Ectopic pregnancy and the hCG discriminatory zone

I’ve written about the use of hCG testing in the evaluation of patients with suspected ectopic pregnancies.  As a reminder, the diagnostic tests used in the work-up of a woman with symptoms of an ectopic pregnancy include a combination of transvaginal ultrasound and the laboratory measurement of serum hCG concentrations.

Transvaginal ultrasound is used to visualize the fluid-filled, gestational sac in which the embryo is growing. Ultrasound may reveal 1) a normal, intrauterine pregnancy, 2) an ectopic pregnancy, or 3) neither. The latter finding would be considered an indeterminate result and occurs in about 10-30% of women being evaluated for ectopic pregnancy. An indeterminate result could represent early intrauterine pregnancy, ectopic pregnancy, or fetal loss.

These indeterminate results are often interpreted against the hCG “discriminatory zone,” the concentration of serum hCG above which a gestational sac in the uterus should be readily observed. The discriminatory zone is often considered to be between 1,500 and 3,000 IU/L.

A recent study investigated how well a serum hCG concentration ≥3,000 IU/L worked to differentiate an ectopic pregnancy from a normal pregnancy in women with symptoms of ectopic pregnancy but with indeterminate ultrasound findings. What they reported was quite interesting.

There were 141 women with an indeterminate ultrasound result and 57 of these had an hCG concentration ≥3,000 IU/L. Of these, 82% actually did have intrauterine pregnancies even though their hCG concentrations were above the discriminatory zone.

There is a very important methodological consideration to consider in this study. Pelvic rather than transvaginal ultrasonography was used and emergency department physicians and not radiologists performed it. The study purposely used pelvic ultrasound because its use in an emergency department allows for the rapid and accurate detection of an intrauterine pregnancy in the majority of cases. It also can result in a shorter stay for the patient, which, presumably, would result in lower costs. In this same study, 99% of 115 women with an intrauterine pregnancy were identified as having one using pelvic ultrasound.

The main conclusion of this study is that in women with an indeterminate pelvic ultrasound, the use of an hCG discriminatory zone does not provide information to help differentiate ectopic from intrauterine pregnancy.

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