Conventional karyotype is a genetic test that examines the size, shape and number of chromosomes in a sample of cells. The turn around time for this test is about 2 weeks, and may take longer if the cell culture is slow.
Common chromosomal disorders such as Down syndrome (Trisomy 21), Edward syndrome (Trisomy 18), Patau syndrome (Trisomy 13), Turner syndrome (45XO) and other sex chromosomal abnormalities (e.g. 47XXX, 47XXY, 47XYY) can be diagnosed by a conventional karyotype as the extra or missing chromosome would be detected by this test.
It also allows the detection of structural abnormalities in the chromosomes such as deletion (i.e. missing parts of the chromosomes), duplication (i.e. extra parts of the chromosomes), translocation (i.e. parts or whole of a chromosome breaks and becomes reattached to a different chromosome) and inversion (i.e. a segment of a chromosome breaks off and reattaches within the same chromosome but in reverse orientation; DNA may or may not be lost in the process). However deletions, duplications, translocation and inversions smaller than 5-10 million DNA bases cannot be detected by karyotype.
Reasons for doing a karyotype
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.
Limitation of doing karyotype
Karyotype used to be the best test we had for checking the genetic make-up of an individual. It is however an old genetic test with limited resolution of 5-10 million DNA bases. This means that genetic changes smaller than 5-10 million DNA bases cannot be detected by this test. This level of resolution is no longer adequate in fetal medicine when no genetic basis has been found yet during investigation of fetal abnormalities. It may be adequate if an genetic abnormality has already been found at this level of resolution.