This case is part of our ongoing Genetic Testing Challenges in Oncology series, which examines how gaps in genetic testing and communication can delay accurate diagnoses. In this case, a well-intentioned retest nearly overshadowed a genuine chromosomal finding.
An Abnormal Result and an Overlooked Warning
A first-time pregnant woman underwent noninvasive prenatal testing (NIPT) during her 11th week of pregnancy, and results flagged a possible chromosome 18 abnormality. Notably, the NIPT lab’s own report explicitly stated that physicians should not redraw and retest a sample, noting that a second test would not provide greater confidence about the presence or absence of a fetal genomic change.
Despite documenting this recommendation in her consultation notes, the patient’s maternal-fetal medicine doctor nevertheless offered a second NIPT through a different laboratory, and the patient agreed to it.
A Genetic Counselor Steps In to Clarify the Confusion
The patient later told her genetic counselor that if the second test came back normal, she planned to skip the amniocentesis her doctor had scheduled. The genetic counselor worked to correct this misunderstanding, explaining that the original abnormal result could not simply be overwritten by a second test.
Indeed, the second NIPT returned a low-risk result. However, the patient’s own testing revealed she harbored a 2.4 kilobase deletion on chromosome 18 that no one had suspected. Subsequently, amniocentesis confirmed that the fetus had inherited this same deletion from the mother and also revealed a region of homozygosity on chromosome 16, raising the possibility of autosomal recessive risk if symptoms emerge.
Ultimately, the referral to genetic counseling was what prevented the diagnostic process from going further off course, the counselor noted.
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