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Genetics

Poster Session 4

(1128) Reliability of FISH by Testing Indication

Wednesday, February 14, 2024
1:00 PM – 2:30 PM  
Location: Prince George's Exhibit Hall CDE, Lower Atrium Level
Objective:

With gestational age limits on abortion, expediting results of prenatal testing is important. Florescence in situ hybridization (FISH) can provide rapid results following CVS or amniocentesis. Our objective was to explore the reliability of FISH in prenatal diagnosis.

Study Design:

This was a retrospective cohort study from 2020-2021. All patients undergoing invasive testing with FISH were included. Patients with uninformative FISH results were excluded. The positive predictive value (PPV) of FISH was calculated for each testing indication, considering only the chromosomes evaluated by FISH (13, 18, 21, X, Y). Fisher’s Exact test was used for statistical comparison.

Results:

703 patients (483 CVS/220 amniocentesis) met inclusion criteria, with 146 abnormal FISH results (20.8%). The most common indication for invasive testing was abnormal NIPT (n = 225) followed by AMA (n = 206). 27 had chromosomal abnormalities beyond the scope of FISH. Results of FISH and karyotype based on procedure and indication are in Table 1. Overall, FISH was not consistent with karyotype in 8 cases (1.1%), all from CVS There were 2 false positive and 6 false negative results. Discrepancy between FISH and karyotype was seen in 8 of 459 CVS vs 0 of 217 amniocentesis (1.7% vs 0%; p = .06). Both false positive results were in cases with abnormal NIPT and normal NT. The 6 false-negative FISH results included one case of T18, one of 47XXY, and four mosaic results, two confined to the placenta based on normal karyotypes on amniocentesis. The PPV of FISH for all indications was 98.6%. PPV was 100% in those with abnormal NT or ultrasound. The overall NPV was 98.9% (524/530), with NPV of 100% with amniocentesis. NPV was lower when CVS was done for abnormal NT than for other conditions (93.6% vs. 99.0%, p = .04)

Conclusion:

While most FISH findings are confirmed by karyotype, PPV with CVS is less than for amniocentesis. Normal FISH results are less reliable with abnormal NT. When NIPT is abnormal but NT is normal, awaiting confirmation with karyotype is recommended.