Category Archives: Ultrasound

Ultrasound

Laboratory testing for premature rupture of membranes


This post is by a guest author, Douglas Stickle, Ph.D.  Dr. Stickle is a professor in the Department of Pathology at Thomas Jefferson University and the director of chemistry and point-of-care testing at Jefferson University Hospitals in Philadelphia, PA.

Rupture of membranes (ROM) is the term used to describe the breaking of the amniotic sac, as normally occurs before the onset of labor. If this happens earlier than the 37th week of pregnancy it is called preterm ROM (PROM). It’s a condition that can lead to a preterm birth, or, if very early, a preterm, premature birth.

Preterm baby 2When PROM happens, there is an increased risk of complications due to intrauterine infection, umbilical cord compression, and the neurodevelopmental disorders that are associated with a preterm delivery. Diagnosis of PROM is particularly important when the gestational age is incompatible with a viable birth, often considered to be a fetal age less than 24 weeks. In such cases, medical intervention is necessary to preserve the chances for a live birth.

Suspected cases of PROM are often investigated by laboratory analysis of fluid obtained from the vagina to detect properties or substances that should otherwise not be present unless the fluid contains amniotic fluid due to PROM. The simplest forms of testing are measurement of acidity (pH) of the fluid, or a test called "fern" testing. Fern testing refers to the fern-like appearance of amniotic fluid when it is dried on a glass slide. Both of these tests aren’t very accurate and so other tests have been developed to better identify patients with ruptured membranes.

These other tests are designed to detect molecules that are normally present in amniotic fluid but not vaginal fluid. For example, tests have been developed that detect alpha-fetoprotein (AFP) or insulin-like growth factor binding protein-1 (IGFBP-1). The presence or absence of these molecules in the specimen are determined by a lateral flow immunoassay. The assay works like commonly performed tests for human chorionic gonadotropin (hCG) (aka pregnancy tests).

These tests are highly sensitive to low concentrations of these molecules, which is both good and bad. It’s good because they can detect small amounts of the molecules and lead to a more accurate diagnosis. It’s bad because these two molecules are also present in maternal blood which means that if a sample is contaminated with blood, the certainty of a positive test to detect amniotic fluid is called into question.

From the doctor’s perspective, a practical advantage of the immunoassays is that their results are binary – the result is either positive or negative — whereas the pH test and the fern test are more subjective and difficult to interpret definitively. However, the AFP and IGFBP-1 tests may be subject to false-positive results as the gestational age of the fetus approaches term. This suggests that, at later stages of pregnancy, these biomarkers may signify imminence of delivery.

The gold standard, or best test, to diagnose rupture of membranes is a dye test, in which a colored fluid is injected into the amniotic fluid followed by direct observation to see if the dye subsequently appears in the vaginal pool fluid. Also, ultrasound imaging of the amniotic fluid volume may also assist in diagnosis of PROM, but in individual cases such imaging may be difficult to interpret. Given the low but finite risk of complications of the dye test, the AFP and IGFBP-1 tests are often preferred as first-line tests for preterm premature rupture of membranes.

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.