Non-Invasive Prenatal Testing (NIPT) screens for certain chromosomal conditions in a developing fetus. This test analyzes cell-free DNA fragments in the pregnant person’s blood, some of which originate from the placenta and reflect the fetal genetic makeup. NIPT is a non-invasive way to identify pregnancies that may benefit from further diagnostic evaluation.
The Role of NIPT
NIPT screens for common chromosomal conditions like Down syndrome (Trisomy 21), Edwards syndrome (Trisomy 18), and Patau syndrome (Trisomy 13). It also screens for sex chromosome aneuploidies, such as Turner syndrome (Monosomy X) or Klinefelter syndrome (XXY). The test detects an imbalance in chromosomal material, indicating an extra or missing chromosome.
NIPT is a screening test, not a diagnostic one. It indicates an increased or decreased risk of a condition, rather than a definitive diagnosis. Its non-invasive nature, requiring only a blood sample, contrasts with diagnostic procedures like amniocentesis or chorionic villus sampling, which carry a small risk of complication. NIPT offers a safe initial step for risk assessment.
Measuring NIPT Accuracy
NIPT accuracy is described using two key metrics: sensitivity and specificity. Sensitivity refers to the test’s ability to correctly identify affected pregnancies, measuring the rate of true positive results. For example, high sensitivity for Down syndrome means the test is very good at detecting its presence.
Specificity measures the test’s ability to correctly identify unaffected pregnancies, indicating the rate of true negative results. High specificity ensures the test rarely gives a positive result when the condition is absent. NIPT generally demonstrates very high sensitivity and specificity for common trisomies.
For Trisomy 21 (Down syndrome), NIPT has sensitivity and specificity exceeding 99%, making it a highly reliable screening tool. While still accurate, sensitivity and specificity may be slightly lower for Trisomy 18 and Trisomy 13, often ranging from 95% to 99%. Test performance can vary depending on the condition and the laboratory.
Understanding Your NIPT Results
A “low-risk” (negative) NIPT result indicates a very low likelihood of the screened chromosomal conditions. While reassuring due to high sensitivity, a low-risk result does not guarantee the fetus is entirely unaffected. Rare false negatives can occur, especially if the fetal DNA fraction in the maternal blood sample is too low.
Conversely, a “high-risk” (positive) NIPT result suggests an increased chance of a screened condition. A high-risk result is not a diagnosis. False positives can occur, meaning the test indicates risk when the condition is absent. The chance of a false positive is higher for rarer conditions or in certain clinical situations.
The positive predictive value (PPV) of NIPT, the probability that a positive result truly indicates the condition, varies based on its prevalence in the tested population. For common conditions like Down syndrome, PPV is higher. For rarer conditions, even with high sensitivity and specificity, PPV can be lower. This highlights why diagnostic testing is essential to confirm any high-risk NIPT finding.
Following Up on NIPT Results
For pregnancies with a low-risk NIPT result, no further action is typically recommended for the screened conditions. Most individuals continue with routine prenatal care. The test’s high accuracy in identifying unaffected pregnancies reduces the need for additional invasive procedures.
If NIPT returns a high-risk or inconclusive result, further steps are recommended for a definitive answer. Genetic counseling is an important first step to discuss results and options. Diagnostic tests, such as chorionic villus sampling (CVS) or amniocentesis, are offered to confirm or rule out a chromosomal condition.
CVS involves taking a placental tissue sample, usually between 10 and 13 weeks of gestation. Amniocentesis involves extracting amniotic fluid, typically after 15 weeks. Both procedures carry a small risk of miscarriage but provide a definitive diagnosis by analyzing fetal cells directly. These diagnostic tests are the gold standard for confirming chromosomal abnormalities identified by NIPT.