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Grand Rounds

NIPS False Positive Rates and PPVs from a Large Reference Laboratory

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BACKGROUND AND PURPOSE:

  • Petersen et al. (AJOG, 2017) provide the PPV and false positive rates from a large reference laboratory database

METHODS:

  • This study reviewed 712 confirmatory diagnostic studies performed on patients who have received an abnormal or inconclusive NIPS result including
    • Common trisomies (492 cases)
    • Sex chromosome aneuploidies (138 cases)
    • Microdeletion (52 cases)
    • Other large genome-wide copy number abnormalities (4 cases)
    • Multiple anomalies (9 cases)
  • Testing modalities included fluorescence in situ hybridization, G-banded karyotype, and/or chromosomal microarray analysis performed on chorionic villus samples, amniotic fluid, or postnatally obtained blood samples

RESULTS:

  • PPV for trisomies were consistent with previous reports
    • Trisomy 13: 45%
    • Trisomy 18: 77%
    • Trisomy 21: 85%
    • Monosomy X (Turner syndrome): 26%
    • 47,XXX: 50%
    • 47,XXY (Klinefelter syndrome): 86%
  • PPV for microdeletion syndrome regions
    • Cri-du-Chat syndrome (7 cases): 0%
    • Prader-Willi/Angelman syndrome (10 cases): 0%
    • 1p36 deletion syndrome (7 cases): 14%
    • 2 deletion syndrome (28 cases): 21%
  • Of 4 cases with genome-wide abnormalities, 2 were confirmed

CONCLUSION:

  • Low PPV (0-21%) and high false positive rates (79-100%) rates for microdeletions likely due to low prevalence of these disorders
  • Possible causes of NIPS false-positive rates include
    • Placental mosaicism
    • Vanishing twin syndrome
    • Maternal chromosomal abnormalities and neoplastic conditions
  • While NIPS PPVs compare favorably to PPVs of standard aneuploidy screening, patients must be counseled regarding limitations of NIPS and the importance of confirmatory testing

Learn More – Primary Sources:

Positive predictive value estimates for cell-free noninvasive prenatal screening from data of a large referral genetic diagnostic laboratory

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How Accurate is NIPT as a Screening Test for Chromosomal Aneuploidies?
Practical info on evidence based medicine for your women's healthcare practice
cfDNA vs. Routine Screening – How Do They Compare?
What Is the Best Follow-Up Diagnostic Test After a High-Risk NIPT Result?
NIPS vs. Microarray for Pathogenic Results

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