Objective. To externally validate five approaches to predict ectopic pregnancy (EP) in pregnancies of unknown location (PUL): the M6P and M6NP risk models, the two-step triage strategy (2ST, which incorporates M6P), the M4 risk model, and beta human chorionic gonadotropin ratio cut-offs (BhCG-RC). Design. Secondary analysis of a prospective cohort study.Setting. Eight UK early pregnancy assessment units.Population. Women presenting with a PUL and BhCG >25 IU/L.Methods. Women were managed using the 2ST protocol: PUL were classified as low risk of EP if presenting progesterone ≤2 nmol/L; the remaining cases returned two days later for triage based on M6P. EP risk ≥5% was used to classify PUL as high risk. Missing values were imputed, and predictions for the five approaches were calculated post hoc. We meta-analysed centre-specific results. Main outcome measures. Discrimination, calibration and clinical utility (decision curve analysis) for predicting EP.Results. Of 2899 eligible women, the primary analysis excluded 297 (10%) women who were lost to follow-up. The area under the ROC curve for EP was 0.89 (95% confidence interval 0.86-0.91) for M6P, 0.88 (0.86-0.90) for 2ST, 0.86 (0.83-0.88) for M6NP, and 0.82 (0.78-0.85) for M4. Sensitivities for EP were 96% (M6P), 94% (2ST), 92% (N6NP), 80% (M4), and 58% (BhCG-RC); false positive rates were 35%, 33%, 39%, 24%, and 13%. M6P and 2ST had the best clinical utility and good overall calibration, with modest variability between centres.Conclusions. 2ST and M6P performed best to predict and triage PUL.Funding. Research Foundation – Flanders (FWO; G0B4716N), Internal Funds KU Leuven (C24/15/037), NIHR Collaboration for Leadership in Applied Health Research & Care, NorthWest London (RDIP033), NIHR Biomedical Research Centre based at Imperial College Healthcare NHS Trust. Keywords Pregnancy of unknown location, ectopic pregnancy, beta human chorionic gonadotrophin (BhCG) ratio, progesterone, prediction model, prediction model validation
