Conventional karyotype

What it detects

Conventional karyotype visualises all 46 chromosomes under a microscope, detecting large structural and numerical chromosomal abnormalities - now largely superseded by chromosome microarray for most indications, but retains specific, important uses
Remains the test of choice for detecting balanced chromosomal rearrangements and for confirming/characterising conditions where the whole-chromosome pattern (not just copy number) matters

Chromosomes are cultured, arrested in metaphase, stained (banding pattern), and examined visually for numerical abnormalities (aneuploidy - e.g. trisomy 21, monosomy X/Turner syndrome) and large structural abnormalities (translocations, inversions, large deletions/duplications) down to a resolution of roughly 5-10 million base pairs

Karyotype detects balanced rearrangements that CMA cannot (since balanced rearrangements involve no net gain/loss of genetic material, which is what CMA specifically detects) - this is the key reason karyotype remains relevant despite CMA's higher resolution for copy number changes
Indicated specifically for: recurrent miscarriage workup (in both partners, to detect a balanced translocation carrier state), suspected Down syndrome/other classic aneuploidy syndromes, and characterising a known structural rearrangement identified by another method

A balanced translocation carrier is usually phenotypically unaffected themselves but at increased risk of unbalanced gametes, causing recurrent miscarriage or an affected child with an unbalanced rearrangement - genetic counselling addresses this specific reproductive risk pattern
Karyotype and CMA are complementary, not interchangeable - the choice depends on the specific clinical question (copy number vs balanced rearrangement)

Down syndrome (trisomy 21), Turner syndrome (monosomy X), recurrent miscarriage, balanced translocation carrier states

Karyotype has been largely replaced by CMA as the first-tier test for most postnatal indications (given CMA's much higher resolution for copy number changes), but remains specifically necessary whenever a balanced rearrangement is suspected or needs to be excluded

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