Category Archives: Integrated Test

Integrated Test

Cell-free DNA screening tests in the general obstetrical population

DNAIt has been several years since cell-free DNA (cfDNA) tests for the detection of fetal aneuploidies became available. The first clinical studies of these tests were reported in women who, because of age or other reasons, were already at increased risk of having an affected pregnancy (i.e. “high risk” women). While these studies demonstrated the superior performance of cfDNA tests compared to traditional biochemical tests, their application to women at low risk was not encouraged because of lack of evidence regarding how well they would work in that population. A recent report on cfDNA screening tests in the general obstetrical population now provides much needed evidence.

Investigators at Brown University described several clinical utility aspects of cfDNA screening for common aneuploidies through the implementation of a statewide program called DNAFirst that offered cfDNA screening tests to the general pregnancy population in the state of Rhode Island. The clinical utility aspects that were investigated were a comparison of screening uptake rates before and after the DNAFirst program, an evaluation of a reflexive serum testing protocol for cfDNA tests that failed to produce a result, and explored women’s decision-making.

Over 11 months, 2,681 women agreed to undergo screening through 72 providers. Prior to undergoing testing, the women received information about cfDNA testing by primary obstetrical care providers. The median maternal age was 31 years and 79% of the women were younger than 35 years of age. There were 16 positive (i.e. abnormal for trisomy 21, 18, or 13) cfDNA results, 12 of which were confirmed as true positive and 4 of which were false-positive. 2,515 women had a negative screening result and all were true-negatives. 150 tests failed to produce a result (none of which were known to have trisomy). Collectively, these data produced a sensitivity of 100%, a positive predictive value of 75% and a false-positive rate of 0.15%. By comparison, the most effective biochemical screening test (the Integrated test) has a 90% detection rate, a 3% false-positive rate, and a positive predictive value of only 6%.

A small number of women who participated in the study (113) completed a survey asking them about their understanding of cfDNA testing. Women reported receiving information from their care provider in 9 minutes or less. While 85% understood that the test identified Down syndrome, 15% incorrectly thought it identified all genetic problems. 79% understood that a negative result did not rule out Down syndrome but 13% thought it did. These survey results suggest that most women do understand the basic concepts of cfDNA screening.

The study’s authors concluded that cfDNA screening tests perform very well in the general pregnancy population and that women understand the basic concepts of screening. Further, the tests were easily incorporated into routine practices. They encouraged clinical laboratories to offer cfDNA screening tests to improve access to better aneuploidy screening for the more than 2 million pregnant women in the United States who choose to undergo such testing each year.

Screening for Down syndrome in the United States

This year marks the 30th anniversary of the use of laboratory tests to screen pregnancies for Down syndrome. The tests have evolved over the last three decades and as have their ability to detect affected fetuses. Currently there are several different lab tests available to screen for Down syndrome during pregnancy, including the recently developed cell-free DNA tests (commonly referred to as non-invasive prenatal testing, or NIPT).

How has the landscape of Down syndrome screening evolved over the last few decades? That was the question considered by investigators of a recent report that sought to determine the number of women screened for Down syndrome in the United States in 2011 and 2012, along with the type of test they received.

The results of this report were based on surveys completed by 168 laboratories that offered Down syndrome screening tests in 2011 and/or 2012 and are rather interesting:

  • Of the 4.13 million pregnancies that occur each year in the United States, approximately 72% are screened for Down syndrome.
  • The most common screening test is the quadruple test (50%) followed by the first-trimester combined test (18%), and the sequential test (14%). The other types of tests (e.g. triple test, full integrated test, and serum integrated test) were less commonly performed.
  • The 6 largest laboratories (those that screened more than 100,000 pregnancies each year) performed 61% of all screening tests.
  • The 32 smallest laboratories (those that screened fewer than 1,500 pregnancies each year) performed only 1% of all screening tests.
  • Between 2011 and 2012, the use of the first trimester and integrated testing increased by 27% and 6% and the use of the quad test decreased by 1.2%.
  • As shown in the figure below, there was an inverse relationship between the percent of laboratories that offered testing only in the second trimester (e.g. triple or quadruple tests) and the number of pregnancies screened each year.

Labs offering only 2nd trimester tests

That last bullet point is an important one because nearly 70% of women have their first prenatal visit in the first trimester and it is recommended that integrated screening be offered at this visit. However, because the number of laboratories offering only second trimester testing is much greater than the number of labs that offer all types of screening tests, this recommendation may not be easily adopted.

DNA-based tests for Down syndrome screening show excellent clinical performance

The use of biochemical screening tests to identify pregnant women who are at high risk of having a fetus with Down syndrome is well established.  Biochemical screening began nearly 30 years ago and, over the years, the tests have evolved and improved.  Now there’s a new kid on the screening test block and it’s name is DNA.

The discovery of cell-free fetal DNA in maternal plasma in 1997 opened up new possibilities for Down syndrome and other aneuploidy screening protocols.  Rather than rely on biochemical testing to determine a biochemical phenotype, DNA-based tests have been developed that can detect the molecular pathology of aneuploidies (e.g. a fetus that has more than the expected 2 copies of chromosomes 21, 18, or 13; the cause of Down syndrome, Edwards syndrome, and Patau syndrome, respectively).

We’ve written about DNA-based screening tests before (here and here) and described the clinical performance of the Sequenom test.  Now, other clinical performance studies have been published for 3 of the 4 tests that are (or will be) commercially available.  As expected, all of them show excellent clinical performance.  As shown in the table below, the detection rates for trisomy 21 are greater than or equal to 99% with very low false-positive results.  Similar performance has been reported for trisomy 18 and 13.

DNA test performance

Table References: Genet Med 2011;13:913-920Genet Med 2012;14:296-305Obstet Gynecol 2012;119:890-901

By comparison, the detection rate of the best biochemical Down syndrome screening test (the Integrated test) is very good at 93%.  However, about 5% of all Integrated test results are false-positive.  A 5% false-positive rate may not seem very high but it is.  For example, consider a population of 100,000 pregnant women who choose Integrated testing in the second trimester.  The prevalence of Down syndrome in the second trimester is about 1 in 500 pregnancies so 200 of those 100,000 women will have a fetus with Down syndrome and 99,800 women (100,000 – 200) will have unaffected fetuses.  Of those 99,800 women with unaffected fetuses, 4,900 will have a false-positive Integrated test result.

Because the false-positive rate of the DNA-based tests is so low (about <0.2%), then if those same 100,000 women were screened there would be only 200 false-positive results, a 96% decrease!

Does this mean that DNA-based tests should replace biochemical screening tests?  Probably not but I’ll leave the explanation as to why for my next post.