A 28-year-old nulliparous woman reports breakthrough bleeding and irregular spotting after 3 months of progestin-only pill (POP) use. She denies pelvic pain, fever, or abnormal discharge. Which investigation is most appropriate to evaluate the cause of her abnormal bleeding?

Explanation

## Evaluation of Breakthrough Bleeding on Progestin-Only Pill ### Clinical Context Breakthrough bleeding (BTB) is the most common reason for POP discontinuation, occurring in 20–30% of users. The first step in evaluation is to **exclude pregnancy**, as POPs are less reliable than COCs (failure rate ~9 per 100 woman-years) and do not reliably suppress ovulation in all cycles. ### Why Serum β-hCG? **Key Point:** Before attributing abnormal bleeding to POP use, pregnancy must be excluded. POPs have higher failure rates than COCs because they rely on cervical mucus thickening and endometrial atrophy rather than ovulation suppression. Ectopic pregnancy is also a consideration. **High-Yield:** Differential diagnosis of BTB on POP: 1. **Pregnancy** (including ectopic) — must rule out first 2. Breakthrough ovulation (inadequate progestin effect) 3. Endometrial atrophy (expected with POP) 4. Structural pathology (polyp, fibroid, infection) — less common 5. Drug interaction (rifampicin, anticonvulsants reduce POP efficacy) ### Investigation Sequence ```mermaid flowchart TD A[Abnormal bleeding on POP]:::outcome --> B{Exclude pregnancy first}:::decision B -->|Positive β-hCG| C[Confirm with ultrasound]:::action B -->|Negative β-hCG| D{Symptoms of infection?}:::decision D -->|Yes: fever, discharge| E[Pelvic exam + culture]:::action D -->|No| F{Severe bleeding?}:::decision F -->|Yes| G[Pelvic ultrasound for structural pathology]:::action F -->|No| H[Counsel on expected BTB with POP]:::outcome H --> I[Consider alternative progestin or COC]:::action ``` ### Why Other Options Are Wrong | Investigation | When Indicated | Why Not First-Line Here | |---|---|---| | **Serum β-hCG** | **Always first** — before any imaging or further workup | Pregnancy is the most common cause of abnormal bleeding | | Pelvic ultrasound | After pregnancy excluded; if heavy bleeding or structural suspicion | Premature — imaging is secondary; pregnancy must be ruled out first | | Cervical Pap/HPV | Routine screening, not for BTB evaluation | Cervical pathology does not cause BTB; this is a screening test | | HSG | Infertility workup, not BTB evaluation | Patient is nulliparous but not presenting for infertility; HSG is not indicated | **Clinical Pearl:** POPs are ideal for breastfeeding women and those with contraindications to estrogen (migraine with aura, VTE history, age >35 smoking). However, BTB is common and often leads to discontinuation. Counseling before initiation improves continuation rates. **Mnemonic:** **POPS** = **P**rogestin-**O**nly — **P**regnancy first, **S**tructural pathology second **Citation:** Park 26e Ch 8 (Family Planning); WHO Medical Eligibility Criteria 5th edition