Laboratory testing in ectopic pregnancy investigation

What’s the leading cause of maternal death in the first trimester? That would be an ectopic pregnancy, which occurs when the fertilized egg implants someplace other than the inside of the uterus. In the vast majority of ectopic pregnancies the zygote implants in the fallopian tube, an organ that, unlike the uterus, can’t expand to accommodate the growing embryo. If not diagnosed early, an ectopic pregnancy can cause the fallopian tube to rupture that can result in massive internal bleeding and death.

 Diagnosing an ectopic pregnancy is challenging. The typical symptoms of abdominal pain and vaginal bleeding are not present in all patients and can be due to reasons other than an ectopic pregnancy. Also, the risk factors of ectopic pregnancy are absent in about half of all women who have an ectopic pregnancy. Fortunately there are laboratory tests that assist in identifying an ectopic pregnancy.

 hCG testing is first used to confirm that the patient is, indeed, pregnant. Although a rapid, point-of-care urine test may be used to detect hCG, I think it is much safer to perform a serum hCG test because it can detect lower concentrations of hCG than a urine test can. If the test is negative, that is hCG is not present, then the patient isn’t pregnant at all. If hCG is detected then pregnancy is confirmed but, all by itself, a single hCG test cannot identify a normal from an ectopic pregnancy.

 To do that, other investigations are commonly put to use:

  1. Transvaginal ultrasound. Ultrasound imaging is used to visually observe if an embryo is in the uterus or the fallopian tube but it can also be inconclusive if no embryo is observed anywhere. If the ultrasound is inconclusive then the concentration of serum hCG can be useful. That’s because an intrauterine pregnancy should always be able to be seen by ultrasound when the serum hCG concentration is greater than 1500 to 2000 IU/L. If that threshold is reached and no embryo is seen in the uterus then the patient is likely to have an ectopic pregnancy. If that threshold is not present at the initial evaluation, then serial hCG testing is considered.
  2. Serial hCG testing. Collecting more than 1 blood sample over time for hCG testing is helpful because in a normal pregnancy serum concentrations of hCG increase by 53% or more every 2 days. An increase that is less than 53% is cause for concern. That’s because an abnormal increase in indicates an abnormal pregnancy. In an ectopic pregnancy, hCG can show a normal or abnormal increase so an abnormal increase by itself it isn’t diagnostic of an ectopic pregnancy.

 These two tests are usually used together in an algorithm that looks something like this:



Ectopic algorithm


The take home message here is that, as helpful as lab tests are, the diagnosis of ectopic pregnancy cannot be accomplished by lab tests alone. All clinical data must be evaluated in order to arrive at the final diagnosis.


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