What initial laboratory tests should be ordered for an adult patient with lower abdominal pain?

Initial Laboratory Tests for Lower Abdominal Pain

Order a complete blood count, C-reactive protein, comprehensive metabolic panel (including electrolytes and renal function), urinalysis, serum lipase, and pregnancy test (β-hCG) in all women of reproductive age. 1, 2

Core Laboratory Panel

The essential initial workup includes:

• Complete Blood Count (CBC): Identifies leukocytosis suggesting infection or inflammation, particularly important for appendicitis, diverticulitis, or bowel obstruction 1, 2
• C-Reactive Protein (CRP): Elevated levels (≥10 mg/L) indicate inflammation and help differentiate urgent from non-urgent causes 1
• Comprehensive Metabolic Panel: Assesses for dehydration, electrolyte imbalances, and renal dysfunction that occur with vomiting and poor oral intake 1, 2
• Urinalysis: Essential to evaluate for urinary tract infection or urolithiasis 1, 2
• Serum Lipase: More specific than amylase for pancreatitis 2
• Pregnancy Test (β-hCG): Mandatory in all women of reproductive age before any imaging to rule out ectopic pregnancy 1, 2, 3

Additional Tests Based on Clinical Suspicion

If bowel ischemia is suspected, immediately add:

• Blood gas analysis and lactate levels (elevated lactate, low pH, and low bicarbonate suggest intestinal ischemia) 1
• D-dimer may provide additional information though lacks specificity 2

If right upper quadrant involvement or hepatobiliary pathology is suspected, add:

• Liver function tests (ALT, AST, alkaline phosphatase, bilirubin) 1, 2

If sepsis is suspected, obtain:

• Blood cultures 2

For potential surgical candidates, check:

• Coagulation profile 1

Condition-Specific Laboratory Findings

Appendicitis

• Elevated WBC with left shift and absolute neutrophil count 1
• CRP ≥10 mg/L is a strong predictor 1

Diverticulitis

• Elevated WBC and CRP are common 1

Bowel Obstruction

• Marked leukocytosis and hyperamylasemia may indicate intestinal ischemia 1

Critical Pitfalls to Avoid

Never proceed with imaging in women of reproductive age without pregnancy testing first - this prevents delayed diagnosis of ectopic pregnancy and avoids unnecessary radiation exposure 2, 3

Do not rely on laboratory tests alone - the American College of Radiology emphasizes that labs are insufficient to diagnose most causes of lower abdominal pain and must be combined with appropriate imaging 1

Reassess patients with normal labs - if both WBC and CRP are normal, re-evaluate after 12 hours rather than assuming no serious pathology 4

Next Steps After Laboratory Testing

Once labs are obtained:

• CT abdomen/pelvis with IV contrast is the preferred imaging for lower abdominal pain in most adults 5, 2
• Ultrasound should be first-line in women of reproductive age to evaluate gynecological causes 5, 2
• Plain radiography has limited value and should not be routinely ordered 5, 2