For naming conventions used throughout this guidance and other general information, please read the Overview page. Show
1. The combined testThe combined test assesses the chance of the baby having trisomy 21 (T21), trisomy 18 (T18) or trisomy 13 (T13) by using:
The NT measurement must be used in combination with a maternal blood sample for calculating the chance result. It must not be used alone. Assessment techniques and biometric charts used for ultrasound measurements must meet nationally agreed standards, as outlined in the British Medical Ultrasound Society (BMUS) Fetal size and dating: charts recommended for clinical obstetric practice (2009) guidelines. The eligibility criteria for the combined test is when the CRL is between 45.0mm and 84.0mm. If the CRL is < 45.0mm, the woman is recalled for a further ultrasound scan to measure the NT and remeasure the CRL. If the CRL is > 84.0mm, the gestational age is calculated using the head circumference (HC). The quadruple test is offered if the criteria is met. If it is not possible to measure the NT, at least one more attempt is offered (‘twice on the couch’). This may be on the same day or at a later date according to local guidelines. The CRL and NT measurement must be taken on the same day. If it is not possible to measure the NT despite ‘twice on the couch’, further attempts do not need to be offered. The woman is offered the quadruple test. The combined test gives the woman more time to consider screening options. In practice, there are 2 ways the combined test can be performed and this determines how the woman receives the chance result. The maternal blood sample can be:
If the sample is taken at the time of the ultrasound scan and sent to the screening laboratory, the chance result report from the combined test is usually available within a few days of the result being authorised by the laboratory. If the sample is taken immediately before the ultrasound scan (from 10 weeks onwards), the screening laboratory makes the biochemistry results available to the ultrasound department; the chance result is calculated in the ultrasound/maternity department. The laboratory is responsible for the chance calculation software and the chance result. Ultrasound/maternity departments must have a process in place to share ultrasound measurements and final screening results with the laboratory. It is recommended that the laboratory take responsibility for auditing all chance results. Chance results can only be recalculated in exceptional circumstances. For example, incorrect data on the screening request form that affects the chance calculation. Every woman who accepts a pregnancy dating scan needs to be informed that unexpected findings may be identified. This must include any woman who declines screening for T21 and/or T18 and T13. The NHS FASP does not provide guidance regarding the clinical care of a woman who has declined the combined test and has an unexpected finding on the ultrasound scan. This may include an NT of ≥ 3.5mm. Local guidance for the care and management of these women should be in place. The Down’s syndrome, Edwards’ syndrome and Patau’s syndrome screening pathway requirements specification outlines failsafe systems providers must have in place. For example, follow-up of attendance at appointments. 2. The quadruple testThe quadruple test screens for T21 only. The quadruple test is offered when the:
The quadruple test uses maternal age and the following 4 biochemical markers measured from 14+2 to 20+0 weeks:
This combination of markers has a lower detection rate (DR) and a higher screen positive rate (SPR) than the combined test. For a woman presenting in the second trimester, an ultrasound scan is required to measure the HC to date the pregnancy and complete the quadruple test. The quadruple test is performed when the HC is between 101.0mm and 172.0mm. If the HC is more than 172.0mm, the quadruple test must not be offered. The woman is offered the 20-week screening scan. If the HC is < 101.0mm and the CRL is > 84.0mm, there are 2 options. 2.1 Option 1Use the HC measurement to calculate the dates that the woman will be between 14+2 and 20+0 weeks. Offer an appointment during this time for a blood sample to be taken for the quadruple test. 2.2 Option 2Use the HC measurement to calculate the date that the woman will be over 14+2 weeks. Rebook the woman for a further ultrasound scan where the HC can be remeasured and a blood sample taken for the quadruple test. The Down’s syndrome, Edwards’ syndrome and Patau’s syndrome screening pathway requirements specification outlines failsafe systems providers must have in place. For example, follow-up of attendance at appointments. 2.3 More informationMore information is available regarding practical solutions to combining dating and screening requirements at the ultrasound scan for the combined or quadruple test in the BMUS ‘A practical solution to combining dating and screening for Down’s syndrome’ (2011) guidelines. The NHS FASP does not provide guidance regarding the clinical care of a woman who has declined the quadruple test and has an unexpected finding on the pregnancy dating scan. Local guidance for the care and management of these women should be in place. 3. Non-invasive prenatal testing (NIPT)NIPT screening for T21, T18 and T13 is offered following a higher chance result (between 1 in 2 and 1 in 150) from either the combined or the quadruple test in both singleton and twin pregnancies. The UK National Screening Committee (UK NSC) has recommended introducing NIPT as an evaluative rollout. This is so any necessary changes to the screening pathway can be made. 4. Markers used in the combined and quadruple testsThere are a number of markers that are used to calculate the chance result. 4.1 Maternal ageAll pregnant women have a chance of having a baby with T21, T18 or T13. This chance increases with age. The older the woman, the greater the chance the baby will have one of these conditions. The graph below shows the chance of a woman having a baby with T21, T18 or T13 (y axis) according to maternal age (x axis). The graph shows that:
The table below shows the chance of a woman having a baby with T21 at 16 weeks of pregnancy.
4.2 Biochemical markersScreening laboratories must use the recommended combination of biochemical markers for both the combined and quadruple tests. Biochemical markers are analytes in the maternal blood that are measured by the laboratory and used to calculate the chance of a woman having a baby with T21, T18 or T13. The levels of these markers are affected by certain factors such as gestational age, ethnicity, smoking, maternal weight and maternal diabetes. To adjust these effects the levels are standardised and expressed as multiples of median (MoM) values. It is important that information about these factors is clearly written on the screening request form so the laboratories can make the correct adjustments. The following analytes are measured by the laboratory to calculate the chance result. Alpha fetoprotein (AFP)A low level of AFP in the maternal blood in the second trimester is associated with T21. Levels of AFP are low in maternal blood in the first trimester and increase until about 32 weeks. Human chorionic gonadatropin (hCG) or free beta hCGAn increased level of hCG and/or the free beta subunit in the maternal blood in the first and second trimester is associated with T21. A decreased level is associated with T18 and T13. Levels of hCG increase with time after conception, reaching a peak at about 9 to 12 weeks. It decreases and evens out at about 20 weeks. Inhibin-AAn increased level of inhibin-A in the maternal blood in the second trimester is associated with T21. Levels of inhibin-A increase during the first trimester. Levels decrease to even out in the second trimester, before increasing again in the third trimester. Unconjugated oestriol (uE3)A decreased level of uE3 in the maternal blood in the second trimester is associated with T21. Levels of uE3 in the maternal blood increase during pregnancy, reaching a peak at term. Pregnancy associated plasma protein-A (PAPP-A)A decreased level of PAPP-A in the maternal blood in the first trimester is associated with T21, T18 and T13. Levels of PAPP-A increase quickly during early pregnancy and continue to increase slowly until term. Effect of vaginal bleeding on biochemical markers used in screening for T21, T18 and T13Current evidence suggests that the biochemical marker levels are not substantially changed in a woman with a history of vaginal bleeding. The NHS FASP recommends eligible women are offered the combined or quadruple test as usual. 4.3 Ultrasound measurementsGestational age is calculated from ultrasound measurements of either the CRL or the HC. The measurements for NT, CRL and HC recorded on the screening request form must be the same as the stored ultrasound images. For purposes of consistency, all ultrasound measurements should be reported:
Nuchal translucency (NT)The NT is the ultrasound appearance of a collection of fluid under the skin at the back of the baby’s neck. The thickness of the NT is measured and used as part of the combined test to calculate the chance of having a baby with T21, T18 or T13. An NT measurement ≥ 3.5mm may be associated with T21, T18 and T13 and serious cardiac conditions. For a woman who accepts the offer of the combined test, an NT measurement ≥ 3.5mm is usually associated with a higher chance result. In these cases, the blood sample must be taken, processed and the chance result calculated in the usual way. Referral, in line with local guidelines, must be made straight away and not delayed until the chance results are known. As soon as the chance results are available, they should be forwarded to the clinician to support discussion with the woman. For a woman who declines the offer of the combined test, local guidelines should be in place for the clinical management of NT measurements ≥ 3.5mm and any unexpected findings on the ultrasound scan. It is important to note a baby with an increased NT may not have any of these conditions. Crown rump length (CRL)The CRL is the ultrasound measurement from the top of the head (crown) to the bottom of the buttocks (rump). The gestational age is calculated from this measurement, as outlined in the BMUS Fetal size and dating: charts recommended for clinical obstetric practice (2009) guidelines. The combined test can only be performed when the CRL measurement is between 45.0mm and 84.0mm. The CRL measurement must be used rather than a gestational age of weeks and days. The CRL measurement is essential for the analysis of the biochemical markers in the combined test. It may also be used in the quadruple test depending on gestational age. Head circumference (HC)The HC is the ultrasound measurement of the circumference of the baby’s head. The gestational age is calculated from this measurement when the CRL is > 84.0mm, as outlined in the BMUS Fetal size and dating: charts recommended for clinical obstetric practice (2009) guidelines. The eligibility criteria for the quadruple test is when the HC measurement is between 101.0mm and 172.0mm. 5. Laboratory information for the combined and quadruple testsAll NHS FASP screening laboratories must:
The laboratory must include an annual vertical audit of an antenatal screening sample in its laboratory audit schedule. The audit should randomly select a higher chance T21, T18 or T13 sample. In laboratories providing both combined and quadruple testing it is acceptable for the annual audit to relate to either. Wherever possible this should alternate between the 2 tests. The audit should include arrival and receipt of an antenatal screening sample at the laboratory and acknowledgment of higher chance results by clinical services. UKAS looks at ISO 15189 and the screening requirements on behalf of PHE Screening QA services and the NHS FASP. All NHS FASP screening laboratories must:
The laboratory must have a viable contingency plan to continue the provision of screening in the event of any failures to the laboratory service. 5.1 Chance calculation softwareScreening laboratories in England must use software that meet the requirements of the NHS FASP chance calculation software specification. This sets out all aspects that need to be included in a software package to give consistent chance results. Some variables that are entered into the software are defined by the local user to take account of the reagents used for screening and the characteristics of the local population. These are decided by the laboratory with support from DQASS. The software used to calculate the chance result from the biochemical and ultrasound markers is complex. This is best provided and supported by commercial suppliers. Laboratories must use chance calculation software that is (CE) marked. It must comply with EU directives and utilise software upgrades. 5.2 Data fields for the screening laboratory request formLaboratory request forms must include the required data fields outlined in the ‘Request form and data fields required when screening for Down’s syndrome, Edwards’ syndrome and Patau’s syndrome’. The data fields highlighted in bold text must be provided to DQASS. However, all data items specified are helpful to DQASS. The NHS FASP recommends all the required data fields are included when current request forms are revised. Software and data entry methods may vary. 5.3 Chance result report contentsOnly results and comments relating to the conditions screened for as part of the Down’s syndrome, Edwards’ syndrome and Patau’s syndrome screening pathway must be included on the chance result report. This report should include all relevant information used for calculating the chance result. This includes:
Test standard FASP-S05 outlines test turnaround times for T21, T18 and T13 screening. Information on reporting chance results in twin pregnancies is available. Lower chance result letter templates are available. 5.4 Blood sample draw, transport and storageBlood samples taken for T21, T18 and T13 screening must be collected in a plain or gel serum separating tube (SST). A screening blood sample must always be taken first if a full blood count sample is being taken at the same time. It is important that the correct order of draw is followed. Tubes containing ethylene-diamine-tetra-acetic acid (EDTA) must be avoided. This is due to significant interference of EDTA in the immunoassays used to measure the biochemical markers for screening. Blood samples must be processed as soon as possible. Free beta hCG can increase in concentration over time due to the dissociation of the intact hCG molecule. This effect is temperature dependent. The rate of deterioration of the sample increases with a rise in temperature. Serum samples are stable up to 72 hours at room temperature. All samples transported or stored at room temperature for longer than 72 hours must be rejected. The stability of the samples is significantly improved by refrigeration at 4 degrees Celsius, with whole blood and serum samples stable for several days. High temperatures (over 30 degrees Celsius) should be avoided as deterioration of free beta hCG occurs within 4 hours of sample collection. Most samples are stored and transported at room temperature. During periods of increased temperatures, providers should consider both storing and transporting samples in cool bags to reduce any effect on the markers. Whole blood samples must not be frozen or transported on dry ice as this makes the sample unsuitable for analysis. ExceptionsIf a gel separator tube is not used, it is good practice for the sample to be centrifuged and separated from the clot within 24 hours of collection. If this is not possible, samples may remain as whole blood at room temperature for up to 48 hours. Samples received as whole blood after 48 hours from sample collection must be rejected. Screening sample stability acceptance criteriaWhole blood (unseparated) samples can be stored at:
Serum (separated) samples can be stored at:
Test standard FASP-S06 measures the proportion of inadequate samples received for T21, T18 and T13 screening. 5.5 Analysers and kitsThere are several suppliers in the UK providing assays and analysers that are CE marked for use in T21, T18 and T13 screening. The NHS FASP does not have a preferred supplier and laboratories must decide which analyser best meets their purpose. Some assays are available on random access analysers which overcome the need to batch samples. All assays should be verified and used according to the manufacturer’s instructions. Any deviations must be fully validated in compliance with Medical laboratories – Requirements for quality and competence (ISO 15189). Analysers must be maintained according to the manufacturer’s instructions and a comprehensive maintenance record kept, preferably in an electronic format. Standard operating procedures that would pass a UKAS assessment must be in place in the laboratory. These should be for each assay and all procedures relating to the screening work of the laboratory. 5.6 Laboratory throughputThe NHS FASP requires a standalone screening laboratory to have a workload of at least 8,000 samples per screening test per year. A high throughput, particularly where technology requires batching of samples, means that laboratories will be able to meet recommended turnaround times for results. Extended turnaround times can delay diagnosis and ongoing management options. All laboratories with a workload of fewer than 8,000 samples per year for a specific screening test must be part of a network screening service for that test. Laboratory network screening serviceThe total number of samples from all laboratories within a network screening service must be a minimum of 8,000 per screening test per year. Each laboratory in a network must have a minimum throughput of 2,000 samples per screening test per year. Having a laboratory network screening service helps to improve the quality of the screening programme. The principle of a network allows laboratories with a smaller throughput to monitor performance while working in a consistent way to a common set of standards. It can develop capacity, technical expertise and proficiency in a safe and efficient way. A laboratory network screening service must:
Each laboratory in a network screening service must have identical:
The laboratory network screening service lead receives the combined 6 monthly DQASS detailed laboratory report. DQASS assessment of laboratory performanceAn annual throughput of 8,000 samples is needed to:
If laboratory throughput is small, DQASS cannot measure performance reliably. 5.7 Internal quality control (IQC)Laboratories must have internal quality control (IQC) procedures in addition to the external quality assurance service provided by DQASS. These should be performed frequently enough to enable timely changes. IQC procedures must be rigorous. They must be able to identify:
Manufacturer recommendations for IQC must be strictly adhered to as a minimum requirement. Additional controls should be included, especially if an assay is particularly difficult. It is good practice to include validated IQC material from another manufacturer. Laboratories should not rely solely on the controls provided by the manufacturer of the assay kit. Long-term trends in IQC data should be analysed to detect assay drift. It is important to compare sample and IQC results when a new batch of reagents is introduced. Significant shifts in population medians were observed on occasions with the introduction of a new reagent batch and after maintenance of an analyser. This can impact on the chance results reported if it is not anticipated and accounted for. Vigilance is required at all times because chance calculation involves several biochemical markers. The measurement of each marker must be as accurate as possible. Small inaccuracies in these measurements can lead to a greater overall inaccuracy of the final chance result. 6. Ultrasound information for the combined and quadruple tests6.1 Ultrasound image archivingEvery provider should save ultrasound scan reports and images onto an electronic reporting and archiving system. This system should be able to provide minimum audit data. All imaging studies should also have an electronic report linked to the images. 6.2 Professional competenceAll ultrasound practitioners must hold minimum certification as specified in the section on education and training. This information is also available in the:
6.3 Ultrasound machinesAll ultrasound practitioners should be aware of the Royal College of Radiologists (RCR) and Society and College of Radiographers (SCoR) Standards for the provision of an ultrasound service. All ultrasound practitioners should adhere to the BMUS Guidelines for the safe use of diagnostic ultrasound equipment which recommends time limits for obstetric scans. Ultrasound machines should be capable of producing images of diagnostic quality and include the following features:
All new ultrasound machines should be correctly set up with the manufacturer’s application specialist to make sure images can be optimised to allow accurate measurements. 6.4 Criteria for measurement of NT for the purposes of the combined testMidline sectionThe criteria for midline section are:
PositionThe criteria for position are:
MagnificationThe criteria for magnification are:
Calliper placementThe criteria for calliper placement are:
Image archivingThe criteria for image archiving are:
The NHS FASP does not recommend screening for nasal bone absence or hypoplasia, thus allowing measurement of the NT in the prone position. 6.5 Criteria for measurement of CRL for purposes of pregnancy dating and the combined and quadruple testsMidline sectionThe criteria for midline section are:
PositionThe criteria for position are:
MagnificationThe criteria for magnification are:
Calliper placementThe criteria for calliper placement are:
Image archivingThe criteria for image archiving are:
7. Screening in twin pregnanciesNHS FASP screening for T21, T18 and T13 is only available for singleton and twin pregnancies. Guidance is produced by National Institute for Health and Care Excellence (NICE) on higher multiple pregnancies (triplets and more). More than one baby in a pregnancy complicates screening. Each feto-placental unit contributes to the levels of the biochemical markers used in the chance calculation. Each baby has an individual NT measurement. 7.1 Combined test in twin pregnanciesThe test of choice for twin pregnancies is the combined test. Every effort must be made to offer and complete this test. Depending on a woman’s personal choice, chance results are reported as:
In a monochorionic twin pregnancy, the chance result is the same for each baby and one ‘pregnancy’ chance result is reported. In a dichorionic twin pregnancy, the chance result is reported for each baby. 7.2 Quadruple test in twin pregnanciesThe quadruple test is offered to a woman with a twin pregnancy to screen for T21 only when one or both of the:
The quadruple test in twin pregnancies is not as sensitive as the combined test and the decision-making process can be more difficult for a number of reasons. Women considering the quadruple test should have a discussion with a healthcare professional with a special interest, experience and knowledge of managing multiple pregnancies. This is to help support personal informed choice. Monochorionic twinsThe performance of the quadruple test in monochorionic twins is comparable to that in singleton pregnancies. Dichorionic twinsIn dichorionic twins where one baby has the condition and the other does not, the performance of the quadruple test is not as sensitive as it is in monochorionic twins. Quadruple test chance results relate to the pregnancy not to individual babies. They are not interpreted in the usual way but used, with a cut-off of 1 in 150 at term, to define a higher chance group. An appropriately trained healthcare professional should interpret and explain these results and pregnancy options due to the complexities involved. 7.3 Prenatal diagnosis in twin pregnanciesChorionic villus sampling (CVS) and amniocentesis sampling options are available. The risk of miscarriage is twice as high in twin pregnancies compared to singleton pregnancies. Prenatal diagnosis (PND) is performed at a tertiary-level fetal medicine unit. This is due to the specialised nature of the procedure and the increased risk of miscarriage. This is in line with the Royal College of Obstetrics and Gynaecology (RCOG) and NICE guidelines. 7.4 Vanished twinThe definition of a vanished twin is when one fetus, in a twin pregnancy, is non-viable. It may be partially or completely reabsorbed. An ultrasound scan for the combined or quadruple test may show either:
Empty second pregnancy sac and the combined testWhen there is an empty second pregnancy sac, the combined test can be used to calculate the chance result. This is because it is assumed the biochemical markers are not significantly different from those in a singleton pregnancy. Second pregnancy sac containing a non-viable fetus and the combined testWhen there is a second pregnancy sac containing a non-viable fetus, the combined test should not be used to calculate the chance result. This is because the non-viable fetus can contribute to the maternal biochemical markers for many weeks. This may affect the sensitivity of the test. The quadruple test can be offered or local guidance should be in place for the specialist clinical management of these pregnancies. For example, use of NT measurement and maternal age alone. The quadruple testThe quadruple test can be offered to a woman with a vanished twin pregnancy. This is because the DR of the quadruple test is unlikely to be reduced in a vanished twin pregnancy. Levels of inhibin and AFP can be increased in these pregnancies. However, given that inhibin is increased and AFP is decreased in a pregnancy with T21, these effects tend to cancel each other out. 8. Screening in in-vitro fertilisation (IVF) pregnanciesThe eligibility criteria for the combined test in IVF pregnancies is when the CRL is between 45.0mm and 84.0mm. The CRL measurement must be used for the purposes of screening. The quadruple test is also performed in IVF pregnancies. Maternal age in IVF pregnancies using donor eggs is calculated from the date of birth or age of the donor at the time of egg collection. This is required for calculation of prior chance. Calculating the estimated due date (EDD) of an IVF pregnancy is not part of the NHS FASP. Local guidelines should be in place to date these pregnancies. 9. Screening resultsChance results from combined or quadruple tests must only be recalculated in exceptional circumstances. For example, incorrect data on the screening request form. All higher chance results and/or significant findings on ultrasound scan must be reported and followed up in line with the screening standards. Referral standard FASP-S07 outlines communication timeframes for higher chance results. Lower chance result letter templates are available. 9.1 Chance cut-offChance cut-off determines whether a woman has a lower chance or higher chance result. The chance cut-off is 1 in 150 at term for both the combined and quadruple tests, as defined by the NHS FASP. A lower chance result is ≤ 1 in 151. A higher chance result is ≥ 1 in 150 (between 1 in 2 and 1 in 150). A woman with a higher chance result is offered a PND test, such as CVS or amniocentesis. The cut-off is based on a chance at term, rather than a chance at the time of the screening test. The chance at the time of screening is not known because there is significant pregnancy loss between the time of screening and birth. This loss rate is not known. 9.2 Combined test chance resultsDepending on the woman’s screening choice, the following chance results are reported:
9.3 Quadruple test chance resultsFor a woman screened using the quadruple test a single term chance of T21 is reported. What is biochemistry test in pregnancy?Biochemical markers are used to assess maternal, placental and fetal health. They help to diagnose and monitor maternal conditions such as gestational diabetes and pre-eclampsia, trophoblastic disease and fetal chromosomal abnormalities such as Down's syndrome (Table 1).
At which point during pregnancy can chorionic villus sampling first be performed quizlet?Chorionic villus sampling is usually done between weeks 11 and 14 of pregnancy — earlier than other prenatal diagnostic tests, such as amniocentesis.
How do you assess a pregnancy patient?Antenatal Care Module: 9.. 9.1 Checking for symptoms of poor nutrition or lack of iodine. ... . 9.2 Checking her weight. ... . 9.3 Checking her temperature. ... . 9.4 Checking her pulse. ... . 9.5 Checking for signs of anaemia. ... . 9.6 Checking for shortness of breath. ... . 9.7 Checking her blood pressure.. What is an indicator for performing a contraction stress test?The contraction stress test is interpreted by the presence or absence of late fetal heart rate decelerations, which are defined as decelerations that reach their nadir after the peak of the contraction and that usually persist beyond the end of the contraction.
|