What labs and initial management are recommended for a patient suspected of having appendicitis?

Laboratory Evaluation and Initial Management for Suspected Appendicitis

In patients with suspected appendicitis, obtain a complete blood count (CBC) with differential and C-reactive protein (CRP), recognizing that normal values do not exclude appendicitis, and proceed with risk-stratified imaging (CT with IV contrast in non-pregnant adults, ultrasound in children/pregnant patients) followed by broad-spectrum antibiotics and surgical consultation once diagnosis is confirmed. 1, 2

Essential Laboratory Tests

Order the following labs in all patients with suspected appendicitis:

• White blood cell (WBC) count with differential - Calculate the absolute neutrophil count, as neutrophil percentage >75% is the most sensitive individual marker (82% sensitivity, 63% specificity) 1, 3
• C-reactive protein (CRP) - CRP ≥10 mg/L is a strong predictive factor, particularly in pediatric patients 1, 2
• Pregnancy test - Mandatory in all females of childbearing potential before imaging 1

The combination of WBC >10,000/mm³ AND CRP ≥8 mg/L has the strongest predictive value (positive likelihood ratio 23.32, negative likelihood ratio 0.03), far superior to either test alone 4. However, appendicitis can occur with completely normal laboratory values - a normal WBC count has a negative likelihood ratio of only 0.25-0.41, meaning it does NOT reliably exclude appendicitis 1, 2.

Critical Pitfall to Avoid

Never rule out appendicitis based solely on normal laboratory values. 2 Laboratory tests must be combined with clinical assessment and imaging - they should never be used in isolation to diagnose or exclude appendicitis 1, 2. The Alvarado score alone should not be used to confirm appendicitis in adults 1.

Risk Stratification Using Clinical Scores

Apply validated clinical scoring systems that integrate symptoms, physical examination, and laboratory findings:

• AIR (Appendicitis Inflammatory Response) score - Incorporates CRP values and has better diagnostic accuracy than the older Alvarado score 1, 2
• Alvarado score - Useful for adults but should not be used alone 1, 4
• Pediatric Appendicitis Score - For children, though diagnosis should not be based on clinical scores alone 1

Risk stratification guides imaging decisions:

• Low-risk patients (Alvarado ≤3): Consider discharge with 24-hour follow-up 4
• Intermediate-risk patients (Alvarado 4-6): Proceed with timely and systematic diagnostic imaging 1, 4
• High-risk patients (Alvarado ≥7, age <40): May proceed directly to surgery without imaging if clinical presentation is classic 1, 4

Imaging Strategy by Population

Non-pregnant adults:

• CT abdomen and pelvis with IV contrast is the recommended initial imaging modality (sensitivity 96-100%, specificity 93-95%) 1, 4
• IV contrast is essential and increases sensitivity; oral contrast is NOT necessary and delays diagnosis 1, 4
• Helical CT with IV contrast (but not oral or rectal contrast) is specifically recommended 1

Pregnant patients:

• Ultrasound first, followed by MRI (not CT) if ultrasound is inconclusive 1, 4
• MRI without IV contrast has 94% sensitivity and 96% specificity 4

Children and adolescents:

• Ultrasound first to avoid radiation exposure 1, 4
• If ultrasound is equivocal, proceed to MRI or CT rather than repeat ultrasound 1
• Point-of-care ultrasound (POCUS) by emergency physicians has 91% sensitivity 4

Initial Management Once Appendicitis is Confirmed

Administer broad-spectrum antibiotics immediately covering aerobic gram-negative organisms and anaerobes: 1, 4, 5

• Preferred regimens:
  • Piperacillin-tazobactam monotherapy, OR
  • Cephalosporin or fluoroquinolone PLUS metronidazole 4, 5

Obtain surgical consultation for appendectomy:

• Within 24 hours for uncomplicated appendicitis 4
• Urgent intervention for complicated appendicitis (perforation, abscess, phlegmon) 4
• Large periappendiceal abscess (>3 cm) may warrant percutaneous drainage rather than immediate appendectomy 4

Special Considerations

Antibiotics-first strategy (non-operative management):

• May be considered in highly selected patients with uncomplicated appendicitis and NO appendicolith on CT 4, 5
• Overall success rate is 63-73% at one year 4, 5
• High-risk CT findings predict antibiotic failure (≈40% failure rate): appendicolith present, appendiceal diameter >13 mm, or mass effect 4, 5

Elderly patients:

• Maintain extremely low threshold for imaging, as atypical presentations are common 4
• Combined normal WBC and CRP in elderly patients showed 100% negative predictive value in one series, though broader validation is needed 2
• CRP >101.9 mg/L suggests perforation 2

Immunocompromised patients:

• Proceed directly to CT with IV contrast if clinical suspicion exists 2
• May not mount normal inflammatory response or demonstrate peritoneal signs 2

Key Clinical Predictors to Assess

High-risk features warranting direct surgical consultation:

• Fever >38°C combined with focal right lower quadrant tenderness 1, 4
• Guarding and abdominal rigidity 1, 6
• Periumbilical pain migrating to right lower quadrant 5, 6

Intermediate-risk features requiring imaging:

• Right lower quadrant pain without clear peritoneal signs 4
• Nausea with focal abdominal tenderness 4
• Rebound tenderness 1

Follow-up for Negative Imaging

If imaging is negative but clinical suspicion persists:

• Follow-up at 24 hours is recommended to ensure resolution of signs and symptoms, due to the low but measurable risk of false-negative results 1
• Consider observation and supportive care, with or without antibiotics 1
• If clinical suspicion remains high, consider surgical intervention 1