What laboratory tests are ordered when suspecting acute appendicitis?

Laboratory Tests for Suspected Acute Appendicitis

Order a complete blood count (CBC) with differential to calculate absolute neutrophil count (ANC), C-reactive protein (CRP), and urinalysis as the core laboratory workup for all patients with suspected acute appendicitis. 1

Essential Laboratory Tests

Standard Panel for All Patients

• White blood cell (WBC) count with differential: This is the foundation of laboratory evaluation, though it cannot be used alone to diagnose or exclude appendicitis 1, 2

  • WBC >10,000/mm³ has only a positive likelihood ratio of 2.47, indicating limited diagnostic power 2
  • Normal WBC does not exclude appendicitis (negative likelihood ratio 0.25) 2
  • Neutrophil percentage >75% or left shift is the most sensitive individual marker (82% sensitivity, 63% specificity) 2

• Absolute neutrophil count (ANC): Calculate this from the differential, as it provides better diagnostic discrimination than WBC alone 1

  • ANC >7,500/mL is a significant risk factor for appendicitis 1

• C-reactive protein (CRP): Essential inflammatory marker that should be routinely ordered 1

  • CRP ≥10 mg/L is a strong predictor in pediatric patients, particularly children <6 years old 1
  • CRP >40 mg/L suggests complicated appendicitis (found in 58% of complicated vs 37% of uncomplicated cases) 1

• Urinalysis: Include routinely to exclude urinary tract pathology that may mimic appendicitis 1

Additional Tests for Specific Populations

Pediatric patients: The same core panel (WBC with differential/ANC, CRP, urinalysis) is strongly recommended 1

• WBC >16,000/mL combined with CRP ≥10 mg/L are strong predictive factors 2
• Consider procalcitonin (PCT) and calprotectin if available, as they significantly improve diagnostic discrimination 1
• PCT has greater value for identifying complicated appendicitis (pooled sensitivity 0.89, specificity 0.90) than for diagnosing simple appendicitis 1

Pregnant patients: Laboratory tests and inflammatory serum parameters should always be requested, as diagnosis cannot be made on symptoms and signs alone 1

Elderly patients: Standard laboratory tests have limited diagnostic accuracy, but combined normal WBC and CRP can exclude appendicitis with 100% negative predictive value 2

• High CRP (>101.9 mg/L) may suggest perforation 2

Critical Clinical Context

Integration with Clinical Assessment

Never use laboratory tests in isolation to diagnose or exclude appendicitis 1, 2. The evidence is clear that:

• Laboratory values must be combined with clinical scoring systems (AIR score, AAS score, or Pediatric Appendicitis Score) rather than interpreted alone 1, 2
• Clinical findings should drive risk stratification, not laboratory values 2
• Even when both WBC and CRP are normal, appendicitis cannot be reliably excluded (negative likelihood ratio 0.08-0.25) 2

Risk Stratification Approach

Use laboratory results within this algorithmic framework 1, 2:

1. Low-risk patients (based on combined clinical scores and labs): May avoid imaging and be safely discharged with close follow-up
2. Intermediate-risk patients: Require timely diagnostic imaging regardless of laboratory values 2
3. High-risk patients: May proceed directly to surgery if clinical suspicion is strong, even with normal labs 2

Common Pitfalls to Avoid

• Do not rule out appendicitis based solely on normal WBC count - this leads to missed diagnoses, as appendicitis can occur with completely normal laboratory values 2
• Do not rely on the Alvarado score alone to confirm appendicitis in adults, as it is not sufficiently specific 1
• Recognize that early appendicitis may not yet demonstrate laboratory abnormalities 2
• In elderly patients, do not dismiss the diagnosis based on laboratory tests alone, as they have limited accuracy in this population 2

Emerging Biomarkers (Not Routinely Available)

While not standard of care, these show promise 1:

• Procalcitonin (PCT): Better for identifying complicated appendicitis than simple appendicitis
• Ischemia-modified albumin (IMA): Strong correlation with distinguishing gangrenous/perforated from uncomplicated appendicitis
• APPY1 test panel (WBC, CRP, myeloid-related protein 8/14): Shows 97.1% sensitivity and 97.4% negative predictive value in pediatric patients