Screening for rare microdeletions is where it falls apart
The most extreme example of the arithmetic above.
Some laboratories offer to screen for rare deletions of small pieces of chromosome. These are not part of standard recommended screening, and they are frequently added to a panel without much discussion.
In a study of 712 positive results sent to a large genetics laboratory for confirmation, the proportion of positives that were real was:
| Condition screened for | Chance a positive result was real |
|---|---|
| Trisomy 21 | 84% |
| Trisomy 18 | 76% |
| Trisomy 13 | 45% |
| 22q11.2 deletion | 21% |
| 1p36 deletion | 14% |
| Cri-du-Chat | 0% |
| Prader-Willi / Angelman | 0% |
The authors' own explanation is the arithmetic: these conditions are rare, so almost everything the test flags is a false alarm. Two of the panels produced no true positives at all in this series.
Before agreeing to an expanded panelAsk which conditions are included beyond trisomy 21, 18 and 13, and what the predictive value is for each. If nobody can tell you, that is your answer.
Where this comes from:
150. Petersen AK, Cheung SW, Smith JL, Bi W, Ward PA, Peacock S, et al. Positive predictive value estimates for cell-free noninvasive prenatal screening from data of a large referral genetic diagnostic laboratory. Am J Obstet Gynecol. 2017;217(6):691.e1-691.e6.
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