Rationalizing the management of pregnancies of unknown location: temporal and external validation of a risk prediction model on 1962 pregnancies

STUDY QUESTION Can we accurately define a group of pregnancies of unknown location (PULs) as low risk in order to safely reduce follow-up for these pregnancies and allocate resources to pregnancies at an increased risk of being ectopic? SUMMARY ANSWER Prediction model M4 classified around 70% of PULs as low risk, of which around 97% were later characterized as failed PULs or intrauterine pregnancies (IUPs), while still classifying 88% of ectopic pregnancies as high risk. WHAT IS KNOWN ALREADY Depending on the level of suspicion of ectopic pregnancy (EP), women with a PUL receive a lengthy follow-up in order to confirm the location and viability of the pregnancy. STUDY DESIGN, SIZE, DURATION A multi-centre diagnostic accuracy study of 1962 patients was carried out between 2003 and 2007 for retrospective temporal validation and between 2009 and 2011 for prospective external validation. The reference standard is the final characterization of PUL as failed pregnancies or IUPs (low risk), or as ectopic pregnancies (high risk). M4 is a multinomial logistic regression model based on the serum human chorionic gonadotrophin (hCG) levels at presentation and 48 h later. PARTICIPANTS/MATERIALS, SETTING, METHODS Temporal validation data from 1341 PULs collected at St George's Hospital in London were available, of which 53% were failed, 39% were intrauterine and 8% were ectopic pregnancies. External validation data from 621 PULs collected at four other London-based teaching hospitals were available, of which 63% were failed, 22% were intrauterine and 15% were ectopic pregnancies. MAIN RESULTS AND THE ROLE OF CHANCE The EP rate varied between 8 and 16% across the five hospitals. At St George's, 980 [73.1%, 95% confidence interval (CI): 70.5–75.4] PULs were considered low risk. Of these, 963 were failed PULs or IUPs (98.3%, 95% CI: 97.2–98.9) and 17 were ectopic pregnancies. At the other four hospitals, 62–75% were considered low risk, with 96–98% of these turning out to be failed PUL or IUP. Eighty-five percent (95% CI: 76.8–90.2) of the ectopic pregnancies were considered high risk at St George's, compared with 80–92% in the other hospitals. LIMITATIONS, REASONS FOR CAUTION Of total, 120 patients had been excluded due to loss to follow-up, and a further 102 patients because of missing hCG levels due to differences in local clinical practice. There are variations in the definition of a PUL used in different countries. WIDER IMPLICATIONS OF THE FINDINGS The suggested prot