Special Feature: Should Every Patient Have Second-Trimester Biochemical Testing Regardless of Amniocentesis?
By John C. Hobbins, MD
We have had many patients decline triple or quad screen testing because "it would not be useful since I am having an amniocentesis" or "the test has too many false positives." Often these statements represent paraphrased comments made by their providers.
The "false positive" concept is one we deal with on a day-to-day basis and represents a misconception about screening tests in general. A false-positive result means that a diagnostic test indicates a problem to be present when it is not. A positive screening test indicates that a given patient is at higher or lower risk for a problem, and often the test can quantify that risk. It would be inappropriate to apply the false-positive concept to a patient with a risk of Down syndrome of 1 in 200 (designated by the lab as a "positive"). This would give her a false-positive rate of 99.5%. Also, by applying triple screening to a population at risk, the test justifiably reassures far more commonly that it raises concerns unnecessarily. That said, the point of this special feature is to demonstrate that there is more to biochemical testing in the second trimester than screening for aneuploidy or neural tube defects. Each analyte contributes selective information about a complex physiological situation involving mother, placenta, and fetus.
Virtually all of the AFP found in the maternal circulation comes from the fetus. The concentration of AFP in the fetal circulation, all emanating from the fetal liver in the second trimester, is in mg percentage. In the amniotic fluid it registers in microgram amounts and in the maternal circulation it is measured in nanograms. Therefore, when the placenta fails on its gate-keeping activity, even the small amounts of transferred AFP will have a major effect on maternal concentrations of the analyte.
Many studies have shown a higher rate of IUGR, maternal hypertension, and intrauterine demise when MSAFP is elevated in the second trimester. A study from Oregon has indicated that in an at-risk group of 113 patients with elevated MSAFP, 35 had an adverse pregnancy outcome. Of these, 12 had heightened surveillance testing and 22 had routine care. The heightened surveillance did not achieve earlier or improved detection of adverse outcome.
A review of the literature has indicated that the dreaded intrauterine demise, although increased above baseline rates in elevated MSAFP, is still very uncommon and very rare in the absence of IUGR.
Human Chorionic Gonadotropin (HCG)
Most second-trimester assays only quantify total HCG, which is an exclusive product of the placenta. In vitro studies show that when placental tissue is hypoxic, HCG is released. This could suggest that a somewhat poorly perfused placenta (perhaps due to inadequate spiral artery invasion) could be responsible for the HCG elevation seen in patients developing higher rates of preeclampsia and IUGR. Also, very low HCG (< 0.20 MoM) has been associated with pregnancy loss.
This is a product of a complex interaction involving mother, placenta, and fetus. Interestingly, it adds little diagnostic clout to the triple screen, accounting at most for a few additional percentage points to the sensitivity of the test (regarding aneuploidy). It also has very little value in prediction of later adverse outcome, perhaps because it only indirectly reflects placental function.
This is a product of the placenta and is the newest analyte now comprising the "quad screen." Individually, it may be the most sensitive screener of Down syndrome and several studies have shown it to be correlated with later IUGR and preeclampsia. When 2 or more analytes are elevated (above 2.0 MoM), the rate of adverse outcome increases appreciably, as do the chances of preterm delivery.
One study from Providence indicated that a combination of maternal age, HCG, and inhibin-A had a sensitivity of 23% and specificity of 95% in the prediction of preeclampsia. Inhibin-A was a better performer individually than HCG.
Role of Doppler Uterine Artery Assessment
Very few studies are available combining biochemical screening in the second trimester with uterine artery Dopplers, but it makes sense to evaluate the uterine arteries in patients with elevations of MSAFP, HCG, or Inhibin-A since this will provide information about trophoblastic invasion of the spiral arteries.
Role of Echogenic Bowel’
The presence of Echogenic fetal bowel, diagnosed through ultrasound, is strongly associated with intrauterine demise in the presence of elevated AFP. However, again this is virtually always in growth-restricted fetuses.
What to do when MSAFP, HCG, or Inhibin-A are elevated? The 2 most common problems in these patients are hypertension (generally preeclampsia) and IUGR. Clinicians should have a high level of sensitivity for early signs of preeclampsia (Proteinuria and increase in blood pressure from baseline values) in these patients. Every patient with any of these elevations should also have a careful clinical assessment of uterine size assessment at each visit and an ultrasound examination at 30-32 weeks to see if fetal growth is lagging. If so, then Doppler assessment of the fetal circulation, and non-stress testing and biophysical profiles should be initiated, along with serial biometry every 2 weeks.
Since intrauterine demise is rare in an appropriately grown fetus with normal amniotic fluid, NSTs and BPPs need not be undertaken in these patients.
Can adverse outcome be prevented in patients with elevations of components of the quad screen? In 2 large NIH-supported studies, low-dose aspirin did not diminish the incidence of preeclampsia in historically "low-risk" and "high-risk" patients. However, in a recent meta-analysis involving patients with abnormal uterine arteries at 18-24 weeks (covered in a previous OB/GYN Clinical Alert), there was an increase in fetal weight and a significant decrease in the development of severe preeclampsia when low-dose aspirin therapy was initiated. This type of information is not yet available regarding aspirin in patients with elevated analytes, but the data from the above meta-analysis make a case for performing uterine artery Dopplers in these patients and treating if abnormalities are found.
Dr. Hobbins is Professor and Chief of Obstetrics, University of Colorado Health Sciences Center, Denver.
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