QF-PCR is a common test used in many laboratories for the quick identification of the common chromosomal aneuploidies like Trisomies 21/18/13 and sex chromosomal aneuploidies such as 45XO, 47 XXX, 47XXY and 47XYY.
It checks on the number of specific chromosomes like 21, 18, 13, X and Y. QF-PCR technique allows the results to be released within 2-3 working days. It is a test performed on amniotic fluid and chorionic villi obtained via amniocentesis and chorionic villus sampling (CVS) respectively.
If the results of non-invasive prenatal test (NIPT) are already available before CVS or amniocentesis, a low risk report on NIPT would generally be consistent with a normal QF-PCR report on the 5 chromosomes (i.e. 21, 18, 13, X and Y). Hence it would be reasonably to skip the QF-PCR test if the NIPT report was low risk.
Reasons for doing QF-PCR
1. On the other hand, a high risk report on NIPT may yield an abnormal or sometimes normal report on QF-PCR. So it is reasonable to do the QF-PCR on the sample taken via amniocentesis or CVS to confirm if the strong suspicion of common chromosomal aneuploidies is correct.
2. If there were risk factors for chromosomal abnormalities for which an amniocentesis or CVS was needed (e.g. patient requests diagnostic test, advanced maternal age, markers like increased nuchal translucency or structural abnormalities on ultrasound scan, high risk on first trimester scan or OSCAR test) and there was no NIPT result, a QF-PCR test should be performed first to screen for the common chromosomal aneuploidies.
Further tests after the QF-PCR test
If the QF-PCR test was abnormal, a karyotype should be performed as it would allow confirmation about whether it was a non-disjunction trisomy (which is most likely a random event and therefore predicts a low risk of recurrence in the next pregnancy of up to 1%) or a potentially inheritable condition like unbalanced translocation (if one of the parents is a balanced translocation carrier, which then predicts a higher risk of recurrence in the next pregnany; sometimes it turns out that the parents are normal and not balanced translocation carriers which imply a new or de novo mutation which could still predict a small risk of recurrence in the next pregnany of about 1%).
If the QF-PCR test was normal, it is better to proceed with the chromosomal microarray test which allows the detection of smaller (up to 50,000 bases in size) copy number variants like microdeletions and microduplications. The further yield of doing a karyotype in such a scenario is low except if the NIPT had showed suspicion of sex chromosomal mosaicism. In such a scenario, a karyotype with extended cell count may be necessary to look for mosaicism.
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