What is the appropriate initial evaluation and treatment approach for a patient presenting with lower abdominal pain, considering factors such as age, sex, medical history, and laboratory results?

Management of Lower Abdominal Pain

CT abdomen and pelvis with IV contrast is the initial imaging modality of choice for most patients presenting with lower abdominal pain, as it has 95% sensitivity and 94% specificity for appendicitis, identifies alternative diagnoses in 94% of cases, and changes management decisions in 25% of patients. 1

Initial Clinical Assessment

Critical Red Flags Requiring Immediate Action

• Hemodynamic instability (tachycardia, hypotension) suggests bleeding or sepsis and requires immediate resuscitation 2
• Peritoneal signs (rigid abdomen, rebound tenderness) indicate perforation or ischemia requiring urgent surgical consultation 2
• Pain out of proportion to exam strongly suggests mesenteric ischemia 2
• Abdominal distension with vomiting indicates bowel obstruction 2
• Fever with severe pain suggests infection, abscess, or perforation 2

Essential History Elements

• Pain location and migration: Right lower quadrant pain suggests appendicitis; left lower quadrant pain in older adults suggests diverticulitis 1, 2
• Associated symptoms: Constipation and distension strongly suggest bowel obstruction 3
• Surgical history: Adhesions account for 55-75% of small bowel obstructions 2
• Age and sex: Women of reproductive age require consideration of ectopic pregnancy and pelvic inflammatory disease 2, 3

Physical Examination Priorities

• Examine all hernia orifices and surgical scars to avoid missing incarcerated hernias 2
• Assess for peritoneal signs systematically 2
• Pelvic examination in women of reproductive age when gynecologic pathology is suspected 1

Mandatory Laboratory Workup

Core Laboratory Tests (Order for All Patients)

• Complete blood count to assess for leukocytosis indicating infection or inflammation 4, 5
• Comprehensive metabolic panel including liver function tests for hepatobiliary pathology 4, 5
• Urinalysis to evaluate for urinary tract infection or nephrolithiasis 4, 5
• Serum lipase (more specific than amylase) if pancreatitis suspected 4, 5
• β-hCG in ALL women of reproductive age before any imaging—failure to obtain this test can delay diagnosis of ectopic pregnancy 2, 4, 5

Additional Tests Based on Clinical Suspicion

• Blood cultures if fever present and sepsis suspected 4, 5
• Lactate and D-dimer if mesenteric ischemia suspected (though lacking specificity) 4, 5

Critical Pitfall to Avoid

Do not over-rely on normal laboratory values early in disease—labs may be normal despite serious pathology, particularly in elderly patients 2

Imaging Strategy by Pain Location

Right Lower Quadrant Pain

• CT abdomen and pelvis with IV contrast is the initial imaging of choice with 95% sensitivity and 94% specificity for appendicitis 1
• CT identifies non-appendiceal diagnoses (gastroenteritis, colitis, inflammatory bowel disease, benign adnexal mass) in 94% of cases 1
• Transvaginal ultrasound should be performed first if gynecologic pathology (ectopic pregnancy, pelvic inflammatory disease) is the primary concern in premenopausal women 1

Left Lower Quadrant Pain

• CT abdomen and pelvis with IV contrast has 98% diagnostic accuracy for diverticulitis and superior ability to detect complications (abscess, perforation, fistula, obstruction) 1
• Imaging may not be required in patients with typical diverticulitis symptoms, prior history of diverticulitis, and no evidence of complications—such patients can be treated medically 1
• However, misdiagnosis based on clinical assessment alone occurs in 34-68% of cases, so imaging is increasingly recommended 1
• Transvaginal ultrasound first if gynecologic pathology suspected in premenopausal women 1

Nonlocalized/Diffuse Abdominal Pain

• CT abdomen and pelvis with IV contrast is the preferred imaging option, changing the primary diagnosis in 51% of cases and altering admission decisions in 25% of patients 1, 2
• This is particularly important in elderly patients where typical signs may be masked and laboratory tests may be normal despite serious infection 1

Imaging Modalities to Avoid

• Plain radiography has limited diagnostic value and should not be routinely ordered—CT is more sensitive and specific for all entities radiography can detect 1, 5
• MRI is not useful for initial evaluation of acute abdominal pain—it is less sensitive for extraluminal air and urinary calculi, more time-consuming, and more subject to motion artifacts 1
• Ultrasound for non-gynecologic lower abdominal pain is less commonly used in the United States and highly operator-dependent 1

Treatment Approach Based on Diagnosis

Appendicitis

• All patients require hospital admission 6
• Surgical consultation for appendectomy 6

Diverticulitis

• Uncomplicated diverticulitis: Medical therapy with antibiotics, can frequently be managed outpatient 1, 6
• Abscess ≥3 cm: Catheter drainage indicated 1
• Free perforation with peritonitis: Surgical intervention required 1
• CT findings determine operative versus nonoperative treatment and inpatient versus outpatient triage 1

Bowel Obstruction

• Adhesions (55-75% of small bowel obstructions) and hernias (15-25%) are most common causes 2
• Colorectal cancer causes 60% of large bowel obstructions 2

Special Population Considerations

Elderly Patients

• Atypical presentations are common—maintain high suspicion even with normal laboratory values 2
• Higher likelihood of malignancy, diverticulitis, and vascular causes (mesenteric ischemia) 2
• More extensive evaluation warranted despite normal initial findings 1, 2

Women of Reproductive Age

• Mandatory β-hCG testing before imaging 2, 4, 5
• Consider ectopic pregnancy, pelvic inflammatory disease, ovarian torsion, and ruptured ovarian cyst 2, 3
• Transvaginal ultrasound is first-line imaging when gynecologic pathology suspected 1

Immunocompromised Patients

• Typical signs of abdominal sepsis may be masked 1
• More extensive workup necessary 4, 5
• High mortality rate if diagnosis delayed 1