Is it time to abandon the hCG discriminatory zone?

I've written about ectopic pregnancy a few times now (see this and this).  The use of hCG testing in the evaluation of a woman with a suspected ectopic pregnancy is invaluable.  For many years doctors have relied upon the concept of a "discriminatory zone."  That is, an hCG concentration above which an intrauterine pregnancy should always be visible using transvaginal ultrasound.  The hCG concentration that is often used as the discriminatory zone is between 1,000 and 2,000 IU/L.  If no fetus is seen then the woman may receive treatment for a presumed ectopic pregnancy.

Early recognition and treatment of an ectopic pregnancy is critical because it is a leading cause of maternal death in the first trimester.  Ectopic pregnancies are terminated by the use of the drug methotrexate or surgery.  Methotrexate is a folic acid antagonist and a powerful teratogen that causes malformations in a developing fetus.  The use of methotrexate has increased substantially in the last few years because it has fewer risks and is less expensive than surgery.  However, methotrexate is sometimes given to women with an erroneous diagnosis of ectopic pregnancy which results in the loss of a viable pregnancy or the delivery of infants with birth defects.

Two recent reports have shown a spotlight on this problem:

  • The first study reported the outcomes of 8 pregnancies that were incorrectly diagnosed as ectopic and in whom the mother was treated with methotrexate.  Sadly and unsurprisingly, all 8 pregnancies had terrible outcomes.  Two pregnancies resulted in severely malformed infants.  One was liveborn at 37 weeks and had Tetralogy of Fallot, pulmonary atresia, congenital scoliosis, 7 ribs on left side and 11 ribs on right side, and a single kidney.  The other was stillborn at 30 weeks and had Tetralogy of Fallot, horseshoe kidney, and a single umbilical artery.  The other 6 pregnancies were aborted, 3 spontaneously and 3 deliberately.
  • The second study addressed the reliability of the hCG discriminatory zone by evaluting 202 patients that met the following criteria: 1) a transvaginal sonogram showing no intrauterine pregnancy; 2) an hCG test performed on the same day as the ultrasound; 3) documentation of a subsequent viable intrauterine pregnancy.  80% had an hCG concentration less than 1,000 IU/L (well below the discriminatory zone), in 9% it was between 1,000 and 1,499 IU/L, in 6% it was between 1,500 and 1,999 IU/L, and in 5% it was 2,000 IU/L or greater (above the discriminatory zone).  The highest hCG concentration observed was 6,567 IU/L.

While the idea of an hCG discriminatory zone is an appealing one, it is clearly not something that can be relied on to make an important therapeutic decision.  The authors of the second study (above) concluded exactly that and recommend using follow-up sonography and serial hCG testing in hemodynamically stable patients before treating for presumed ectopic pregnancy.

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