What labs to order for a 17-year-old female (17F) with generalized lower abdominal pain, last menstrual period (LMP) 3 weeks ago, and no fever or associated symptoms?

Laboratory Workup for 17-Year-Old Female with Generalized Lower Abdominal Pain

Order a urine or serum β-hCG pregnancy test immediately, followed by a complete blood count (CBC), urinalysis, and consider sexually transmitted infection (STI) testing if sexually active. 1

Essential Initial Laboratory Tests

Pregnancy Testing (Highest Priority)

• β-hCG testing (urine or serum) is mandatory before any imaging in all women of reproductive age with abdominal pain, regardless of reported sexual history or contraceptive use 2
• With LMP 3 weeks ago, this patient could have an early intrauterine pregnancy or ectopic pregnancy, both of which can present with lower abdominal pain even without vaginal bleeding 1, 3
• A negative β-hCG effectively rules out pregnancy-related causes and guides subsequent imaging decisions 1

Complete Blood Count (CBC)

• Obtain CBC to evaluate for leukocytosis suggesting infection (appendicitis, pelvic inflammatory disease, tubo-ovarian abscess) or anemia from hemorrhage (ruptured ovarian cyst, ectopic pregnancy) 4, 3
• While laboratory values may be normal despite serious pathology, they provide important baseline data 4, 2

Urinalysis

• Perform urinalysis to evaluate for urinary tract infection, pyelonephritis, or urolithiasis—common causes of lower abdominal pain in adolescent females 1, 3
• Microscopic hematuria may suggest nephrolithiasis 5

Additional Testing Based on Clinical Context

STI Testing (If Sexually Active)

• Nucleic acid amplification tests (NAATs) for Chlamydia trachomatis and Neisseria gonorrhoeae can be performed on urine samples without requiring pelvic examination 1
• Consider testing given that pelvic inflammatory disease is a critical differential in sexually active adolescents with lower abdominal pain 1, 3

When Pelvic Examination Is Indicated

• Complete pelvic examination is always indicated for evaluation of lower abdominal pain to identify pelvic inflammatory disease, ovarian masses, ovarian torsion, or ectopic pregnancy 1
• Cervical or vaginal swab specimens may be more sensitive than urine for STI detection depending on the specific test used 1

Critical Clinical Pearls

Avoid Common Pitfalls

• Never assume a patient is not pregnant based on history alone—always obtain objective β-hCG testing 2
• Do not delay pregnancy testing to obtain other laboratory results, as this fundamentally changes the diagnostic approach and imaging safety 1, 2
• Laboratory values may be completely normal despite serious intra-abdominal pathology, especially in young patients 4, 2

Life-Threatening Diagnoses to Consider

• Ectopic pregnancy (can rupture causing hemorrhagic shock) 1, 3
• Ovarian torsion (requires urgent surgical intervention to preserve ovarian viability) 3, 6
• Appendicitis (most common surgical emergency in this age group) 1, 5
• Ruptured hemorrhagic ovarian cyst 3

Next Steps After Laboratory Results

• If β-hCG is positive or equivocal, proceed with transvaginal and transabdominal ultrasound as the single best diagnostic modality for pregnancy-related complications 1
• If β-hCG is negative and gynecologic etiology suspected, ultrasound remains the initial imaging modality of choice in this young patient to avoid radiation exposure 1
• CT abdomen/pelvis with IV contrast is reserved for when ultrasound is inconclusive or life-threatening non-gynecologic pathology is suspected 1