NIPT Test PPV Calculator
NIPT test results can be confusing to understand given it is a screening test rather than a diagnostic test. Use the tool below to visualise the performance of NIPT for each condition.
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Where do these figures come from?
This calculator uses Bayes’ theorem to estimate the positive predictive value (PPV) and negative predictive value (NPV) of your NIPT result — in other words, the probability that a high-risk result is a true positive or that a low-risk result is a true negative. The age-related prevalence figures for trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome) are derived from established epidemiological data by Snijders et al (1999) and Hook (1981), reflecting estimated prevalence at approximately 16 weeks’ gestation. For conditions where prevalence does not vary significantly with maternal age — such as monosomy X (Turner syndrome) and 22q11.2 deletion (DiGeorge syndrome) — fixed population estimates are used. The default sensitivity and specificity values are drawn from the most widely cited meta-analysis in the field: Gil et al (2017), published in Ultrasound in Obstetrics & Gynecology, which pooled data from 35 studies encompassing over 224,000 pregnancies.
Where available, the calculator also offers test-specific performance data for the NIPT tests we provide at London Pregnancy Clinic, including KNOVA (Fulgent Genetics), Panorama (Natera) and PrenatalSAFE (Eurofins Genoma), sourced from each manufacturer’s published validation studies. It is important to understand that PPV depends heavily on the background prevalence of a condition — a positive NIPT result for a younger patient, where trisomy is rarer, is more likely to be a false positive than the same result for an older patient. This is why NIPT is a screening test, not a diagnostic test, and a high-risk result should always be confirmed with diagnostic testing such as amniocentesis or CVS. If you have received an NIPT result and would like help understanding what it means for your pregnancy, our genetic counsellors are available to talk you through your options in detail.
What affects the accuracy of NIPT?
Several factors can influence how reliable your NIPT result is. One of the most important is your age — because conditions like Down syndrome become more common as maternal age increases, a positive NIPT result is more likely to be correct in an older patient than in a younger one. This is because of something called positive predictive value (PPV), which depends not just on how good the test is, but on how common the condition is in the first place. Another key factor is fetal fraction — the proportion of your baby’s DNA circulating in your blood compared to your own. If the fetal fraction is too low, usually below 4%, the test may not be able to give a reliable result. Fetal fraction can be affected by how early in pregnancy the blood is drawn, maternal weight (higher BMI is associated with lower fetal fraction), and certain pregnancy complications. This is one reason why NIPT is typically performed from 10 weeks of pregnancy onwards.
Other factors that can lead to unexpected results include confined placental mosaicism (CPM), where the placenta has a different chromosomal makeup to the baby — since NIPT analyses DNA from the placenta rather than the baby directly, this can occasionally cause a false positive or false negative. Vanishing twin pregnancies, where a second twin has been lost early on, can also affect results because residual DNA from the lost twin may still be present in the mother’s blood. The type of NIPT technology used matters too — different laboratories use different methods such as whole-genome sequencing or SNP-based analysis, and each has slightly different detection rates and false positive rates depending on the condition being screened for. If your NIPT result is high-risk, it is essential to confirm the finding with a diagnostic procedure such as amniocentesis or chorionic villus sampling (CVS) before making any decisions about your pregnancy.
You have a question? We have an answer.
NIPT is the most accurate screening test currently available for Down syndrome (trisomy 21). According to the largest pooled analysis of clinical studies (Gil et al 2017), NIPT detects over 99% of pregnancies affected by Down syndrome, with a false positive rate of just 0.04%. However, accuracy also depends on your age and background risk. For a 35-year-old with a positive NIPT result for Down syndrome, the chance that the baby is truly affected is around 80–90%. For a 25-year-old, the same positive result may only have a 50–60% chance of being correct, because the condition is much rarer at that age. This is why a positive NIPT result should always be confirmed with a diagnostic test such as amniocentesis or CVS before making any decisions.
Yes — like all screening tests, NIPT can occasionally give incorrect results. A false positive means the test says high-risk but the baby is actually unaffected, and a false negative means the test says low-risk but the baby does have the condition. False positives are more common than false negatives and are most often caused by confined placental mosaicism (CPM), where the placenta has a different chromosomal makeup to the baby. Because NIPT analyses DNA from the placenta rather than the baby directly, this can sometimes lead to a misleading result. Other causes include a vanishing twin, certain maternal conditions, or very low fetal fraction. Although false negatives are rare — particularly for Down syndrome — no screening test can guarantee a result with 100% certainty, which is why NIPT is classified as a screening test rather than a diagnostic test.
The test itself performs the same regardless of your age — the sensitivity and specificity do not change. What does change is the positive predictive value (PPV), which is the chance that a high-risk result is truly correct. This happens because conditions like Down syndrome, Edwards syndrome and Patau syndrome are less common in younger women. When a condition is rarer in the population being tested, any positive result is statistically more likely to be a false alarm. For example, the PPV for trisomy 21 might be over 90% for a 40-year-old but closer to 50% for a 25-year-old — meaning that roughly half of positive results in younger women could be false positives. This does not mean younger women should avoid NIPT; it simply means that understanding what a positive result actually means is especially important, and genetic counselling can help you interpret your result in context.
Your weight can indirectly affect NIPT reliability. Women with a higher BMI tend to have a lower fetal fraction — the proportion of the baby’s DNA in the mother’s blood. Most NIPT laboratories require a minimum fetal fraction (usually around 3–4%) to produce a reportable result. If the fetal fraction is too low, the laboratory may be unable to give a result at all, or the result may be less reliable. Studies have shown that women with a BMI over 30 are more likely to receive a no-call or inconclusive result on their first blood draw. In these cases, a repeat blood sample at a later gestational age — when fetal fraction is naturally higher — often resolves the issue. If you are concerned about how your weight might affect your NIPT, speak with your clinician about the best time to have the test and which NIPT platform may be most suitable for you.
Yes — NIPT is significantly more accurate than the NHS first trimester combined screening test for the three main trisomies. The combined test, which uses a nuchal translucency ultrasound measurement and blood markers (PAPP-A and free beta-hCG), detects approximately 85–90% of Down syndrome cases with a false positive rate of around 3–5%. By comparison, NIPT detects over 99% of Down syndrome cases with a false positive rate below 0.1%, meaning far fewer women receive an unnecessary high-risk result. This dramatically reduces the number of women who go on to have invasive procedures like amniocentesis, which carry a small risk of miscarriage. It is worth noting that the combined test does provide additional information — such as the nuchal translucency measurement and PAPP-A level — that can flag other pregnancy complications beyond chromosomal conditions, so many clinicians recommend having both an early scan and NIPT for the most complete picture of your pregnancy.
