Understanding how to do a karyotype begins with recognizing that this laboratory technique visualizes an individual’s complete set of chromosomes. By arranging stained chromosome pairs according to size, banding pattern, and centromere position, cytogeneticists can detect numerical abnormalities, large structural rearrangements, and certain microscopic deletions or duplications. The procedure integrates cell culture, controlled cell division arrest, chromosome condensation, harvesting, dropping, staining, and meticulous microscopic analysis to produce a standardized map of human karyotype 46, XX or 46, XY.
Clinical Purpose and Indications
Clinicians order a karyotype to investigate recurrent pregnancy loss, suspected chromosomal syndromes, infertility, hematologic disorders, or solid tumors with characteristic translocations. For example, Down syndrome is suggested by trisomy 21, while chronic myeloid leukemia often reveals the Philadelphia chromosome, a translocation between chromosomes 9 and 22. The test can also clarify ambiguous genitalia or identify balanced translocations in parents with recurrent miscarriages, providing essential information for genetic counseling and family planning decisions.
Sample Collection and Transport
Peripheral blood is the most common specimen, collected into sterile tubes containing heparin or lithium heparin to prevent clotting. Bone marrow, amniotic fluid, chorionic villi, or tissue biopsies are processed similarly after appropriate consent and institutional oversight. Rapid transport to the cytogenetics laboratory at 37°C preserves cell viability, and prewarming transport media minimizes cold shock. Proper patient identification, meticulous chain of custody, and timely processing within 24 to 48 hours are critical to maintain culture success and accurate results.
Cell Culture and Mitotic Arrest
In the laboratory, cells are inoculated into culture media containing phytohemagglutinin for lymphocytes to stimulate division, or tissue explants for solid samples. After approximately 72 hours, colchicine or colcemid arrests cells in metaphase, where chromosomes are maximally condensed. This synchronization step ensures that cells accumulate at the stage optimal for microscopic analysis, reducing the risk of analyzing early or late anaphase cells that could obscure structural details.
Harvesting, Fixing, and Slide Preparation
Technicians gently treat the culture with a hypotonic solution, causing cells to swell and chromosomes to spread. Fixation with a methanol-acetic acid mixture then preserves chromosome morphology and removes residual cytoplasm. The fixed cell suspension is dropped onto clean glass slides, where cells adhere and chromosomes begin to banding after controlled trypsin treatment. Uniform, high-quality slides with well-spread, overlapping chromosomes in distinct bands are essential for reproducible G-banding analysis.
Staining, Microscopy, and Image Analysis
Giemsa or similar stains generate G-bands, light and dark alternating stripes along each chromosome that reflect chromatin density and base composition. Under a high-resolution microscope, cytogeneticists identify each homologous pair by size, banding pattern, and centromere index, capturing images with cooled CCD cameras. Digital karyotyping software aligns pairs according to the International System for Human Cytogenomic Nomenclature, builds ideograms, and annots rearrangements such as deletions, duplications, inversions, or translocations with precise cytogenetic nomenclature.
Quality Control and Reporting
Rigorous quality assurance includes reviewing banding quality, confirming cell count and identity, verifying that at least 20 well-banded metaphases are analyzed, and documenting any mosaicism. Reports specify the karyotype notation in ISCN format, highlight clinically significant findings, and distinguish constitutional from acquired abnormalities. Clear communication with clinicians ensures that patients receive accurate interpretations, appropriate follow-up testing like fluorescence in situ hybridization or chromosomal microarray, and informed discussions regarding prognosis and recurrence risks.
