What is the differential diagnosis and initial workup for a 45-year-old female (F) presenting with lower abdominal pain, nausea, and vomiting, with a last menstrual period (LMP) 7 weeks ago and significant physical exam findings of lower abdominal tenderness?

Differential Diagnosis and Initial Workup for 45-Year-Old Female with Lower Abdominal Pain

This patient requires immediate serum β-hCG testing to fundamentally alter the diagnostic pathway, followed by transvaginal ultrasound as the initial imaging study given her reproductive age, delayed menses (LMP 7 weeks ago), and localized pelvic pain. 1

Top 5 Differential Diagnoses

1. Ectopic Pregnancy

• Most critical diagnosis to exclude given LMP 7 weeks ago and reproductive age 1, 2
• Presents with lower abdominal pain, nausea, vomiting, and requires positive β-hCG for diagnosis 3, 4
• Can be life-threatening if ruptured, leading to hemorrhagic shock 4, 5

2. Ovarian Torsion

• Characterized by acute onset lower abdominal pain with nausea and vomiting 3, 5
• Physical exam finding of lower abdominal tenderness with potential palpable adnexal mass is highly suggestive 3
• Critical pitfall: Normal Doppler flow does NOT exclude torsion, as 38-60% of confirmed cases show normal flow 3
• Requires emergent surgical exploration to preserve ovarian function 3

3. Pelvic Inflammatory Disease (PID)

• Common cause of lower abdominal pain in reproductive-age women with significant morbidity 6
• Caused by ascending infection from Neisseria gonorrhoeae, Chlamydia trachomatis, and anaerobic bacteria 6
• Can present with lower abdominal tenderness, though typically includes fever and vaginal discharge (absent in this case) 4

4. Hemorrhagic Ovarian Cyst (Ruptured or Rupturing)

• Frequent cause of acute pelvic pain in women of reproductive age 4
• Presents with acute lower abdominal pain, nausea, and vomiting 4
• Ultrasound shows variable intracystic echoes depending on blood clot quality and quantity 4

5. Appendicitis

• Most common surgical emergency that must always be considered if appendix not previously removed 2, 4
• Presents with lower abdominal pain, nausea, and vomiting with abdominal tenderness 2
• Less likely if discrete pelvic mass present on exam, but cannot be excluded clinically 3

Essential Laboratory Tests

Immediate Priority Labs

• Serum β-hCG (quantitative): Mandatory first test in all reproductive-age women with acute pelvic pain, becomes positive ~9 days post-conception 1
  • Negative result essentially excludes intrauterine or ectopic pregnancy 1
  • Positive result immediately prioritizes pregnancy-related causes and eliminates CT as first-line imaging 1

Additional Core Labs

• Complete blood count (CBC): Evaluate for infection, anemia from hemorrhage 2
• C-reactive protein (CRP): Assess inflammatory process 2
• Urinalysis with urine culture: Detect urinary tract infection or hematuria; obtain culture even with negative dipstick 1, 2
• Comprehensive metabolic panel: Electrolytes, creatinine, glucose 2

Supplementary Labs Based on Clinical Suspicion

• Lipase: If upper abdominal component or pancreatitis suspected 2
• Hepatobiliary markers: If right upper quadrant involvement 2

Imaging Algorithm

If β-hCG is Negative

• Transvaginal ultrasound with Doppler: Initial imaging of choice for suspected gynecologic pathology in reproductive-age women 7, 1

  • Provides excellent visualization of ovarian cysts, ovarian torsion, and pelvic inflammatory disease without radiation 1
  • Include color and spectral Doppler to evaluate internal vascularity of pelvic structures 8
  • Ultrasound findings for torsion: enlarged edematous ovary, presence of ovarian cyst/mass, potentially decreased or absent flow 3

• CT abdomen/pelvis with IV contrast: If ultrasound inconclusive or non-gynecologic cause suspected 7, 1

  • Superior sensitivity (89% vs 70% for ultrasound) for urgent abdominopelvic diagnoses 8, 1
  • Provides ~88% overall accuracy compared with surgical diagnosis 8
  • Excellent for detecting appendicitis, diverticulitis, and other gastrointestinal pathology 1

If β-hCG is Positive

• Transvaginal ultrasound is MANDATORY as first-line imaging 1
• CT is contraindicated due to fetal radiation exposure 1
• Ultrasound detects intrauterine pregnancy, ectopic pregnancy, threatened/spontaneous abortion 4
• Ectopic pregnancy shows uterine and adnexal signs with high sensitivity and specificity on transvaginal sonography 4
• If ultrasound inconclusive and serious pathology remains concern, MRI is preferred over CT in pregnancy 7

Problem-Solving Imaging

• MRI abdomen/pelvis without contrast: Consider after initial ultrasound or CT if endometriosis or fistulizing disease suspected 8
• Provides excellent soft tissue contrast without radiation 8

Critical Pitfalls to Avoid

• Never skip β-hCG testing in reproductive-age women—inadvertent CT in pregnant patients exposes fetus to unnecessary radiation 1
• Do not rely on normal Doppler flow to exclude ovarian torsion—38-60% of confirmed cases have normal flow 3
• Never use plain radiographs for pelvic pain evaluation—they have extremely limited utility 8, 1
• Do not assume appendicitis is excluded by presence of pelvic mass—both can coexist 3
• Avoid delaying surgical consultation for suspected ovarian torsion—early laparoscopic management correlates with ovarian preservation 3