What initial laboratory tests are recommended for patients presenting with lower abdominal pain?

Initial Laboratory Tests for Lower Abdominal Pain

For patients presenting with lower abdominal pain, initial laboratory tests should include a complete blood count, C-reactive protein, renal function tests, electrolytes, urinalysis, and pregnancy testing in women of reproductive age. 1, 2

Core Laboratory Tests

• Complete blood count (CBC) - Leukocytosis may indicate infection or inflammation, which is particularly important in suspected appendicitis, diverticulitis, or bowel obstruction 1
• C-reactive protein (CRP) - Elevated levels suggest inflammation and can help differentiate urgent from non-urgent causes 1, 2
• Renal function tests and electrolytes - To assess for dehydration, pre-renal acute renal failure, and electrolyte imbalances that may occur with vomiting and poor oral intake 1
• Blood gas analysis and lactate levels - Particularly important when bowel ischemia is suspected, as low serum bicarbonate levels, low arterial blood pH, and high lactic acid levels may indicate intestinal ischemia 1
• Urinalysis - To evaluate for urinary tract infection or urolithiasis, which can present with lower abdominal pain 1, 2
• Pregnancy test - Essential in all women of reproductive age to rule out ectopic pregnancy and other pregnancy-related complications 2

Additional Tests Based on Clinical Suspicion

• Liver function tests - When right upper quadrant pathology is suspected that may radiate to lower abdomen 1
• Lipase - When pancreatitis is in the differential diagnosis 2
• Coagulation profile - Should be tested due to the potential need for emergency surgery 1

Laboratory Tests for Specific Conditions

For Suspected Appendicitis

• White blood cell count with differential and absolute neutrophil count (ANC) - Elevated in most cases of appendicitis 1
• CRP level ≥ 10 mg/L - Strong predictor of appendicitis, especially in pediatric patients 1

For Suspected Bowel Obstruction

• CBC, electrolytes, and renal function - To assess for dehydration and electrolyte imbalances 1
• Marked leukocytosis and hyperamylasemia - May suggest intestinal ischemia in bowel obstruction 1

For Suspected Diverticulitis

• CBC and CRP - Elevated white blood cell count and CRP are common in diverticulitis 1

Important Considerations

• Laboratory tests alone are insufficient to diagnose most causes of lower abdominal pain and should be combined with appropriate imaging studies 1, 3
• CBC and CRP alone cannot reliably discriminate between urgent and non-urgent diagnoses but can help guide further workup 1, 3
• In cases of suspected sepsis from intra-abdominal sources, antibiotic treatment should be initiated within the first hour after recognition 3

Imaging Considerations After Laboratory Testing

• CT with intravenous contrast is often the preferred imaging modality for lower abdominal pain after initial laboratory evaluation 1
• Ultrasound may be preferred as first-line imaging in women of reproductive age to evaluate for gynecological causes 1
• Plain radiography has limited value in the initial evaluation of lower abdominal pain 1

Remember that laboratory findings should always be interpreted in the context of the patient's clinical presentation, as no single test can definitively diagnose the cause of lower abdominal pain 1, 2, 3.