What is the initial work-up for a patient suspected of having acute appendicitis?

Work-up for Acute Appendicitis

In adults with suspected acute appendicitis, obtain CT abdomen/pelvis with IV contrast as the initial imaging study, combined with complete blood count (CBC) and C-reactive protein (CRP). 1, 2

Clinical Assessment

History and Physical Examination

• Right lower quadrant pain is the most reliable clinical finding for ruling in appendicitis in adults 3
• Look specifically for periumbilical pain that migrates to the right lower quadrant, accompanied by anorexia, nausea, and low-grade fever 4, 3
• Abdominal rigidity and rebound tenderness are highly predictive when present 3
• In children, assess for positive psoas sign, obturator sign, and Rovsing sign as these are most reliable 3

Important caveat: Clinical diagnosis alone has only 80% accuracy and results in unacceptably high negative appendectomy rates (up to 25%) without imaging 1, 5. Clinical scoring systems like the Alvarado score have not significantly improved diagnostic accuracy and should not replace imaging 1, 3.

Laboratory Testing

Essential Laboratory Work-up

• White blood cell count (WBC) with differential to calculate absolute neutrophil count 1
• C-reactive protein (CRP) - levels ≥10 mg/L are strongly predictive 1
• Urinalysis to exclude urinary tract pathology 1

Biochemical markers show promise but require further validation: Procalcitonin (PCT) has greater value in identifying complicated appendicitis (sensitivity 89%, specificity 90%) rather than diagnosing uncomplicated disease 1. In children, CRP >40 mg/L is found in 58% of complicated cases versus 37% of uncomplicated cases 1.

Imaging Strategy

Adults (Non-Pregnant)

CT abdomen/pelvis with IV contrast is the gold standard initial imaging modality with sensitivity 97% (range 83-100%) and specificity 94% (range 64-100%) 1, 2.

Technical specifications:

• IV contrast is usually appropriate and improves sensitivity to 96-100% 2
• Oral contrast is NOT recommended - it delays diagnosis by 40 minutes to 2+ hours without improving accuracy and is poorly tolerated by patients with nausea/vomiting 2
• Unenhanced CT remains highly accurate (sensitivity 93%, specificity 98%) if IV contrast is contraindicated, though it has limitations in characterizing complicated appendicitis 2

If CT is negative but clinical suspicion persists: Consider observation with supportive care ± antibiotics, or surgical consultation if suspicion is high 1, 2. Do not dismiss appendicitis based on negative imaging alone if clinical suspicion remains elevated 2.

Alternative initial imaging: Ultrasound when definitively positive or negative has sensitivity 98% and specificity 97%, and MRI has comparable accuracy (sensitivity 96%, specificity 97%) 1. These may be used first depending on patient circumstances and availability 1.

Children and Adolescents

Ultrasound is the preferred initial imaging modality with sensitivity 99% and specificity 96% when yielding definitive results 1.

If initial ultrasound is equivocal/non-diagnostic:

• Obtain MRI or CT with IV contrast as subsequent imaging rather than repeating ultrasound 1
• MRI has sensitivity 98% (range 92-100%) and specificity 97% (range 89-100%) 1
• CT has sensitivity 96% (range 91-98%) and specificity 96% (range 87-100%) 1

Critical pitfall: When ultrasound results include equivocal interpretations, sensitivity drops from 99% to 82%, necessitating additional imaging 1. Do not rely on equivocal ultrasound results alone 1.

Pregnant Patients

Ultrasound is the initial imaging modality due to absence of fetal radiation exposure 1, 6.

If ultrasound is inconclusive or negative with persistent clinical suspicion:

• MRI without IV contrast is the next step with sensitivity 93% (range 18-100%) and specificity 96% (range 54-100%) 1, 6
• Avoid CT when possible due to fetal radiation exposure; use only as last resort if MRI unavailable and diagnosis urgent 6

Management consideration: Do not delay surgery more than 24 hours from admission if appendicitis is confirmed, as laparoscopic appendectomy is safe during pregnancy 6.

Diagnostic Accuracy Summary

The 2024 IDSA guidelines provide the most comprehensive diagnostic accuracy data:

• CT in adults: Sensitivity 97%, Specificity 94% 1
• Ultrasound in children (definitive results): Sensitivity 99%, Specificity 96% 1
• MRI in pregnant patients: Sensitivity 93%, Specificity 96% 1

Common Pitfalls to Avoid

• Do not rely on clinical examination alone - this results in 25% negative appendectomy rate 1
• Do not use oral contrast with CT - it delays diagnosis without improving accuracy 2
• Do not repeat ultrasound if initial study is equivocal - proceed to MRI or CT instead 1
• Do not dismiss appendicitis based solely on negative imaging if clinical suspicion remains high 2
• In women of childbearing age, maintain high suspicion for gynecological causes as alternative diagnoses 7