Genetics - Cytogenetics & Molecular
The importance of Clinical Details
Clinical details are very important when providing genetic analysis. The more clinical information that is available (e.g. details of ultrasound information, phenotypic features or family history) the better the service we can provide. Failure to provide this information for cytogenetic studies may result in an inaccurate analysis.
Cytogenetic analysis is performed according to the Professional Guidelines for the Association of Clinical Cytogeneticists and the recommendations provided are dependent on the clinical indications given for each case.
Clinical details inform the investigation at all stages:
- Prior to analysis, clinical details may indicate, for example, that specialist procedures such as chromosome breakage or leukaemic studies are required, which must be referred to a specialist centre.
- During analysis they may indicate that extra cells should be screened to investigate the possibility of mosaicism, for example in a diagnosis of suspected Turner syndrome, or that particular chromosomes must be targeted for high-resolution study, for example chromosome 8 in suspected Langer-Giedion syndrome.
- When the analysis has been completed they may help to provide an accurate interpretation of the findings and in some instances prompt further investigations, for example FISH or molecular genetic studies.
When clinical details are not available a routine analysis will be performed and a conditional report issued.
Clinical details can be extremely important for clinical interpretation of a molecular genetic test.
For example, the clinical comments accompanying a cystic fibrosis screening report will vary depending on whether the patient is a potential gamete donor or a person exhibiting a cystic fibrosis phenotype.
Similarly, the interpretative comment accompanying Factor II and V studies may vary depending on
whether the investigation is prompted by a history of recurrent miscarriage or the need to determine a thrombotic risk.
It may also be crucial, where a mutation has already been shown to be segregating in a family, to be provided with information concerning the mutation and a family pedigree to ensure the correct analysis is performed and reliable risk figures calculated.
Notes for Cytogenetics
As cytogenetic studies require living cells, please ensure that samples reach the laboratory quickly. If a delay before despatch is unavoidable, samples may be stored in a refrigerator (4°C) but they must not be frozen.
Information concerning packaging, transportation, and labelling of samples is provided on the inside cover of our TDL Genetics Request Form Pad.
On completion of analyses, fixed cell suspensions are stored for a minimum of three months and are available for additional follow-up studies (for example, FISH), if necessary.
Requesting additional tests
Any further tests not requested at the time of sample receipt must be requested within:
- 2 weeks for DNA testing
- 2 weeks for cell culture testing
- 3 months for FISH testing
Samples can be stored for longer periods if specifically requested at the time of sample receipt.
Postnatal Diagnosis (Blood Culture)
Reasons for analysis: Chromosome studies are requested where problems that may have a cytogenetic basis are suspected, e.g. babies with birth defects; children with developmental delay and physical handicaps, or adults with fertility problems. Additionally, prospective gamete donors are screened to detect carriers of balanced chromosome rearrangements.
Sample requirements: Lithium heparin whole blood specimens are required - gently mixed to prevent clotting and must not be frozen. Sample volumes may be reduced for children (2-4ml) and neonates (1-2ml).
Turnaround time: The usual turnaround time is 2-3 weeks however the laboratory will endeavour to respond to urgent requests. Where a major trisomy is suspected, a rapid PCR screen may be performed to provide an urgent provisional result.
a) Rarely, blood samples fail to culture (<1%);
b) The culture may yield chromosomes of insufficient quality. This will be indicated on the report and a repeat study suggested;
c) The laboratory should be informed if the patient has recently received a blood transfusion.
Leukaemic Studies (Bone Marrow)
Sample requirements: 5-10ml bone marrow in preservative free heparin and RPMI medium. This can be supplied by the laboratory.
Clinical information: Please complete the Leukaemic Studies Request form at the back of the laboratory guide, including WBC, reason for referral, stages of disease/treatment and analysis required i.e. karyotype and/or FISH.
Reasons for analysis: Chromosome studies are requested where pregnancies are identified as being at risk of a cytogenetic abnormality e.g. advanced maternal age; positive maternal serum screening; fetal abnormalities found on ultrasound; or where a parent is a known carrier of a chromosome anomaly.
a) amniotic fluid - 10ml+ in a plain sterile, leak-proof container. Suitable containers can be provided by the laboratory. The specimen must not be frozen.
b) chorionic villus - 5mg+ in sterile transport medium. Suitable containers containing medium can be provided by the laboratory. The specimen must not be frozen.
c) fetal blood - 1-2ml LITHIUM HEPARIN whole blood, gently mixed to prevent clotting.
The specimen must not be frozen.
Turnaround time: This is dependant on the rate of cell growth, however, the usual turnaround time is approximately two weeks. Fetal blood results will usually be reported within 7 days.
a) Maternal contamination, and mosaicism may complicate the analysis and may lead to the suggestion that a second invasive test is performed.
b) Rarely, cultures fail to grow (overall <1%)
c) Very small chromosome abnormalities may not be detected (this is why the phrase 'No trisomies or major chromosome abnormalites detected...' is used in our reports).
Reasons for analysis: Fibroblast cultures may be used in addition to blood cultures, for example where tissue specific mosaicism is suspected, or where blood samples cannot be obtained. Analysis may be requested for early spontaneous miscarriages, stillbirths, or to confirm a prenatal diagnosis.
Sample requirements: All specimens should be placed in a sterile container, preferably containing transport
medium. This can be supplied by the laboratory. Sterile normal saline can be used if transport medium is
not available. Samples must not be placed in formaldehyde or other preservative and must not be frozen.
Turnaround time: This is dependant on the rate of cell growth, however, the usual turnaround time is
approximately 4 weeks.
a) Material from miscarriages has a relatively high culture failure rate (around 20%)
b) If no villus or fetal parts are identified in supposedly POC material and a normal female chromosome result is found, this may indicate that maternal tissue has been cultured (this will be noted on our report)
c) Disposal of material from miscarriages will be by incineration. Other arrangements can be made if requested at the time of sample receipt
Fluorescence In Situ Hybridisation (FISH)
Where FISH studies for specific microdeletion syndromes are required this must be indicated on the request form.
Note: FISH studies for a prenatal aneuploidy screen have now been superceded in our laboratory by multiplex-PCR technology and subtelomeric screens are now performed by an MLPA assay.