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Mosaicism Ratio

Personalized information for patients with positive NIPT results.

MaterniT 21 PLUS with Mosaicism Ratio helps to refine your patient’s risk for aneuploidy using personalized data specific to their results.

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Counseling patients with positive NIPT results can be complicated.

After nearly 10 years’ experience with NIPT testing, we know that not all positive results are created equal, and by definition screening tests may have the risk of a false-positive result.

The new Mosaicism Ratio result, only available with MaterniT 21 PLUS (at no extra cost), helps differentiate between a positive result that is more likely to be a true positive, and one with an increased chance to be a false positive.1

While most laboratories work hard to deliver the best possible results, experience, sequencing technology, and capability varies from lab to lab. Take advantage of our experience - Mosaicism Ratio adds an additional level of clarity. Rely on Integrated Genetics/LabCorp as your single-source laboratory partner for both best-in-class screening and follow-on diagnostic testing and ALL your needs across the entire pregnancy.

What is Mosaicism Ratio?

  • A metric calculated in the event of a positive NIPT result for trisomy 21, 18, or 13
  • Requires two data points:
    • Affected fraction – the percentage of cell-free DNA that is impacted by aneuploidy
    • Fetal fraction – the percentage of cell-free DNA contributed by the pregnancy
Mosaicism Ratio graphic

How is Mosaicism Ratio interpreted?

Positive results are classified into one of three Mosaicism Ratio groups:

GroupsLikelihood of POSITIVE result being a TRUE POSITIVE
High mosaicPositive NIPT result is more likely to be a true positive, associated with a higher PPV (see report below)
Low mosaicPositive NIPT result is more likely to be a false positive, associated with a lower PPV (see report below)
Non-mosaicPositive NIPT result that is very likely to be a true positive, associated with the highest PPV. No ‘mosaic’ language will display on the report.

In general, as Mosaicism Ratio increases, so does the positive predictive value (PPV) of the result. The PPV associated with an individual patient result depends on the trisomy involved and the Mosaicism Ratio group associated with the results (i.e. nonmosaic, high mosaic, or low mosaic).

How is Mosaicism Ratio reported? (Singleton pregnancies only)

For patients with a positive result for trisomies 21, 18, and 13, a Mosaicism Ratio comment will feature in the Lab Director Comments box on the MaterniT 21 PLUS lab report.

Test ResultPositive 
Trisomy 13

Lab Director Comments

This specimen showed an increased representation of chromosome 13, suggestive of high mosaic trisomy 13, which may affect the reported PPV (Rafalko et al, 2020). In placental testing, trisomy 13 is a common finding that is often confined to the placenta (CPM), Grafi et al, 2014. However, true fetal involvement is associated with phenotypic abnormality. Genetic counseling, confirmatory diagnostic testing, and clinical correlation are recommended.

Test Result

Positive 
Trisomy 18

Lab Director Comments

This specimen showed an increased representation of chromosome 18, suggestive of low mosaic trisomy 18, which may affect the reported PPV (Rafalko et al, 2020). In placental testing, trisomy 18 is a common finding that is often confined to the placenta (CPM), Grafi et al, 2014. However, true fetal involvement is associated with phenotypic abnormality. Genetic counseling, confirmatory diagnostic testing, and clinical correlation are recommended.

Positive Predictive Value (PPV) by trisomy and Mosaicism Ratio group

Positive Predictive Value (PPV) by trisomy and Mosaicism Ratio group chart

What are the potential causes of a low Mosaicism Ratio?

Though it may be difficult to pinpoint the exact cause of a low Mosaicism Ratio in any particular case, in general, a depressed Mosaicism Ratio may be the result of placental mosaicism, prior co-twin demise, or other biological phenomena.

Summary

  • Mosaicism Ratio allows clinicians to provide customized counseling for patients with positive results for trisomy 21, 18, and 13 in singleton pregnancies
  • The lower the mosaicism ratio, the greater the chance for a discordant result from diagnostic testing
  • MaterniT 21 PLUS with Mosaicism Ratio identifies when a positive result is more likely to be a true positive
  • Regardless of Mosaicism Ratio, diagnostic testing and genetic counseling are recommended for all positive NIPT results according to ACOG, Practice Bulletin 2262
Test nameTest no.Fetal sex opt-out

MaterniT 21 PLUS Core (Trisomy 21, 18, 13, fetal sex)

451927

451951

MaterniT 21 PLUS Core + SCA* (Singletons only)

451934

452112

MaterniT 21 PLUS Core + ESS**

451931

452136

MaterniT 21 PLUS Core + ESS + SCA* (Singletons only)

451937

452122

GENOME-Flex (Add On)

452104

n/a

GENOME-Flex (Add On) Redraw

452114

n/a

MaterniT GENOME (Singletons only)

451941

452106

* Sex chromosome aneuploidies ** Enhanced sequencing series (microdeletions, trisomies 16 & 20)

Reference

  1. Rafalko et al. Impact of Mosaicism Ratio on positive predictive value of cfDNA screening. Prenatal Diagnosis, 2020.
  2. American College of Obstetricians and Gynecologists. Screening for Fetal Chromosomal Abnormalities. Practice bulletin no. 226. Obstet Gynecol. 2020;136(4):859-867