elderly patients specifically, unelevated WBC and CRP together can exclude appendicitis with 100% negative predictive value 2

Optimal Laboratory Strategy

Pediatric Patients

• Routinely order WBC with differential (including absolute neutrophil count) and CRP in all children with suspected appendicitis (strong recommendation, 1D) 1
• WBC >16,000/mL combined with CRP ≥10 mg/L are strong predictive factors 1
• The combination of WBC >12,000/mm³ AND CRP >3 mg/dL increases odds of appendicitis 7.75-fold 6

Adult Patients

• Order WBC with differential and CRP, but recognize that biochemical markers have lower quality evidence in adults 1
• Use laboratory values as part of clinical scoring systems (AIR score or AAS score) rather than in isolation 1
• Never rely on laboratory tests alone—they must be integrated with clinical assessment 2, 3

Pregnant Patients

• Laboratory tests and inflammatory serum parameters should always be requested—do not diagnose based on symptoms and signs alone (weak recommendation, 2C) 1

Integration with Clinical Decision-Making

Risk Stratification Approach

• Use laboratory values within validated clinical scoring systems (AIR or AAS scores for adults; Pediatric Appendicitis Score for children) rather than interpreting labs in isolation 1
• Patients with intermediate risk based on combined clinical and laboratory assessment should undergo timely diagnostic imaging 1, 2
• High-risk patients <40 years old with strongly positive clinical scores may proceed directly to surgery without imaging, even with normal labs 1

When to Image Despite Normal Labs

• CT scanning may be appropriate even with normal laboratory values when clinical suspicion exists 2
• Do not let normal laboratory results prevent appropriate imaging in clinically suspicious cases 2

Tests That Are NOT Useful

• Alvarado score alone should not be used to confirm appendicitis in adults (weak recommendation against, 2B) 1
• Sedimentation rate has lower diagnostic accuracy than WBC differential 3
• Eosinophil count has not been identified as a significant marker 3
• D-lactate is not a useful laboratory adjunct 6
• Procalcitonin (PCT) shows elevated levels in appendicitis but is not routinely recommended in current guidelines 6