What laboratory tests are recommended for diagnosing acute appendicitis?

Laboratory Tests for Acute Appendicitis

Order a complete blood count (CBC) with differential to calculate the absolute neutrophil count (ANC) and C-reactive protein (CRP) as the essential laboratory workup for suspected acute appendicitis, but never use these tests alone to diagnose or exclude the condition—they must be combined with clinical assessment and risk stratification scores. 1

Essential Laboratory Tests

Adults

• White blood cell (WBC) count with differential is the primary laboratory test, with neutrophil percentage >75% or left shift being the most sensitive individual marker (82% sensitivity, 63% specificity) 1, 2
• C-reactive protein (CRP) should be routinely ordered alongside WBC 1
• The combination of WBC >10,000/mm³ and CRP >8 mg/L has a positive likelihood ratio of 23.32 for acute appendicitis 3
• Absolute neutrophil count (ANC) demonstrates superior sensitivity compared to total WBC alone, with ANC >7,500/mL showing 97.8% sensitivity 2

Pediatric Patients

• WBC with differential and calculation of ANC is a strong recommendation (1A evidence) 1
• CRP ≥10 mg/L is a strong predictive factor for appendicitis in children, particularly those <6 years old 1, 4
• WBC >16,000/mL combined with CRP ≥10 mg/L are strong predictive factors in pediatric populations 1, 4
• Procalcitonin (PCT) has greater diagnostic value for identifying complicated appendicitis (pooled sensitivity 0.89, specificity 0.90) but limited value for diagnosing uncomplicated appendicitis 1
• Urinalysis should be included in the routine workup 1

Elderly Patients (>60-65 years)

• Order the same tests (WBC with differential and CRP), but recognize their limitations 1
• Unelevated WBC and CRP together can exclude appendicitis with 100% negative predictive value 1, 4
• CRP >101.9 mg/L suggests perforation with an AUC of 0.811 1
• Leukocyte response is not affected by age, and elderly patients often have elevated WBC counts 1, 3

Critical Limitations and Pitfalls

Do NOT Rely on Laboratory Tests Alone

• Appendicitis can occur with completely normal laboratory values 4
• A normal WBC count (negative likelihood ratio 0.25) does not reliably exclude appendicitis 4
• An elevated WBC count has only a positive likelihood ratio of 2.47, indicating limited diagnostic power when used alone 4, 3
• 8.4% of patients with confirmed appendicitis have low Alvarado scores (below 5), which incorporate laboratory values 4

Integration with Clinical Decision-Making

• Use laboratory values within validated clinical scoring systems rather than interpreting them in isolation 1, 4
• The Appendicitis Inflammatory Response (AIR) score incorporates CRP and has better diagnostic accuracy than the older Alvarado score 1
• The Adult Appendicitis Score (AAS) and AIR score are the best performing clinical prediction scores (1A recommendation) 1
• In pregnant patients, laboratory tests and inflammatory serum parameters should always be requested alongside clinical assessment (2C recommendation) 1

Risk Stratification Algorithm

Low-Risk Patients

• When both WBC and CRP are normal, the negative likelihood ratio is approximately 0.08-0.25, but this still does not reliably exclude appendicitis 4
• Patients with low clinical scores and normal labs can be observed or discharged with precautions 1

Intermediate-Risk Patients

• Require systematic diagnostic imaging regardless of laboratory values 1, 4
• Use ultrasound first, followed by CT if ultrasound is negative or inconclusive 1

High-Risk Patients

• Patients <40 years old with strongly positive clinical scores may proceed directly to surgery without imaging, even with normal labs 4
• In elderly patients, elevated inflammatory markers should prompt adequate diagnostic workup with CT imaging 1

Special Considerations

Markers for Complicated Appendicitis

• Fever, right lower quadrant tenderness, and neutrophilia are the three most sensitive markers for predicting complicated appendicitis (88.6%, 82.3%, and 79.7% respectively) 1
• CRP >40 mg/L was found in 58% of patients with complicated appendicitis versus 37% with uncomplicated disease 1
• WBC >15 × 10⁹/L was found in 58% of complicated cases versus 43% of uncomplicated cases 1

Tests That Are NOT Useful

• Erythrocyte sedimentation rate (ESR) has lower diagnostic accuracy than WBC differential and CRP 2, 5
• Eosinophil count has not been identified as a significant marker 2
• Alpha-1-antiproteinase concentration shows lower diagnostic efficiency 5

Pregnant Patients

• Laboratory tests and inflammatory serum parameters should always be requested (2C recommendation) 1
• Do not make the diagnosis based on symptoms and signs alone 1

Key Clinical Pearls

• Early appendicitis may not yet demonstrate laboratory abnormalities, so do not rule out appendicitis based solely on normal initial labs 4
• CT scanning may be appropriate even with normal laboratory values when clinical suspicion exists 4
• The combination of clinical parameters, laboratory tests, and ultrasound may significantly improve diagnostic accuracy and reduce the need for CT scan 1
• In patients with intermediate risk based on combined clinical and laboratory assessment, proceed to timely diagnostic imaging rather than repeating laboratory tests 4