What are the differential diagnoses for a 20‑year‑old woman presenting with right lower‑quadrant abdominal pain, vomiting, diarrhoea, normal CT scan and ultrasound, normal C‑reactive protein, normal white‑cell count, normal fecal calprotectin, and urinalysis showing infection but a negative urine culture?

Differential Diagnosis for Right Lower Quadrant Pain with Atypical Features

The most likely diagnosis in this 20-year-old woman is early appendicitis despite normal imaging and inflammatory markers, followed by gynecologic pathology (ovarian torsion, ruptured cyst, pelvic inflammatory disease), right-sided colonic diverticulitis, inflammatory bowel disease (particularly Crohn's disease), and urinary tract infection with sterile pyuria.

Primary Diagnostic Considerations

Appendicitis (Most Common Surgical Cause)

• Appendicitis remains the leading diagnosis even with normal CT/ultrasound, as imaging sensitivity is only 85.7-100% and early appendicitis may not show classic findings 1
• Fever is absent in approximately 50% of appendicitis cases, and normal WBC does not exclude the diagnosis 1, 2
• The combination of right lower quadrant pain with vomiting is the classic early presentation before migration of periumbilical pain 3
• Normal inflammatory markers (CRP, WBC) significantly reduce but do not eliminate appendicitis probability, with a negative likelihood ratio of 0.25 3

Gynecologic Pathology (Critical in Young Women)

• Ovarian torsion, ruptured ovarian cyst, tubo-ovarian abscess, and pelvic inflammatory disease must be excluded in all reproductive-age women with RLQ pain 2
• CT detects gynecologic pathology in approximately 21.6% of women presenting with RLQ pain 2, 3
• These conditions can present with identical symptoms but require entirely different management 2

Right-Sided Colonic Diverticulitis

• Increasingly common even in younger patients and can precisely mimic appendicitis 4
• May present with normal initial inflammatory markers 1

Inflammatory Bowel Disease (Crohn's Disease)

• Normal fecal calprotectin makes active inflammatory bowel disease unlikely but does not completely exclude it 5
• Terminal ileitis can present identically to appendicitis 6

Urinary Tract Infection with Sterile Pyuria

• Urinalysis showing infection with negative culture suggests either:
  • Early infection (culture not yet positive)
  • Partially treated UTI
  • Sterile pyuria from adjacent inflammatory process (appendicitis, diverticulitis)
  • Urethritis from sexually transmitted infection 7

Recommended Diagnostic Algorithm

Immediate Next Steps

1. Obtain beta-hCG pregnancy test immediately to exclude ectopic pregnancy before any further imaging 1, 2

2. Perform transvaginal ultrasound as the next imaging study in this reproductive-age woman with inconclusive transabdominal imaging 2, 3

   • TVUS is rated "appropriate" (rating 5) by the American College of Radiology for women with pelvic pain manifesting as RLQ symptoms 3
   • Achieves 97.3% sensitivity and 91% specificity for gynecologic pathology in adult women 4
   • Avoids radiation exposure while evaluating ovarian torsion, cysts, and pelvic inflammatory disease 3

3. If TVUS is nondiagnostic or negative but symptoms persist, proceed immediately to CT abdomen/pelvis with IV contrast 1, 2

   • CT achieves 95% sensitivity and 94% specificity for appendicitis 2, 4
   • Identifies alternative diagnoses in 23-45% of cases with RLQ pain 1, 3
   • Use IV contrast only (no oral contrast needed) for rapid acquisition 1, 3

Clinical Observation Protocol

If imaging remains negative but clinical suspicion persists:

• Admit for 24-hour observation with serial abdominal examinations every 6-12 hours 3
• Repeat complete blood count to monitor for evolving leukocytosis 3
• NPO status with IV hydration 4
• Return immediately if developing fever, worsening pain, peritoneal signs, or inability to tolerate oral intake 3
• Mandatory follow-up within 24 hours if discharged, as false-negative rates exist even with negative imaging 2, 3

Critical Pitfalls to Avoid

Do Not Rely on Absence of Fever or Normal Labs

• Clinical assessment alone misdiagnoses appendicitis in 34-68% of cases 3
• Fever is absent in 50% of appendicitis, and normal WBC occurs frequently in early disease 1, 2
• Negative appendectomy rates reach 25% when relying on clinical assessment without imaging 3

Do Not Discharge Without Clear Safety Net

• Symptoms can evolve rapidly, and early appendicitis may initially present with normal imaging 1, 3
• NSAIDs can mask evolving symptoms and delay diagnosis 3
• Establish explicit return precautions: fever, vomiting, worsening pain, or inability to eat/drink 3

Do Not Skip Gynecologic Evaluation

• Ovarian torsion is a time-sensitive emergency requiring immediate surgery 2
• Ectopic pregnancy can rupture and cause life-threatening hemorrhage 2
• Transvaginal ultrasound is essential and should not be skipped in favor of CT alone 3

Additional Differential Diagnoses to Consider

• Mesenteric lymphadenitis (viral gastroenteritis with reactive nodes) 6
• Epiploic appendagitis (self-limited inflammation of colonic fat appendages) 6
• Omental infarction 6
• Urethritis from sexually transmitted infection (Chlamydia, Gonorrhea) causing sterile pyuria 7
• Abdominal wall pain (musculoskeletal) 5