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ACB Training Course- Plymouth

ACB Training Course- Plymouth. Down’s syndrome screening David Worthington (Laboratory Advisor - National Screening Programme). National Screening Programme. What is the National Screening Programme? The problem in screening – lack of uniformity Standard setting

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ACB Training Course- Plymouth

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  1. ACB Training Course- Plymouth Down’s syndrome screening David Worthington (Laboratory Advisor - National Screening Programme)

  2. National Screening Programme • What is the National Screening Programme? • The problem in screening – lack of uniformity • Standard setting • How many markers should be used? • Deciding the risk cut-off • Imprecision of the risk • DQASS • The future?

  3. Organisational structure of the UK National Screening Committee and its subgroups UK National Screening Committee DOH Screening Policy Unit Programme Director – Dr Anne Mackie Antenatal Screening Child Health Screening Adult Screening Cancer Screening Communicable diseases Sickle cell and thalassaemia Fetal anomaly Diabetes and heart disease Diabetic retinopathy UK Newborn Screening Programme Centre Sickle cell Cystic fibrosis Newborn hearing screening National Cervical Screening National Breast Screening Prostate cancer risk management Bowel cancer pilot

  4. Why is there a problem? • Down’s Syndrome screening evolved from NTD screening using maternal serum AFP • Screening programmes developed in late 1980s • Started in academic departments and 'marketed' • Start of the 'competitive' NHS - Trusts • Loss of Regional NHS structures • No National Screening framework • Each Trust does its own thing !!

  5. What laboratory tests? First trimester tests:- • Pregnancy Associated Plasma Protein -A (PAPP-A) • Free Human Chorionic Gonadotrophin (HCG) Second trimester tests:- • Alpha FetoProtein (AFP) • Free HCG or Total HCG • Unconjugated Estriol (uE3) • Inhibin-A

  6. Laboratory markers Different combinations of markers in second trimester screening: • Double test (AFP + HCG (total or free )) • Triple test (AFP + uE3 + HCG (total or free )) • Quadruple test (AFP + uE3 + HCG + inhibin-A)

  7. Laboratory markers First trimester screening: • 'Combined test' (NT + freeHCG + PAPP-A) Adding first and second trimester screening: • 'Integrated test' (NT + PAPP-A + quadruple test) • 'Serum integrated' (PAPP-A + quadruple) • 'Contingency testing' (Using risk cut-offs in first trimester to decide who requires second trimester testing) • 'Repeated measures' (Analysing same markers in both trimesters)

  8. MarkersNo ofHighestLowest‘Higher’ labsriskriskrisk group AFP + tHCG 16 181 693 4/16 AFP + fHCG 23 183 710 11/23 AFP + tHCG 43 91 910 20/43 +UE3 AFP + fHCG 21 24 770 7/21 +UE3 UK NEQAS (Dist 567 - May 2007)for 2T Maternal Serum Screening

  9. Calculation of Risk Different analytical methods + Different combinations of markers + Different risk calculation software = Different risks reported for the same woman. NOT IDEAL

  10. The Problem Wide variations in clinical practice "There is nothing lawyers like better than differences in clinical practice"

  11. Lessening lab diversity How do you make all laboratories do the same (or at least a similar) thing?

  12. Specifications vs Standards Specification:- What has to be done! Standard:- How well it has to be done!

  13. Specifications vs Standards Specification:- Use quadruple test of AFP, fHCG, uE3 and inhibin-A Standard:- Achieve a 60% detection rate for a 5% screen positive rate

  14. Standards and Guidance Standard:- Achieve a 75% detection rate for a 3% screen positive rate Guidance:- This maybe achieved using a, b, and c protocols but not x, y, or z protocols.

  15. How many markers? As many as it takes to reach the standard! Factors to consider: • Cost • Practicality • Equipment required • Convenience for the woman • State of the art

  16. Standards from October 2001 Laboratories should:- • be accredited • have satisfactory EQA performance • operate appropriate internal QC • participate in multidisciplinary audit • turn round 97% results in 3 days

  17. Standards from April 2007 Laboratories should:- • have a documented risk management policy • have a consultant responsible for the service with defined accountability • comply with national standards regarding risk cut-offs

  18. Workload Standard Laboratory Size:- • 'Stand-alone' labs - at least 10000 specs/year • Less than 10000 specs/year must be part of a 'managed network' of no less than 3 labs with at least 5000 specimens each, using the same screening package (Neonatal HbO standard > 25000 ideally 50000)

  19. Benefit:hazard ratio Down’s syndrome diagnosed : unaffected fetuses lost (Detection rate : false positive rate)

  20. Benefit:hazard ratio A large percentage of a small number is still a small number (DR) A small percentage of a large number can still be a large number (FPR)

  21. Why was the 1 in 250 second trimester cut-off chosen? • In 2000 most women were being screened in the second trimester by double testing. • There was a range of cut-offs being used, many determined by the effect it had on amnio rate and cyto labs. • Originally thought to give a SPR of about 5%.

  22. Why was the 1 in 250 second trimester cut-off chosen? Nothing magical about the cut-off value! It simply defines the 'higher risk' or 'screen positive' group

  23. Risk and cut-offs 1 in 100 Has FPR of 3% and DR of 65% Down’s Unaffected Frequency 1 10 100 1000 10000 Risk (1 in x)

  24. Risk and cut-offs Has FPR of 5% and DR of 75% 1 in 250 Down’s Unaffected Frequency 1 10 100 1000 10000 Risk (1 in x)

  25. Risk and cut-offs Has FPR of 25% and DR of 90% 1 in 500 Down’s Unaffected Frequency 1 10 100 1000 10000 Risk (1 in x)

  26. Receiver operator curves (ROC) 100 50 0 Detection Rate (%) 1 in 500 1in 250 1in 100 0 20 40 60 80 100 False positive rate (%)

  27. Risk and cut-offs Down’s Unaffected Frequency 1 10 100 1000 10000 Risk (1 in x)

  28. Risk and cut-offs Down’s Unaffected Frequency 1 10 100 1000 10000 Risk (1 in x)

  29. Risk and cut-offs Down’s Unaffected Frequency 10 1000000 Risk

  30. Imprecision of the risk • Difficult complex area! • As more markers added, imprecision is increased BUT populations move further apart • Only really important in borderline zone • If populations totally separated then imprecision is unimportant

  31. Imprecision of the risk Down’s Unaffected Frequency 10 1000000 Risk

  32. Receiver operator curves (ROC) Quad test 100 75 50 Triple test Detection Rate (%) Double test Cut-off of 1 in 250 0 2 4 6 8 10 False positive rate (%)

  33. Quad Test:-AFP+tHCG+uE3+Inhibin-A Triple:-AFP+tHCG+uE3 Double:-AFP+tHCG Threshold Risks 250 200 150 100 (Data from SURUSS)

  34. Integrated Quad Test:-AFP+tHCG+uE3+Inhibin-A Triple:-AFP+tHCG+uE3 Double:-AFP+tHCG Threshold Risks 250 200 150 100 (Data from SURUSS)

  35. Down’s Quality Assurance Support Service • Labs send raw screening data twice/year • Statisticians calculate medians and correction equations for weight, GA, etc • Compare with lab values • Suggest improvements • Has shown 'suboptimal' performance in nearly all labs • Work in conjunction with NEQAS

  36. The future? • First trimester screening will increase • NT and ultrasound will become more widespread • Second trimester screening will still be needed • Audit and monitoring will increase • DQASS influence will become more apparent • Better quality assays/software as standards bite • More 'managed networking' to improve medians • Why have a cut-off?

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