The diagnosis of Williams syndrome generally has two parts:
- Clinical diagnosis based on a variety of characteristics. See the What is Williams syndrome section for details.
- Medical/genetic test confirmation through a DNA test which will be performed on a small amount of blood from the child. There are two DNA tests that can determine if a person has Williams syndrome.
The FISH test
You can obtain a blood test to confirm the clinical diagnosis of Williams syndrome. A laboratory can use the technique known as fluorescent in situ hybridization (FISH).
FISH is a type of specialized chromosome analysis utilizing specially prepared elastin probes. If a patient has 2 copies of the elastin gene (one on each of their chromosomes #7), they probably do not have WS. If the individual only has one copy, the diagnosis of WS will be confirmed.
Virtually all (98-99%) persons with typical features of WS will have a deletion of the elastin gene.
In more technical terms:
- Williams syndrome is the result of a deletion of the 7q11.23 region of chromosome #7 containing the elastin gene. Williams syndrome is a contiguous gene syndrome - all of the deleted genes "line up" in the Williams syndrome "critical region". Diagnosis of Williams syndrome is confirmed by dual colour FISH using a specific probe and a control probe.
The laboratory will need 5 ml of blood drawn in a Sodium heparin tube. The sample should arrive in the lab the same day it was drawn or on the following day. Results are usually available in 2-4 weeks.
The FISH test is readily available at major hospitals and Cytogenetics laboratories around the country, but it is not a routine test. Therefore not all labs will do FISH-based diagnosis. Families or their physicians should call the lab in advance to make sure they can perform the test.
If you have any doubts that a child may have Williams syndrome, the FISH test will give you a clear cut answer in most cases.
It is important to stress that WS is a genetic diagnosis and an individual who does not have the gene deletion does not have Williams syndrome (i.e. a person who was clinically diagnosed with WS but was later found not be have a deletion in fact does NOT have WS).
NOTE: It is extremely unlikely that any other family member also has Williams syndrome. On the other hand, if the individual with Williams syndrome plans to become a parent, there is a 50/50 chance that his or her child will have Williams syndrome. If such a situation should arise, consult an obstetrician about using the FISH test for prenatal testing of the embryonic cells.
A chromosomal microarray is a newer diagnostic tool that uses millions of markers to determine if there are missing pieces or extra pieces of DNA anywhere in the person’s chromosomes. This test is slower but gives more information about how big the missing piece is, which may affect how severe the child’s condition will be.