What laboratory tests are drawn for a patient presenting with possible appendicitis?

Laboratory Tests for Suspected Appendicitis

The essential laboratory tests for evaluating possible appendicitis are complete blood count (CBC) with differential and C-reactive protein (CRP), though these should never be used alone to diagnose or exclude appendicitis and must be combined with clinical assessment and imaging. 1

Core Laboratory Panel

White Blood Cell Count with Differential

• Order a CBC with differential calculating absolute neutrophil count in all patients with suspected appendicitis 1
• WBC >10,000/mm³ has limited diagnostic power when used alone (positive likelihood ratio only 2.47), and a normal WBC does not exclude appendicitis (negative likelihood ratio 0.25) 1
• Neutrophil percentage >75% or neutrophil shift to the left is the most sensitive individual laboratory marker, with 82% sensitivity and 63% specificity 1
• WBC >16,000/mL combined with CRP ≥10 mg/L are strong predictive factors for appendicitis 1

C-Reactive Protein

• CRP should be ordered routinely in all patients with suspected appendicitis 1
• CRP ≥10 mg/L is a strong predictive factor, particularly in pediatric patients 1
• CRP is superior to ESR for evaluating acute appendicitis and should be the inflammatory marker of choice 1
• ESR has poor sensitivity (51-60%) and should not be used 1

Optimal Combined Laboratory Strategy

Most Powerful Combination

• The combination of WBC >10,000/mm³ AND CRP ≥8 mg/L provides the highest diagnostic accuracy with a positive likelihood ratio of 23.32 and negative likelihood ratio of 0.03 2
• When both leukocytosis and CRP are absent, the negative likelihood ratio is approximately 0.08-0.25, but appendicitis still cannot be reliably excluded based on normal laboratory values alone 1

Triple Test for Ruling Out Appendicitis

• The combination of normal WBC count, normal neutrophil percentage, and normal CRP (all three negative) has demonstrated 100% negative predictive value in some studies, though this requires broader validation 1, 3
• This triple test combination was valid in 32% of patients free from appendicitis and could potentially reduce unnecessary surgeries 3

Critical Clinical Integration Points

Laboratory Tests Must Be Combined with Clinical Assessment

• Never diagnose or exclude appendicitis based solely on laboratory values—clinical findings should drive risk stratification 1
• Laboratory values should be incorporated into validated clinical scoring systems (Alvarado score, AIR score) rather than interpreted in isolation 1
• Appendicitis can occur with completely normal laboratory values, as test results may be normal especially in early appendicitis 1

Population-Specific Considerations

Pediatric Patients:

• Routinely order WBC with differential and CRP in all children with suspected appendicitis (strong recommendation) 1
• CRP ≥10 mg/L is particularly predictive in pediatric populations 1

Elderly Patients:

• Unelevated WBC and CRP values together can exclude appendicitis with 100% negative predictive value in elderly patients 1
• High CRP (>101.9 mg/L) may suggest perforation in this population 1

Pregnant Patients:

• Laboratory tests and inflammatory serum parameters should always be requested (weak recommendation) 1
• Do not base diagnosis solely on laboratory values in pregnant patients 1

Management Algorithm Based on Laboratory Results

High-Risk Laboratory Profile (WBC >10,000/mm³ AND CRP ≥8 mg/L)

• Proceed with imaging (CT in adults, ultrasound in children) or surgical consultation based on clinical context 2
• This combination has a positive likelihood ratio of 23.32 2

Intermediate Laboratory Profile (Either WBC or CRP elevated)

• Obtain diagnostic imaging for intermediate-risk patients 1
• Use clinical scoring systems to guide decision-making 1

Low-Risk Laboratory Profile (Normal WBC, neutrophils, and CRP)

• Appendicitis is unlikely but not excluded 1
• Clinical assessment remains paramount—if clinical suspicion is high, proceed with imaging despite normal labs 1
• Consider discharge with 24-hour follow-up for very low clinical suspicion 2

Common Pitfalls to Avoid

• Do not rule out appendicitis based solely on normal WBC count—this leads to missed diagnoses 1
• Early appendicitis may not yet demonstrate laboratory abnormalities 1
• Certain populations (elderly, pregnant, immunosuppressed) may present atypically with normal laboratory values 1
• The Alvarado score alone should not be used to confirm appendicitis in adults 1
• 8.4% of patients with appendicitis have low Alvarado scores (below 5) 1

Additional Laboratory Considerations

• Neutrophil count >7.88 × 10⁹/L or neutrophil percentage >75% are the single best individual tests with 81-84% sensitivity 4
• Neutrophil-to-lymphocyte ratio (NLR) is helpful in diagnosing appendicitis and measuring severity 5
• Mean platelet volume (MPV) is not recommended as it has low diagnostic rate and contradictory results 5