Screening for Down’s syndrome is offered routinely to thousands of women each year as part of standard antenatal care. For those women identified as being at high risk of carrying a Down syndrome foetus, chorionic villus sampling (taken at10-12 weeks) or amniocentesis (taken at 12-16 weeks) is offered. Standard cytogenetic techniques involving tissue culture and microscopic analysis can take up to 14 days to provide a diagnosis. Fluorescent in-situ Hybridisation (FISH) using interphase cells is expensive, time consuming and unsuitable for
high throughput use.

Quantitative Fluorescence PCR

This new test now offered by Carigen utilizes the QF-PCR (Quantitative fluorescence- Polymerase Chain Reaction) technique. Using PCR amplification, fluorescent dye labelled primers target highly polymorphic regions of DNA sequence, short tandem repeats (STRs), that are located on the chromosomes of interest. Each targeted STR marker is specific to the chromosome on which it is located, thus the copy number of the STR marker can be diagnostic of the copy number of the chromosome.

A normal sample has the normal complement of two of each of the autosomal chromosomes and two sex chromosomes, thus two alleles of a chromosome specific STR are determined by the QF-PCR technique as two peaks in a 1:1 ratio. The observation of an extra STR allele as either a three peak pattern in a 1:1:1 ratio or two peak pattern in a 2:1 peak ratio is diagnostic of the presence of an additional sequence which in turn may represent an additional chromosome, as in the case of a trisomy.

Our systems involve a combination of tests which allows for the most accurate
representation of the Trisomy data. Our main screening system is QST*R plus. This is a highly multiplexed DNA fluorescent-based assay containing a total of 20 markers. It contains markers for chromosomes13, 18, 21, X and Y and will detect both the most common viable autosomal trisomies and sex chromosome aneuploidies in a single test.

Our autosomal markers are used to detect the three most common viable autosomal trisomies:

Trisomy 21 (Down syndrome)
Trisomy 18 (Edwards syndrome)
Trisomy 13 (Patau syndrome)

More information may be obtained by using additional markers on specific systems for trisomy 18, 21 or 13 for use on the very few occasions where there are insufficient informative markers to make an accurate determination with the QST*R plus alone.

Additional markers on the sex chromosomes X and Y can detect the most common sex chromosome aneuploidies. Our QST*R-XY systems detect sequences on both the X and Y chromosome. It can be used to detect sex chromosome aneuploidies including Turner syndrome (monosomy X) and Klinefelter syndrome (predominantly 47, XXY]

This series of tests have been fully validated for in vitro diagnostic use and suitable for both chorionic villus and amniotic fluid analysis. Studies have shown a greater than 99% concordance* with standard karyotyping. The testing may be used as a preliminary test before karyotyping or for final analysis.

Individual results can be obtained within a few days of receipt of samples. In routine use. The turnaround time for this test is three to five working days.