February 2023 Moms

How accurate is the NIPT test?

Just wondering if anyone's NIPT test was incorrect?

Re: How accurate is the NIPT test?

  • You’re unlikely to find someone in a group this small with a false result, because the conditions aren’t that common and the cell free fetal DNA testing is pretty good. That doesn’t mean it doesn’t and can’t happen, though.

    Generally it’s very accurate, although exact numbers depend on positive vs negative, specific chromosomal abnormality, your age/baseline risk, etc. You’ll often see it cited as around 99% accurate for everything, but it may be even more or slightly less depending on your baseline risk and your result, and depending on the quality of the DNA isolation.

    For example, a positive for Trisomy 21/Downs syndrome in a 42 year old mother is incredibly predictive and wildly unlikely to be wrong (but should still be confirmed with amniocentesis or chorionic villa sampling before making decisions), while a positive for XXY in a 25 year old without any additional risk factors is likely somewhere around 40% predictive, if it’s that high. This is because the Trisomy 21 detection is what the tests are meant for and is better validated than the XXY, and because the chances that a random 42 year old’s baby has Trisomy 21 are much higher than the chances that a random 25 year old’s baby is XXY.

    If you’ve gotten a result that you’re worried about, it’s best to discuss your specific situation with your provider. NIPT is a screening tool, not a diagnostic test, so it’s totally appropriate to do amniocentesis or chorionic villa sampling for more certainty. And your provider will know what your options are as far as what to do next based on the diagnosis and your location.
  • @knottie… I agree with @fertile-turtle above. I would just like to add that the fetal fraction percentage is important in terms of reliability of the results. Ideally it should be 4% or higher but I found another resource stating that the fetal fraction should be at least 2%. So there is a gray area there. The fetal fraction should be listed on your lab report. 

    There are factors that can affect the percentage such as timing (the earliest you can draw is at 10 weeks but the ideal time is 12-13 weeks and thereafter), race (African American, Southeast Asian), weight, certain types of medications like lovenox, physical activity, and pre-existing hypertension. 

    Ultimately, review the results with your provider since they are the ones to guide you through your care. 
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  • Thank you! So weight could be a factor that can affect the accuracy correct?
  • Ultimately, check your fetal fraction percentage. If it’s 4% or above, you are fine regardless of weight. But if it is lower than 4%, weight can be a factor in having a lower fetal fraction percentage. 
  • Not directly, but like @ttc3y said, it can affect fetal DNA fraction, which can affect results. If you already have a result that’s abnormal or inconclusive, talk to your doctor about additional testing that isn’t affected by weight or anything else. Don’t assume that the results are wrong just because of your weight, especially if you've gotten a result that would change the course of your pregnancy. If you’ve gotten a normal result, you can still talk to them about the accuracy if you’re still worried. And if you’re asking about sex of the baby as one of the NIPT results, male is basically definitive (because you don’t have a Y chromosome), bht there’s a slight chance that female is wrong and they missed the Y chromosome, and you’d find this out at the anatomy scan.

    If you’re just trying to decide whether you should do it and you’re heavier, you can go a little later than 10 weeks to improve the fetal fraction and hopefully avoid that confusion. NIPT is still the most accurate screening tool for chromosomal abnormalities, so if that’s something you’re worried about and your insurance covers it, it’s definitely worth it. 
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