[Factors affecting the failure of non-invasive prenatal testing and the feasibility analysis of retesting]

To explore the cause for the failure of non-invasive prenatal testing (NIPT) and feasibility of repeated testing.

Clinical data, test results and pregnancy outcomes of 40 311 pregnant women who received NIPT test from January 2011 to December 2018 were reviewed.

Among all the pregnant women, 1116 cases failed in the first test, 9 cases (0.81%) had fetal free DNA concentration lower than 4%, 663 cases (59.41%) were retested after the establishment of Z value gray area, and the remainder 444 cases (39.78%) needed to be retested after the blood collection due to the fetal free DNA concentration lower than 4%. After retesting, 1069 cases (95.78%) obtained effective NIPT results. The results showed that 53 cases were at high risk (6 cases for trisomy 21, 6 cases for trisomy 18, 13 cases for trisomy 13, 16 cases for sex chromosomal abnormality, 12 cases for chromosomal copy number variation). Forty-eight cases were selected for invasive prenatal diagnosis, and 2 cases of 47, XXY and 2 CNV were confirmed. A total of 47 cases (0.12%) did not obtain results because the concentration of fetal free DNA was lower than 4%. Only 16 cases (34%) chose invasive prenatal diagnosis.

Repeated detection of the gray area of Z value can reduce the false positive rate of NIPT and invasive prenatal diagnosis, and the feasibility of repeated detection is high. In the case of fetal free DNA concentration lower than 4%, the success rate of obtaining effective NIPT results by re-sampling and re-detection increases with the increase of gestational age, but may delay the diagnosis for fetal aneuploidies. Therefore, personalized estimation should be made according to gestational age and clinical indications. It is suggested that pregnant women should choose invasive prenatal diagnosis when they have failed in the retest.