Objective: To determine a discriminatory level for serum progesterone (SP) in pregnant patients with no definite intrauterine pregnancy (IUP) on endovaginal ultrasonography (US) in the differentiation of ectopic pregnancy from normal IUPs.
Methods: A prospective observational study in a convenience sample of women at risk for ectopic pregnancy was performed at an urban teaching hospital from May 1991 until May 1994. Women aged > or =18 years presenting to the ED with a positive pregnancy test in combination with pelvic or abdominal pain, vaginal bleeding, orthostasis, adnexal mass or tenderness, or any historical risk factor for ectopic pregnancy were eligible. Hypotensive or unstable patients were excluded. Endovaginal US was performed and patients with no definite IUP had a serum beta-hCG and SP measured.
Results: 314 patients were enrolled, with 14 excluded for lack of follow-up or incomplete SP data, yielding 300 patients. The initial endovaginal US diagnoses included 169 definite IUP, 31 abnormal IUP, 5 definite ectopic pregnancy, and 95 no definite IUP. 68/95 had SP measured, with values of 22.8 +/- 13.4 ng/mL (mean +/- SD) for IUP; 4.9 +/- 6.5 for spontaneous abortion, and 7.5 +/- 7.2 for ectopic pregnancy. The mean values were significantly different (2-tailed t-test) for ectopic pregnancy vs IUP and for spontaneous abortion vs IUP. An SP of > or =11 ng/mL (sensitivity 91%; specificity 84%) was post hoc the best cutoff value for suggesting an IUP when the endovaginal US was not definite for IUP.
Conclusions: SP cannot reliably discriminate ectopic pregnancy vs spontaneous abortion in pregnant patients with no definite IUP on endovaginal US; however, a low SP (<11 ng/mL) in this sonographic category suggests an abnormal pregnancy.