The blood test for Down syndrome, also called cell-free DNA screening, is a highly accurate screening test that estimates the risk of the fetus having Down syndrome and some other chromosomal abnormalities. The test analyzes small fragments of DNA from the fetus that are present in the pregnant woman’s bloodstream. Here are some quick answers about the accuracy of this test:
What is the detection rate of the blood test for Down syndrome?
The blood test detects around 99% of Down syndrome cases. This means it has a very low false negative rate of less than 1%. However, false negative results can still occur in rare cases.
What is the false positive rate of the test?
The false positive rate of the standard blood test is around 0.1-0.2%. This means that about 1 in 500 to 1 in 1000 positive results will be wrong. The false positive rate can be higher for women at lower risk.
How accurate are low risk and high risk results?
For women who receive a low risk result, the chance of the baby having Down syndrome is much lower than the background risk for her age group. A high risk result does not mean that Down syndrome is definitely present, but indicates a higher chance that it may be.
Can the blood test diagnose Down syndrome?
No, the cell-free DNA test cannot definitively diagnose Down syndrome or other chromosomal conditions. It simply provides a probability. A diagnostic test like amniocentesis or CVS is needed for a confirmed diagnosis.
What affects the accuracy of the blood test?
Accuracy is affected by maternal weight, gestational age, fetal fraction of DNA, and presence of chromosomal mosaicism. Low fetal fraction or high maternal weight can increase false positives and negatives. Mosaic Down syndrome is more likely to be missed than full trisomy 21.
Conclusion
The cell-free DNA blood test for Down syndrome is highly accurate, with detection rates over 99% and a false positive rate of 0.1-0.2%. No screening test is perfect, but it is an excellent noninvasive way to assess risk early in pregnancy. Low and high risk results are not definitive, and confirmatory diagnostic testing is recommended if a chromosome abnormality is suspected. Speak to your doctor about your individual risk factors when interpreting results.
How does the test work?
The cell-free DNA test analyzes DNA from the placenta that is circulating in the mother’s bloodstream. During pregnancy, some of the placenta breaks down and releases DNA from the fetus into the mother’s blood. The test looks at fragments of this cell-free fetal DNA for extra or missing chromosome material associated with Down syndrome and other trisomies.
It does this by comparing amounts of DNA from different chromosomes – too much or too little DNA from a given chromosome points to a chromosomal anomaly. The test is typically done after 10 weeks of pregnancy using a regular blood draw.
There are two main techniques used:
Shotgun sequencing: Thousands and thousands of DNA fragments are sequenced and mapped to their chromosome of origin. The number of fragments from each chromosome is counted and analyzed to detect any excess or shortfalls.
Targeted sequencing: This technique only looks at DNA fragments from the specific chromosomes involved in the conditions being tested for, like chromosomes 21, 18 and 13. Abnormal amounts point to a higher risk for a trisomy.
What conditions can it detect?
The cell-free DNA test can check for:
– Down syndrome (trisomy 21) – caused by an extra copy of chromosome 21
– Trisomy 18 (Edwards syndrome) – an extra chromosome 18
– Trisomy 13 (Patau syndrome) – an extra chromosome 13
– Sex chromosome aneuploidies – extra or missing X or Y chromosomes
Some labs also offer screening for microdeletions of chromosomes and other rare trisomies. Talk to your doctor about what specific conditions are included in the version of the test they offer.
Detection rates for various trisomies:
| Condition | Detection Rate |
|---|---|
| Down syndrome (trisomy 21) | 99% |
| Trisomy 18 | 97% |
| Trisomy 13 | 92% |
Who should have the test?
Professional guidelines generally recommend offering Down syndrome screening, including the cell-free DNA test, to all pregnant women. The test performs best for high-risk pregnancies, but can also give valuable information for lower risk women who want early screening.
Some reasons a woman may choose cell-free DNA screening include:
– Advanced maternal age – 35 years or older
– Previous pregnancy affected by Down syndrome or Trisomy 18/13
– Positive first trimester screening test
– Ultrasound findings indicating higher risk
– Family history of chromosome abnormalities
Talk to your doctor about whether you should consider cell-free DNA screening based on your specific risk factors and preferences. The test is not routinely recommended for twin pregnancies or egg donor pregnancies at this time.
How early can the test be done?
The cell-free DNA test is typically performed after 10 weeks gestation. Earlier than this, there may not be sufficient cell-free fetal DNA in the maternal bloodstream to get a reliable result.
A few specialty labs may offer testing as early as 9 weeks. The optimal timing is between 10-22 weeks. Testing too early or too late increases the chances of a failed sample or inaccurate result.
Recommended timing:
– 10-22 weeks gestation is ideal
– Can be done as early as 9 weeks at some labs
– Less accurate before 10 weeks or after 22 weeks
Talk to your doctor about the best timing for you. Earlier testing around 10 weeks provides risk information sooner, but waiting until after 12 weeks reduces the chances of a failed test due to low fetal fraction.
How long do results take?
Standard turnaround time for results is approximately 5-7 calendar days from the time the sample arrives at the lab. Some laboratories may offer faster testing options with results in as little as 2-3 days for an additional fee. Talk to your doctor about available timing options at the lab they use.
The required processing time at the lab is roughly 2-4 days. The exact time frame depends on factors like batching frequency, the lab’s workload, and insurance approval. Courier transit time must also be accounted for.
Rush results or same day testing is not possible with current technology. The fetal DNA must be extracted from the sample and analyzed through a complex process that cannot be shortened. If you need genetic results immediately for any reason, diagnostic tests like CVS may be recommended over cell-free DNA screening.
Typical result timeframes:
– Standard: 5-7 days
– Faster expedited: 2-3 days (extra fee may apply)
– Stat results not available for this test
How much does the test cost?
The out-of-pocket cost for a cell-free DNA prenatal screening test ranges from about $200 to $800 USD depending on the lab and insurance coverage. Pricing is affected by the lab performing the test, additional genetic conditions being screened for, and your health plan benefits.
Many insurance plans cover this screening as part of routine prenatal care for women over age 35 or those at high risk. Coverage for lower risk patients varies. Government programs like Medicaid often fully cover the test in eligible women. Contact your insurance provider for your specific coverage details.
Some labs offer financial assistance or capped costs for women who are underinsured or uninsured. Your doctor’s office can provide information on pricing for your circumstances.
Cost range:
– $200 to $800, on average
– Varies by lab, additional screens, and insurance
– Often covered for women 35+ or high risk
– Financial assistance may be available
What is a high risk vs low risk result?
The test provides a probability score that indicates your risk of carrying a fetus with Down syndrome or another trisomy. It calculates this risk based on the measured chromosomal material in your blood sample.
Low risk results indicate a risk below the general population risk for your age – usually less than 1 in 10,000. High risk results indicate a risk above your baseline age-related risk, typically above 1 in 100.
Risk result interpretation:
Low risk: Less than 1 in 10,000 chance of trisomy 21 or 18. Very reassuring.
High risk: Greater than 99% chance of trisomy 21. Further testing recommended.
Talk to your doctor about how risk estimates apply specifically to your pregnancy based on your age and other factors. Results close to the cutoffs in a “grey zone” may require additional evaluation as well.
Does a high risk result mean the baby definitely has Down syndrome?
No, a high risk cell-free DNA result does not definitively mean the baby has Down syndrome or Trisomy 18/13. It indicates an increased risk based on the DNA analysis, but this screening cannot diagnose or confirm chromosomal abnormalities.
A high risk result should be followed up by a diagnostic test like amniocentesis or chorionic villus sampling (CVS) to check the baby’s chromosomes directly. Only a chromosome analysis on fetal or placental cells can definitively confirm Down syndrome.
Around 80-90% of high risk cell-free DNA results are true positives that are confirmed positive by diagnostic testing. In 10-20% of cases, the baby does not actually have the condition. Talk to your doctor about recommended next steps to definitively diagnose any chromosome anomalies suggested by screening.
What does it mean if I have a low risk result?
A low risk cell-free DNA result indicates your risk of having a baby with Down syndrome or Trisomy 18/13 is lower than the average background risk for your age group. For example, a risk score of 1 in 10,000 or less is generally considered very reassuring.
However, while a low risk result reflects a reduced chance that your baby has these conditions, it does not guarantee the absence of Down syndrome or other chromosomal issues. The test only estimates probability – it cannot entirely rule out the chance of rare false negatives.
Your doctor will help interpret your specific low risk result in context with other factors like ultrasound findings, age, and family history. Repeat testing later in pregnancy may occasionally be recommended for very high-risk patients to maximize detection.
Can I get a “no result” from the test?
It is possible to receive a “no result” from cell-free DNA screening in around 2-5% of cases. This occurs when the test is unable to measure the fetal fraction or chromosomes clearly enough to report a high or low risk score.
Some reasons this can happen include:
– Low levels of cell-free fetal DNA
– High maternal body mass index (BMI)
– Insufficient fetal or maternal DNA in sample
– Rare biological causes
If you receive a no result, your doctor will discuss whether repeating the blood draw, testing a new sample, or moving directly to diagnostic testing is recommended based on your history. In some cases, no further testing is needed if you are already past the gestational age where fetal anatomy is clearly seen on ultrasound.
Reasons for “no result”:
- Low fetal fraction
- High maternal BMI
- Poor DNA sample
- Rare biological factors
Does the test cause miscarriage?
No, a cell-free DNA blood test does not increase the risk of miscarriage when performed by an experienced phlebotomist. The test poses no direct physical risk to the pregnancy. It involves only a simple blood draw from the mother’s arm, like any other routine blood test during prenatal care.
In contrast, diagnostic tests like amniocentesis and CVS do carry around a 1 in 300 to 1 in 500 risk of pregnancy loss. The cell-free DNA test is done to help determine whether these riskier invasive tests will be recommended to confirm a high risk screening result.
The absence of miscarriage risk with cell-free DNA testing makes it a preferable first-line screening option vs older screening methods that require riskier follow-up testing, like the quad screen.
Miscarriage risk:
Cell-free DNA – No risk of miscarriage
Amniocentesis/CVS – Around 0.5% procedure-related loss rate
How is the blood test different from a first trimester screening?
While both are screening options for Down syndrome in pregnancy, there are some key differences:
Cell-free DNA testing
– Noninvasive blood test
– High accuracy – detects >99% of Down syndrome cases
– Results take about 1 week
– Can be done anytime after 10 weeks gestation
– No risk of miscarriage
First Trimester Screen
– Combines blood work and ultrasound markers
– Lower 80% detection rate
– Done between 11-13 weeks
– Higher false positive rate
– May require risky follow-up testing
The cell-free DNA test is more accurate and can be done earlier, but also costs more. Talk to your provider about which option may be preferred for your pregnancy.
Why choose cell-free DNA vs quad screen?
Both the cell-free DNA test and conventional quad screen are options for Down syndrome screening in pregnancy. Here’s how they compare:
Detection Rate:
Cell-free DNA – 99% for Down syndrome
Quad screen – 80% for Down syndrome
False Positives:
Cell-free DNA – 0.1-0.2%
Quad screen – 5%
Timing:
Cell-free DNA – from 10 weeks
Quad screen – second trimester
Miscarriage Risk:
Cell-free DNA – None
Quad screen – Due to need for amnio if positive
Limitations:
Cell-free DNA – Doesn’t screen neural tube defects
Quad screen – Lower accuracy, higher false positives
Overall, cell-free DNA is more accurate and provides results earlier with no miscarriage risk. But it costs more and does not detect neural tube defects. Your doctor can help decide which option may be preferred for your pregnancy based on your history and priorities.
How do I get the test?
The cell-free DNA prenatal screening test requires a doctor’s order and is performed by specialized genetic testing laboratories. Follow these steps:
1. Make an appointment with your obstetrician or other prenatal care provider.
2. Discuss whether you should undergo Down syndrome screening based on factors like your age, risk factors, and preferences.
3. If recommended, your doctor will arrange for the cell-free DNA blood test and place the order through the lab they use.
4. The lab will send a test kit to your doctor’s office. The kit contains tubes and forms for collecting and labeling your blood sample.
5. A routine blood sample will be drawn at your provider’s office and shipped to the lab according to the instructions.
6. Results are sent back to your doctor to discuss with you once complete – usually in about 1 week.
7. Any positive high risk result should be followed up with a diagnostic test like amniocentesis or CVS for confirmation.
The test is prescribed by your prenatal doctor and performed behind the scenes at a specialty laboratory certified for prenatal genetic screening. Contact your OB/GYN or healthcare provider to learn more about accessing this test.
Conclusion
The cell-free DNA blood test provides a highly accurate way to screen for Down syndrome and other common chromosome abnormalities as early as 10 weeks in pregnancy. Detection rates exceed 99% for Down syndrome, with false positive rates of only around 0.1-0.2%. No currently available prenatal screening test is 100% perfect, but cell-free DNA offers a safer and more accurate option than previous screening methods like the quad screen. Speak to your doctor about whether this screening test may be right for your pregnancy based on your age, risk factors, and preferences. While a high or low risk result still requires careful interpretation, this innovative noninvasive technology gives families valuable information about the genetic health of a pregnancy earlier and with greater accuracy than ever before.