Additional Information

Method:
Whole blood is cultured with an additive to stimulate cell division. The cells are incubated for 48 hours at 37° Celsius. Cell cultures are synchronized with excess thymidine, arrested in metaphase with Colcemid, harvested, and fixed with methanol and acetic acid. Giemsa stain and trypsin are used to generate a characteristic banding pattern (G-banding) which permits unambiguous identification of individual chromosomes. (Arsham et al., The AGT Cytogenetics Laboratory Manual, Fourth Edition) 
Twenty metaphases with an average band-level resolution of 400-700 are routinely examined. An additional 30 metaphases are analyzed in cases of suspected mosaicism. Two or more digital karyograms are prepared and stored using a computer-based imaging system. An abbreviated study of 5 metaphases may be performed for testing ordered to confirm a known familial chromosome abnormality or as an add-on to a normal CMA result. Chromosome abnormalities are confirmed in at least 2 different cultures. A minimum of 3 metaphases with the same chromosome loss, and a minimum of 2 metaphases with the same chromosome gain or structural abnormality are required to authenticate findings. (ACMG, Technical Standards for Clinical Genetics Laboratories (2021 Revision)) 

Limitations: 
Chromosomes at the 400-700 band-level of resolution generally have a detection limit of 5-10 megabases in regions where the banding pattern is distinctive. The detection limit may be adversely affected in regions lacking contrasting bands. Smaller genomic abnormalities may not be detected via G-banded chromosome studies (cryptic abnormalities). Some chromosomal anomalies hinder cell growth and may be selected against during routine cell culture processes resulting in underrepresentation or exclusion during final analysis. This test is not appropriate for detecting acquired chromosome abnormalities.