University of North Carolina at Chapel Hill

Positive Predictive Value of Cell Free DNA Calculator

Baseline Risk
Maternal Age (20)

      Gestational Age in Weeks (10)


          Trisomy 21 Trisomy 18 Trisomy 13
          Age-related risk
          Test Sensitivity
          Test Specificity


          • The calculator is a counseling tool and is meant to be an adjunct to, not a replacement of, in-person counseling.
          • The calculator is an estimate of the positive predictive value and does not account for errors in estimation of the maternal age/gestational age-related risk of aneuploidy or the confidence intervals around each tests’ sensitivity and specificity.
          • Age and gestational age-related risk tables were used from: Snijders RJ, Sebire NJ, Nicolaides KH. Maternal age and gestational age-specific risk for chromosomal defects. Fetal Diagn Ther 1995;10:356–67.
          • This calculator is designed as a teaching tool to demonstrate the relationship between a priori risk, sensitivity and specificity and to underline that cell-free DNA screening is not a diagnostic test. Therefore, it is critical for patients with abnormal test results to be offered diagnostic testing such as a CVS or amniocentesis.
          • The sensitivity and specificity for each test is based on clinical validation trials. As these studies vary significantly in their sample size, study design, and data analysis, this calculator should not be used as a tool to compare the different testing platforms.
          • These estimates are based on high risk results (“aneuploidy detected”, “Positive”, or increased risk results (99/100)) and PPV for results reported with an intermediate increased risk (such as “aneuploidy suspected”, >1% but
          • For more information about this calculator please see: Grace, M. R., et al. (2015). "Cell free DNA testing-interpretation of results using an online calculator." Am J Obstet Gynecol 213(1): 30 e31-34.

          • Verifi® is a registered trademark of Illumina, Inc.

            Harmony® is a registered trademark of Ariosa Diagnostics.

            Materniti21® is a registered trademark of Sequenom.

            Panorama® Prenatal Screen is a registered trademark of Natera, Inc.