Down’s syndrome screening: quality assurance support service

This document explains the Down’s syndrome screening quality assurance support service (DQASS) and how to use it.

The Down’s syndrome screening quality assurance support service (DQASS) is a service commissioned by Public Health England (PHE) to support the NHS Fetal Anomaly Screening Programme (FASP).


The main aim of DQASS is to monitor and support the quality and effectiveness of Down’s syndrome, Edwards’ syndrome and Patau’s syndrome screening in England.

DQASS provides feedback and support to laboratories, sonographers, the Screening Quality Assurance Service (SQAS) and to the NHS FASP. The analyses provided by DQASS are used to improve the performance of the screening programme through feedback on all aspects of the test to laboratories, ultrasound departments and commercial suppliers.

The software used to calculate the Down’s syndrome (T21) and/or Edwards’ syndrome (T18) and Patau’s syndrome (T13) chance result from the biochemical and ultrasound markers is complex and best provided and supported by commercial suppliers. The screening programme has developed a specification for the chance result(s) calculation software for laboratories in England. Software used in screening laboratories must meet the requirements of this specification.

How DQASS works

DQASS works on a rolling audit of screening test data on a 6-monthly cycle. The statistical analyses monitor the screening process at various levels: from the overall standardised screen positive rate (SPR) at the top level, to specific adjustments for ethnicity, smoking and other factors applied to individual biochemical markers. Through meta-analyses, DQASS provides information on effects of factors such as smoking that is used to improve screening performance.

The NHS FASP section 7a service specification requires all screening laboratories providing NHS screening to participate in DQASS. Currently the service is provided to 20 screening laboratories and approximately 2,500 individual ultrasound practitioners across England, based in more than 150 NHS hospitals/providers.

New screening laboratories

Any new screening laboratories, or laboratories wishing to set up a new network, should contact DQASS to register an intention to supply data. Following registration, DQASS will issue a date for data submission.

DQASS code for ultrasound practitioners

To request a trainee DQASS code, the screening support sonographer (SSS) should email DQASS stating that they require a trainee identity code. The email should also include the practitioner’s name and their associated hospital unit(s). If the practitioner already has a Fetal Medicine Foundation (FMF) code, this can be used in place of a DQASS code.

When sending 25 paired nuchal translucency (NT) / crown rump length (CRL) measurements for assessment, the SSS should return the measurements and diagnostic plot in the Excel spreadsheet provided by DQASS along with the practitioner’s name, identity code and hospital unit(s). Feedback will be sent to the SSS for onward distribution to the individual practitioner.

Data needed for DQASS

Laboratories providing NHS screening submit data every 6 months based on the DQASS schedule. It is important that screening laboratories submit data on time (the first working day of the scheduled month) to enable timely reporting and feedback. DQASS should be contacted if data submission is anticipated to be late.

A designated person in each laboratory must provide the data specified for a 6-month period in spreadsheet format with anonymised individual patient data contained in separate rows. Data provided should only include screening performed as part of NHS FASP using either combined screening or quadruple screening testing strategies. Data related to other testing strategies, such as the integrated test, non-routine or private screening arrangements, should be removed by the laboratory before the data is submitted to DQASS.

The laboratory should liaise with the SSS to make sure the ID codes for the sonographers are up to date and that the codes can be matched to an ultrasound department, to enable feedback to be given.

Each column should correspond to a specific data field. The first row should contain the variable label. For the combined test, each row should correspond to a fetus. For the quadruple test each row should correspond to a pregnancy.

If some of the specified data fields are not recorded then a blank column, or a column with N/A in each cell, should be reported. Although it is not used for the statistical analyses, additional information for comparison purposes should include:

  • current medians by day, or regression equation used, indicating both the gestational age and weight adjustment equations used
  • current algorithm parameters, means, standard deviations, correlation coefficients for both the unaffected and affected (T21 and T18/T13) outcomes
  • dates of lot and median changes and corresponding lot numbers
  • calibration or quality-control changes
  • analyser and software used

Any specific queries should also be attached.

Table showing data that screening laboratories need to provide to DQASS

Submission dates for laboratories (start of month)

October and April

Birmingham Women’s NHS Foundation Trust

Mid Essex Hospitals NHS Trust (Broomfield Hospital)

Norfolk and Norwich University Hospitals NHS Foundation Trust

Nottingham University Hospitals NHS Trust

Bolton NHS Foundation Trust

Bolton/Nottingham second trimester network

November and May

Oxford University Hospitals NHS Foundation Trust

Kettering General Hospital NHS Foundation Trust

Barking, Havering and Redbridge University Hospitals NHS Trust (King George Hospital)

January and July

Cambridge University Hospitals NHS Foundation Trust

King’s College Hospital NHS Foundation Trust

Sheffield Teaching Hospitals NHS Foundation Trust (Northern General Hospital)

University College London Hospitals NHS Foundation Trust

February and August

Portsmouth Hospitals NHS Trust (Queen Alexandra Hospital)

Royal Devon and Exeter NHS Foundation Trust

Wolfson Institute of Preventive Medicine

March and September

Cambridge/Oxford/Leeds/Newcastle second trimester network

Newcastle Hospitals NHS Foundation Trust

North Bristol NHS Trust (Southmead Hospital)

The Leeds Teaching Hospitals NHS Trust

University Hospitals Coventry and Warwickshire NHS Trust

Report types

DQASS undertakes a range of statistical analyses on the data provided and produces reports summarising activity and performance.

Four reports are produced for each 6-monthly data submission (reports 1 to 4 below). Two national cycle reports are produced at the end of each 6-month cycle, one for laboratory data and another for ultrasound data (reports 5 and 6 below).

1. Detailed laboratory report for each screening laboratory

This report contains information on all aspects of the screening data submitted by the laboratory, including:

  • summary statistics on the demographic information of the population screened
  • estimated standardised screen positive rates (SPR) for each test provided
  • details of each biochemical marker used by the laboratory
  • a summary of the ultrasound data from the ultrasound department(s) that the laboratory provides a service for
  • DQASS parameter estimates for log MoM values from routine samples

Laboratories that are part of a network will have one combined DQASS report for the screening strategy that is part of the network. This report is sent to the named laboratory lead for the network.

2. Summary laboratory report

The content of this report is agreed on a follow-up phone call, between the laboratory lead and the lead statistician from NHS FASP. This document outlines information such as:

  • screening test(s) provided with associated software and equipment used
  • estimated annual number of pregnancies screened with each test
  • estimated standardised SPR for each test
  • agreed list of recommendations and actions
  • providers served for each screening test

3. Detailed ultrasound report for each individual ultrasound practitioner

These reports are in the form of a graph/plot known as an individual’s NT distribution plot and are circulated to individuals by their SSS’s. If any data sets suggest that further support or training may be required, through a red flag for bias or throughput, the associated plots are also shared with the national and regional SQAS. The plots provide:

  • ultrasound practitioners with feedback on their individual paired NT and CRL distributions in relation to the Fetal Medicine Foundation (FMF) reference curve
  • the number of scans performed in the 6-month period and the associated hospital(s)

4. An ultrasound department summary report for each ultrasound provider

This report outlines:

  • the screening laboratory used and time period covered by report
  • for each practitioner – identity code, number of scans, median NT, median CRL, estimated bias and flag status
  • flag status allocated in the previous cycle
  • a plot of the estimated bias for each practitioner with 95% confidence intervals

5. Laboratory cycle report

The laboratory end-of-cycle report includes aggregated results for all laboratories and allows comparison of performance cycle on cycle. This can be seen for individual markers and for the overall screening programme. These reports also provide summary statistics for the distribution of biochemical markers, including effects of factors such as ethnicity and smoking status.

6. Ultrasound cycle report

The ultrasound end-of-cycle plots report includes aggregated results for all ultrasound practitioners and gives feedback on the overall performance of sonography over time. The plots give the mean level of bias in NT measurements relative to the NT reference curve nationally.

DQASS report recipients

The 6 reports produced by DQASS are sent to numerous stakeholders including laboratory leads, individual sonographers via SSSs, NHS FASP, SQAS (national and regional), Screening and Immunisation Leads (SILs) via regional SQAS and chief executives of screening laboratory providers. Recipients for each of the reports are as follows:

Detailed laboratory report recipients

Laboratory lead

Summary laboratory report recipients

Laboratory lead

SSS and deputy

Chief executives of all providers using the laboratory


SQAS national and regional

SILs via regional SQAS

Individual NT distribution report recipients

Individual ultrasound practitioners via SSS

NHS FASP (only red flagged data)

SQAS national and regional (only red flagged data)

Ultrasound department summary report recipients

SSS and deputy


SQAS national and regional

SILs via regional SQAS

Laboratory cycle report recipients (reports published on GOV.UK)

Laboratory lead


SQAS national team

Ultrasound cycle report recipients (reports published on GOV.UK)

SSS and deputy


SQAS national team

Contact DQASS

Down's syndrome screening Quality Assurance Support Service (DQASS)

Room N15
Plymouth Science Park
1 Davy Road



Fetal anomaly screening: programme handbook

Fetal anomaly screening laboratory handbook: Down’s, Edwards’ and Patau’s syndromes

Fetal anomaly screening: ultrasound practitioners handbook

e-Learning modules

e-Learning modules can be accessed on e-Learning for Healthcare. Relevant modules are:

  • screening for Down’s syndrome, Edwards’ syndrome and Patau’s syndrome
  • nuchal translucency
  • fetal cardiac e-Learning
  • 18+0 to 20+6 week fetal anomaly ultrasound scan resource

NT/CRL diagnostic plot

A Nuchal translucency diagnostic plot self-assessment tool (MS Excel Spreadsheet, 110KB) is available for practitioners to enter their 25 paired NT/CRL measurements. This spreadsheet tool must be used when the SSS sends data to DQASS for either new starter feedback or for follow-up as part of a red flag action plan.

Software specification

The software specification sets out in detail all the aspects that need to be incorporated into the software package to provide consistent chance results across England. 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 they are screening. These would normally be decided by the laboratory in collaboration with DQASS.

Measuring nuchal translucency: using ultrasound equipment

Published 1 January 2014
Last updated 13 September 2019 + show all updates
  1. Updated DQASS detailed guide.

  2. First published.