Factors affecting performance of tests
The main factors affecting performance of Molecular Genetics tests are the quality and quantity of DNA extracted. Provided guidelines provided are followed, it is unusual for additional samples to be required. The main causes of failure to extract DNA are: Blood clotting (due to poor mixing with EDTA anticoagulant or use of serum tube), use of anticoagulants other than EDTA, or excessive haemolysis following poor sample storage or excessive delays in sample transport.
In the event of failed extractions a report with a request for an additional sample will be sent to the referring clinician.
Certain genetic diseases, particularly those that require testing of several genes, may require more DNA than is obtained from the initial sample.
Interpretation of some test results may require, or be aided by, availability of samples from other family members.
A document “Uncertainty of Measurement in Molecular Genetics Tests” (GENE-W5) is available to referrers on request. This document describes the likely or important sensitive issues in laboratory procedures that lead to uncertainty of measurement and could lead to the failure of qualitative tests. Where appropriate, this document contains the acceptable limits of sensitive or critical stages of procedures or analysis.
Prenatal samples: Maternal Cell Contamination
For prenatal samples, excessive maternal contamination of foetal samples can produce problems with interpretation of tests. Samples of maternal blood are routinely requested to allow foetal DNA to be identified by its different genotype; without such samples it may be impossible to determine the identity of the DNA tested.
Mosaicism
In rare cases, mosaicism, the presence of two populations of cells with differing chromosome complements, may be detected when analysing the DNA of a tissue sample. The extent of mosaicism can vary substantially between different parts of an individual: The level in crucial organ(s) or system(s) may not reflect that in the sample tested, with consequent uncertainty in expression of phenotype(s) associated with the observed abnormality.
NGS sequencing sensitivity for detection of mosaic variants is variable and dependent on a number of factors: This technique cannot reliably detect low level mosaicism and applied analysis parameters will not detect <10% mosaicism.
