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Trisomy 21 by FISH*

Test Code

F T21*

Test Synonyms

+21, CEP 21


Down syndrome


Fluorescence in situ Hybridization (FISH)
*Performed by affiliate laboratory

Turnaround Time

7-9 days

Specimen Requirements

5mL peripheral blood in sodium heparin
3mL bone marrow in sodium heparin
Fixed cytogenetically prepared cells in sterile centrifuge tube with pellet visible in 3:1, Methanol:Acetic Acid

Specimen Stability
Blood and bone marrow = 4°C to 25°C, specimens are stable up to 72 hours
Fixed cell pellets are stable for years when stored at -28°C to 15°C
Storage & Handling

4°C to 25°C during transit, but specimens may be transported on refrigerated gel packs. Do not allow the gel pack to come in contact with the specimen. Do not freeze. Extreme temperatures should be avoided.

Causes for Rejection

Clotted specimen; Specimen exposed to extreme temperature; Anticoagulant toxic to cells; Insufficient number of cells

Reference Range

See report

Conventional cytogenetic evaluation is required for prenatal and newborn cases when aneuploidy FISH is requested (typically chromosomes X,Y,13,18 and 21).  If the FISH result is abnormal, chromosome analysis can determine whether the abnormality is due to aneuploidy or a complex structural abnormality and will enable a more accurate recurrence risk for the family. If the FISH result is normal, a chromosome analysis allows identification of more complex abnormalities and the less common numeric abnormalities of other chromosomes.


An extra copy of chromosome 21 is the cause of Down syndrome. An increase risk is seen with the increase in maternal age.

  1. NCBI. Down Syndrome