Maternal screening is offered to all expectant women during the first or second trimester of pregnancy. The purpose of this screening is to test for fetal abnormalities including chromosomal abnormalities such as Down’s syndrome, Trisomy 18 and neural tube defects such as spina bifida. Testing is performed by taking a blood sample from the patient’s arm which is then tested for a combination of biomarkers. Clinical results in addition to the maternal age are considered and used to calculate the risk of Down’s syndrome.
by Leah Hoencamp and Lynsey Adams
Down’s syndrome is a genetic condition and occurs when an individual inherits an extra copy of one chromosome. This means that affected people have three copies of chromosome 21, where there should be only two. The extra chromosome causes characteristic physical and intellectual features. The reasons why an extra copy of chromosome 21 causes Down’s syndrome are not known, which is why screening is so essential.
A combination of tests is used to screen for Down’s syndrome. Two types of screening are available and which is used depends on the stage of pregnancy of the patient. These stages are divided into first and second trimester.
First trimester screening includes:
• Free beta-hCG
• Pregnancy associated plasma protein (PAPP-A)
Second trimester screening includes:
• Double test AFP and hCG)
• Triple Test (AFP, hCG and uE3)
• Quadruple Test (AFP, hCG, uE3 and inhibin A)
If the results generated from this screening appear within the ‘higher risk’ category, more definitive tests are needed to confirm a diagnosis, such as amniocentesis or a chorionic villus sample. These tests provide a definitive result and involve taking samples of fluid from around the unborn baby. However, it is a highly invasive procedure and carries a small risk of miscarriage.
Internal quality control in maternal screening
Quality control (QC) is a crucial part of any clinical testing programme to ensure the accuracy and reliability of patient test results. Quality control is designed to detect, reduce and correct deficiencies in the laboratory’s internal analytical process prior to the release of patient results and to improve the quality of the results reported by the laboratory. Quality controls are manufactured to mimic a patient sample and contain one or more analytes of known concentration. They are made using a base material normally human serum, bovine serum, urine or spinal fluid. A laboratory will use quality controls to validate the patient samples. If QC results are within their target range then patient results should also be accurate. Once validated, the patient results can be used for diagnosis, prognosis and treatment planning. If QC values are outside the target range, it may indicate a number of issues including inaccurate calibration, instrument failure, operator error or reagent issues. In the field of maternal screening, the main aim is to minimise the risk of false positive and false negative results, ultimately ensuring results obtained are accurate and reliable.
In any type of screening the majority of errors take the form of false positive or false negative results. In other areas false negative results are of more concern as the patient will be perceived as healthy and will therefore not receive the required treatment. However, in prenatal screening, false positive results are also of major concern. If a patient tests positive they may have to undergo an invasive amniocentesis procedure with risk to the fetus in order to confirm if a chromosomal disorder like Downs’s syndrome is present. It is clear that such screening requires a robust and reliable quality control procedure in order to avoid potential errors.
To facilitate the increased screening for Down’s syndrome, trisomy 18 and neural tube defects, Randox has developed the only commercially available multi-analyte; tri-level control specifically designed to cover both first and second trimester prenatal screening, with the following benefits:
• The unique combination of inhibin A and PAPP-A in addition to AFP, total hCG, free B-hCG and uE3 reduce the need to purchase separate controls thus saving money
• Manufactured from 100% human serum providing a matrix similar to the patient sample while reducing cross reactivity and ultimately shifts in QC values
• Three distinct levels of control are available, accurately covering the complete clinical range. The level one control contains suitably low levels of AFP whereas the level three control contains high levels of hCG. Moreover, the uE3 levels are in line with those typically found during the first twenty weeks of pregnancy
• True third party control providing an unbiased, independent assessment of performance. Highly accurate instrument specific target values and ranges are provided for the most popular analysers used in maternal screening
• Excellent reconstituted stability of seven days at +2–8 oC
• Excellent vial-to-vial homogeneity (%CV <1 %) • Suitable for first trimester double screen and second trimester triple and quad screens.
Internal quality control (IQC) will help ensure results are reliable. An inter-laboratory data management package such as Acusera 24.7 can be used to further ensure quality. An effective IQC and peer group reporting scheme will help improve your laboratory’s analytical performance, help meet regulatory requirements and most importantly ensure the accuracy and reliability of patient test results. Acusera 24.7 enables laboratories to monitor analytical performance, access peer group reports and compare results with other laboratories using the same quality controls, method and instrument.
External quality assessment in maternal screening
To further assess the performance of maternal screening tests, laboratories should also be involved in an external quality assessment (EQA)/proficiency testing (PT) scheme. External quality assessment (EQA) is an essential aspect of any laboratory operation. EQA measures a laboratory’s accuracy using ‘blind’ samples that are analysed as if they were patient samples. EQA provides a means of assessing the analytical performance of a laboratory compared to other laboratories utilising the same methods and instruments. Participation in an EQA scheme will help produce reliable and accurate reporting of patient results. Quality results will reduce time and labour costs, and most importantly provide accurate patient diagnosis and treatment. Such a scheme is of paramount importance during testing such as maternal screening.
Randox International Quality Assessment Scheme (RIQAS) offers a Maternal Screening Programme which is capable of monitoring all 6 parameters involved in first and second trimester screening. RIQAS is the world’s largest global EQA scheme with more than 20 000 participants in over 100 countries worldwide.
Effective screening is essential for the detection of fetal abnormalities including Down’s syndrome, trisomy 18 and spina bifida. However, equally important in this process for laboratories responsible for processing the results is quality control. Effective quality control will help reduce false positives and false negatives, thereby ensuring reliable results and improving care of the patient overall.
AFP, alpha-fetoprotein; hCG, human chorionic gonadotropin; uE3, unconjugated estriol.
Leah Hoencamp BSc & Lynsey Adams BSc
55 Diamond Road, Crumlin,
Co. Antrim, UK BT29 4QY