Interpreting Results

Interpreting Harmony Prenatal Test Results

Non-invasive prenatal testing (NIPT) is an accurate genetic screening test for common trisomies.

Conventional prenatal screening methods using serum proteins and ultrasound have higher false-positive rates, necessitating additional invasive testing to distinguish false positive from true positive results.

The performance of non-invasive prenatal testing varies with the prior risk that the woman has an affected fetus.

Three general scenarios are considered below.

Woman who is at low risk (1 in 1,000, or 0.1%) of her fetus having a chromosomal abnormality.
This may apply to a young woman with normal fetal ultrasound, or a woman with a low risk result from conventional first trimester screen.
If the NIPT result is NORMAL

It is highly likely that the result is correct, and that the fetus does not have any of the chromosome abnormalities tested.

In such a low risk setting, a normal result from NIPT is correct in more than 999 cases out of 1000. The power of NIPT to correctly identify a normal fetus is the main clinical benefit of this test.

If the NIPT result is ABNORMAL

It is likely but not definite that the result is correct, and that the fetus has a chromosome abnormality. The probability of the result being correct varies from 30-90% for different chromosomal abnormalities.

An abnormal NIPT result is more likely to be correct than an abnormal conventional first trimester screen. Nonetheless, an abnormal NIPT result must be confirmed by invasive testing by CVS or amniocentesis before making major medical decisions.

 

Woman who is at increased risk (1 in 100, or 1%) of her fetus having a chromosomal abnormality.
This may apply to an older woman, or a woman with an increased risk result from conventional first trimester screen.
If the NIPT result is NORMAL

It is likely that the result is correct, and that the fetus does not have any of the chromosome abnormalities tested. In this setting, a normal result from NIPT is correct in more than 99.5 cases out of 100. The power of NIPT to correctly identify a normal fetus is the main clinical benefit of this test.

If the NIPT result is ABNORMAL

It is highly likely but not definite that the result is correct, and that the fetus has a chromosome abnormality. The probability of the result being correct varies from 80-95% for different chromosomal abnormalities. In this setting, an abnormal NIPT result must be confirmed by invasive testing by CVS or amniocentesis before making major medical decisions.

 

Woman who is at high risk (1 in 10, or 10%) of her fetus having a chromosomal abnormality.
This may apply to a woman whose fetus has malformations on ultrasound, or who has a very high risk from conventional first trimester screening.
NIPT may be less useful in this setting. A normal result for the tested chromosomes is still very likely to be correct, but the risk of a ‘false negative’ result is increased. It is also possible that the fetus has a chromosomal abnormality that is not tested by NIPT. Invasive testing by CVS or amniocentesis should be considered.

 

NIPT is a very good test, but it is a screening test. Abnormal results should be confirmed by invasive testing before acting on that result. Invasive tests may also be appropriate in the case of an unexpectedly normal result.