Identity testing is a rapid test that can be used to provide information on the identity of the individual a DNA sample is derived from. There are three key situations in which identity testing may be used, which are described below.
- Where confirmation of the biological relationships between family members may influence the clinical interpretation of results of other genetic testing. For example, if a variant found in an affected child is not found in either parent (it is de novo), this evidence is deemed more powerful if the biological relationships between the child and both parents can be confirmed, as this excludes the risk of misinterpretation due to non-paternity or non-maternity.
- In testing of prenatal samples, to rule out the presence of substantial maternal DNA contamination of the fetal (chorionic villus/amniotic fluid/fetal blood/cord blood) sample. Such maternal cell contamination is routinely tested for to prevent misdiagnosis.
- In the investigation of possible sample mix-up.
How does it work?
Rapid testing to establish sample identity is performed using quantitative fluorescent-polymerase chain reaction (QF-PCR).
- QF-PCR exploits regions of chromosomes that contain short tandem repeats (STRs). These repeat regions are not themselves associated with disease. However, the number of repeats is highly variable between people. This means it can be used to identify whether a DNA sample matches a previous DNA sample for the same individual; whether two individuals are likely to be biologically related; and whether a DNA sample is contaminated with DNA from someone else.
- Primers (short single-stranded DNA with a particular sequence designed to target a unique part of the genome) are designed for each STR marker and are labelled with a fluorescent tag.
- The STR markers are amplified by quantitative PCR.
- The fluorescently-labelled PCR products are separated by size by capillary electrophoresis.
- The unique pattern of STR markers in the individual tested can then be determined and compared to previous samples from the same individual (to confirm sample identity), family members (to provide information on biological family relationships) or a maternal sample (in the case of prenatal samples, to rule out contamination with maternal DNA).
Advantages and limitations of identity testing
Identity testing is:
- cost effective;
- accurate; and
- robust – can cope with poor-quality samples and small samples.
- Maternal cell contamination below 10% may not be detected.
- It is not designed to provide information on whether the samples tested have a genetic defect.
- Rare, unexpected results may be observed for a specific STR marker, for example apparent loss or gain of a marker. These will be investigated by the laboratory and there is a small chance of an incidental finding of a deletion or duplication in the region of the STR marker.
- A blood sample in an EDTA tube is required: 5-10ml of blood from adults, 2-5ml from children and 1-2ml from babies.
- Target reporting time is dependent on the reason for testing, and can be as little as three days if, for example, identity testing is used in the context of prenatal testing (to rule out maternal cell contamination).
- Mann K and Ogilvie CM. ‘QF-PCR: Application, overview and review of the literature‘. Prenatal Diagnosis 2012: volume 32, issue 4, pages 309-14. doi: 10.1002/pd.2945