Beta-HCG of Little Value For Ectopic Pregnancy Dx

July 2008

By Michele G. Sullivan
Elsevier Global Medical News

SAN FRANCISCO - A combination of ultrasound and β-human chorionic gonadotropin is the best way to diagnose extrauterine pregnancy, although neither one on its own can be completely relied upon as a diagnostic tool.

Ultrasound - either transvaginal or transabdominal - will identify only 20% of extrauterine pregnancies, whereas the much-touted "β-HCG discriminatory zone" offers little more than the most rudimentary diagnostic guidance, said Dr. Eric Snoey, an emergency physician at Alameda County Medical Center, Oakland, Calif.

"There are not many things in medicine that are worse [than the ability of ultrasound to identify ectopic pregnancy]," Dr. Snoey said at the 12th International Conference on Emergency Medicine. "What we end up doing most of the time is ruling in intrauterine pregnancy instead."

Another ultrasound clue to an ectopic pregnancy is free fluid near the adnexa, although that isn't an ironclad diagnostic tip-off. "About half the time, this is from a leaking or ruptured extrauterine pregnancy," he said, "but the other half [of the time], it's from a ruptured physiologic cyst."

When ultrasound picks up an adnexal mass, of course, the conclusions are much more obvious. "If you see a complex adnexal mass with a heartbeat, obviously, it's an ectopic pregnancy," Dr. Snoey said. But 80% of women with an ectopic pregnancy will have an initial ultrasound that is either normal or nondiagnostic, with nothing showing in the uterus or adnexa.

In those cases, β-HCG levels offer little guidance, because the discriminatory zone (i.e., the β-HCG level at which it is assumed that all viable intrauterine pregnancies can be visualized by ultrasound) varies widely, Dr. Snoey said. The zone can be anywhere from 1,000 to 2,000 mIU/mL, depending on the institution setting the rule.

"The discriminatory zone causes a lot of consternation, and people tend to give it an unusual importance," he said. "While it's true that if you have a β-HCG level below the discriminatory zone you are unlikely to see an intrauterine pregnancy, it's also true that you won't necessarily see one if the level is above the zone." In addition, studies have shown that ectopic pregnancies tend to produce lower amounts of β-HCG, he said, further eroding the diagnostic certainty of the hormone level.

A 2003 study points up the problem, Dr. Snoey added. That retrospective study included 730 women with early pregnancy who presented to an emergency department with pain and/or bleeding.

A β-HCG level less than 1,500 mIU/mL more than doubled the odds of ectopic pregnancy, Dr. Snoey said. Of the 158 patients with β-HCG below 1,500 mIU/mL, 25% had ectopic pregnancies, and 16% had normal intrauterine pregnancies (Acad. Emerg. Med. 2003;10:119-26). "Almost half of the ectopics in this study fell below the discriminatory zone," Dr. Snoey said.

Other studies confirm the association of low levels and ectopic pregnancy. "One found that 40% of women with a ruptured ectopic had a β-HCG of less than 1,000 mIU/mL, so there is some evidence that the lower the number, the worse the outcome," he said.

However, ectopic pregnancy can occur at any level of β-HCG, he stressed. "There are plenty of reports with levels in the 50,000-75,000 mIU/mL range, and plenty where it is extremely low, in the range of 10, 15, and 25 mIU/mL."

Although ectopic pregnancy is rare - accounting for only 2% of pregnancies overall - it is fairly common among women presenting to the emergency department with first trimester pain or bleeding (up to 15% of cases). Risk factors for ectopic pregnancy are present in about 40% of cases. The usual symptoms (pain and bleeding) are not discriminatory of ectopic pregnancy, and more bleeding and more pain are in no way predictive of it.

"Of course, if the cervical os is open and you see products of conception being expelled, you can rule it out," Dr. Snoey said. That is true except for heterotropic twins, in which one embryo implants in the uterus and the other outside it, he added.

Algorithm for Ectopic Diagnosis

Dr. Eric Snoey offered a treatment algorithm that relies completely on the history, physical, and ultrasound. "In my algorithm, there is no place for β-HCG, because I feel it really has no role in the decision-making process for ectopic pregnancy," he explained. The algorithm includes the following steps:

arrow redDetermine if the symptomatic patient is hemodynamically stable. If she is not stable, go to resuscitation and the FAST (Focused Abdominal Sonography for Trauma) exam. If she is stable, try to determine the location and development of the embryo by using transvaginal or transabdominal ultrasound.

arrow redIf the patient has a threatened abortion (i.e., intrauterine pregnancy confirmed by ultrasound with pain, possible bleeding, and closed os), discharge her with a recommendation for bed rest and a quick follow-up with the obstetrician, or a quick return to the ED if symptoms worsen.

arrow redIf no intrauterine pregnancy can be seen, then scan the adnexa. If the scan shows a complex adnexal mass or an unusual amount of free fluid, get an immediate obstetric consult.

arrow redWith a negative or indeterminate scan, discharge the patient home if vital signs are stable and pain and bleeding are acceptably low. Advise a follow-up β-HCG test within 48 hours.

arrow redIf the patient is experiencing unacceptable levels of pain and bleeding, then admit her for observation.

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