Beta Subunit of Human Chorionic Gonadotropin

Beta Subunit of Human Chorionic Gonadotropin

The mean plasma concentration of human chorionic

gonadotropin (hCG) is significantly lower for an ectopic

pregnancy than for a viable intrauterine pregnancy, but

there is no definitive laboratory level permitting distinc-

tion between the two. A consistently declining hCG level

indicates a nonviable pregnancy.

Conventionally, serial hCG testing to diagnose sus-

pected ectopic pregnancy is performed at 48-hour

intervals; a 66% or greater increase should be observed in

a normal pregnancy. Approximately 15% of normal intra-

uterine pregnancies are associated with less than a 66%

increase in hCG, and 17% of ectopic pregnancies have

normal doubling times (13). Limitations of serial hCG

testing include its inability to distinguish a failing intra-

uterine pregnancy from an ectopic pregnancy and the

inherent 48-hour delay. A prospective study of asymp-

tomatic patients described a 36% sensitivity and a 63–71%

specificity (14). However, most reports and clinicians have

found serial hCG testing useful in the early diagnosis of

ectopic pregnancy. The rate of hCG doubling decreases

from every 1.4–1.5 days in early pregnancy to every 3.3–

3.5 days at 6–7 weeks of gestation, at which point the

reliability of serial testing may be diminished.

The elimination of hCG after treatment of ectopic

pregnancy follows a two-phase distribution. The major

elimination has a half-life of 5–9 hours and a second, longer

phase has a half-life of 22–32 hours.

The quantitation of hCG has been complicated by the

existence of three different reference standards for hCG

assays, the existence of multiple antibodies in commercial

assays, and confusing nomenclature. These complicating

factors can cause varying and inconsistent results, both from

one laboratory to another and within the same laboratory,

and affect interpretation of the results and clinical manage-

ment.

  1. Kadar N, Caldwell BV, Romero R. A method of screening

for ectopic pregnancy and its indications. Obstet Gynecol

1981;58:162–166 (Level II-2)

  1. Shepherd RW, Patton PE, Novy MJ, Burry KA. Serial beta-

hCG measurements in the early detection of ectopic preg-

nancy. Obstet Gynecol 1990;75:417–420 (Level III)