nsw forensic & analytical science service€¦ · • forensic biology & dna • forensic...

14
21/09/2015 1 Two Years Later: A reflection on the implementation of STRmix™ in a high throughput DNA laboratory ISHI 2015 Zane Kerr Senior Forensic Biologist NSW Forensic & Analytical Science Service, Sydney, Australia Outline Intro to NSW Forensic & Analytical Science Service Impetus for change Intro to STRmixValidation Training Challenges NSW Forensic & Analytical Science Service

Upload: others

Post on 26-Apr-2020

24 views

Category:

Documents


0 download

TRANSCRIPT

Page 1: NSW Forensic & Analytical Science Service€¦ · • Forensic Biology & DNA • Forensic Toxicology • Drug Toxicology • Illicit Drugs • Chemical Criminalistics Forensic biology

21/09/2015

1

Two Years Later:

A reflection on the implementation of STRmix™ in a high throughput DNA laboratory

ISHI 2015

Zane Kerr

Senior Forensic Biologist

NSW Forensic & Analytical Science Service, Sydney, Australia

Outline

• Intro to NSW Forensic & Analytical Science Service

• Impetus for change

• Intro to STRmix™

• Validation

• Training

• Challenges

NSW Forensic & Analytical Science

Service

Page 2: NSW Forensic & Analytical Science Service€¦ · • Forensic Biology & DNA • Forensic Toxicology • Drug Toxicology • Illicit Drugs • Chemical Criminalistics Forensic biology

21/09/2015

2

A statewide forensic service provider

• Forensic Medicine

• Forensic Biology & DNA

• Forensic Toxicology

• Drug Toxicology

• Illicit Drugs

• Chemical Criminalistics

Forensic biology & DNA laboratory

FB/DNA Laboratory

Evidence Recovery Unit

DNA Laboratory

Casework Laboratory

Person Sample Laboratory

Specialist DNA Laboratory

Case Management

Unit

Database Management

Team

R&D Laboratory

69 FTE employees

DNA laboratory

• Fully-automated DNA laboratory

• Average TAT of ~2 days

Page 3: NSW Forensic & Analytical Science Service€¦ · • Forensic Biology & DNA • Forensic Toxicology • Drug Toxicology • Illicit Drugs • Chemical Criminalistics Forensic biology

21/09/2015

3

Forensic biology & DNA laboratory

• High throughput lab

• In one year:

– 11,660 cases received

– 35,531 samples tested

– 17,680 person samples tested

– 1822 court statements completed

– 8871 database links reported

Impetus for change

Impetus for change

• Trace/touch DNA samples → weak/complex mixtures

• Inconsistency between labs → DNA evidence

suspended in Victorian courts in December 2009

• ANZPAA NIFS standardisation of DNA interpretation

project

• Formation of StatSWG

Page 4: NSW Forensic & Analytical Science Service€¦ · • Forensic Biology & DNA • Forensic Toxicology • Drug Toxicology • Illicit Drugs • Chemical Criminalistics Forensic biology

21/09/2015

4

Impetus for change cont.

• StatSWG recommended move towards continuous

probabilistic model for DNA interpretation

• National standardisation could not be envisaged without a

software solution

• STRmix™ developed to meet needs of Australasian

forensic community

STRmix™

What is STRmix™?

• Fully continuous probabilistic expert system

• Uses Markov Chain Monte Carlo to thoroughly ‘explore’ profile

• Genotype Probability Distribution produced following deconvolution

• Genotype weights used to calculate Likelihood Ratio

• Models stochastic effects, drop-out, drop-in → suitable for weak &

complex profiles

STRmix™ logo used with the permission of the Institute of Environmental Science and Research Limited

Page 5: NSW Forensic & Analytical Science Service€¦ · • Forensic Biology & DNA • Forensic Toxicology • Drug Toxicology • Illicit Drugs • Chemical Criminalistics Forensic biology

21/09/2015

5

Validation

Validation overview

• April – June 2012: beta testing (STRmix™ v1.01, Profiler Plus®)

• January – March 2013: validation (STRmix™ v1.05,

PowerPlex® 21)

• Known samples: single source, 2 & 3 contributor mixtures

• Reproducibility

• Determination of model parameters

Validation overview cont.

March 2013: STRmix™ v1.05 & PP21 implemented for

casework use

• July 2013: Known samples: 4 contributor mixtures

• Ongoing validation & testing when new versions released

• Currently using STRmix™ v2.06 and about to implement

STRmix™ v2.3.06

Page 6: NSW Forensic & Analytical Science Service€¦ · • Forensic Biology & DNA • Forensic Toxicology • Drug Toxicology • Illicit Drugs • Chemical Criminalistics Forensic biology

21/09/2015

6

Validation overview cont.

• STRmix™ v2.3.06 validation → determination of model

parameters, comparison of results with STRmix™ v2.06,

reproducibility

• Future validation → mixture samples with five known

contributors

Key findings

• Rapid → most profiles deconvoluted within several minutes

on a standard PC/laptop

• Accurate → genotypes of known contributors generally

assigned highest weight

• Reproducible

Key findings cont.

• Provides more informative match statistics than existing

binary methods

• Artefacts can significantly effect results if not removed

Page 7: NSW Forensic & Analytical Science Service€¦ · • Forensic Biology & DNA • Forensic Toxicology • Drug Toxicology • Illicit Drugs • Chemical Criminalistics Forensic biology

21/09/2015

7

Training

Training overview

• January 2012: Intro to software and training course

• July 2012: ‘Train the trainer’ course

• August 2012: In-house training of senior reporting biologists

Training overview cont.

• August 2012 – January 2013: theory and practical exams

completed by senior staff, authorised to use STRmix™

• Ongoing training: lectures, discussion of papers

• Lectures to Police, prosecutors & other parties

Page 8: NSW Forensic & Analytical Science Service€¦ · • Forensic Biology & DNA • Forensic Toxicology • Drug Toxicology • Illicit Drugs • Chemical Criminalistics Forensic biology

21/09/2015

8

Challenges

Challenges

• Implementation of new kit & interpretation software was not

without difficulty

• Two of the biggest issues encountered:

– Assigning number of contributors

– Increased workload

Assigning number of contributors

• STRmix™ requires user to assign number of contributors

• Can be difficult to do, particularly for weak/complex

results

• Increased sensitivity of PP21 kit, ABI 3500xL analyser,

extraction chemistry → more mixtures recovered & most

with >2 contributors

• Increased stochastic effects & artefacts such as ‘post

stutter’ & ‘stutter of stutter’ complicate interpretation

Page 9: NSW Forensic & Analytical Science Service€¦ · • Forensic Biology & DNA • Forensic Toxicology • Drug Toxicology • Illicit Drugs • Chemical Criminalistics Forensic biology

21/09/2015

9

Potential solutions

• Analyse using lower analytical threshold, re-test sample

• Deconvolute assuming different number of contributors to

determine effect on LR

• LRs for major contributors generally insensitive if number of

contributors varied

• Can have a significant effect on LRs for minor contributors

however

Page 10: NSW Forensic & Analytical Science Service€¦ · • Forensic Biology & DNA • Forensic Toxicology • Drug Toxicology • Illicit Drugs • Chemical Criminalistics Forensic biology

21/09/2015

10

16,17 15,17 11,19 8,13 16,18

14,15 16,24 10,12 9,139,12

18,20 31.2,33.2 8,10 10,10 8,88,9.3

12,13 17,20 14,15 19,28

Assuming two contributors:16,17

Contrib 1: 89%Contrib 2: 11%

STRmix™ v2.06 results

Page 11: NSW Forensic & Analytical Science Service€¦ · • Forensic Biology & DNA • Forensic Toxicology • Drug Toxicology • Illicit Drugs • Chemical Criminalistics Forensic biology

21/09/2015

11

If 2 contributors assumed, LR = zero (i.e. exclusion)

If 3 contributors assumed, LR approx. 16 billion favouring

inclusion

How to report???

Increased workload

• STRmix™ has greatly expanded our ability to interpret weak

& complex results → increased workload

• Rapid… but can be very time consuming to review results

• Expert system for experts: need to scrutinise output to check

that it ‘makes sense’

• May not model profile appropriately, even for mixtures that

appear fairly straightforward

High stutter (16%)

High stutter (15%)

3rd contributor? Combination ofpost stutter/stutter of stutter?

Page 12: NSW Forensic & Analytical Science Service€¦ · • Forensic Biology & DNA • Forensic Toxicology • Drug Toxicology • Illicit Drugs • Chemical Criminalistics Forensic biology

21/09/2015

12

High stutter (14%)

Contrib 1: 3%Contrib 2: 32%Contrib 3: 65%

STRmix™ v2.06 results

STRmix™ v2.06 results cont.

Page 13: NSW Forensic & Analytical Science Service€¦ · • Forensic Biology & DNA • Forensic Toxicology • Drug Toxicology • Illicit Drugs • Chemical Criminalistics Forensic biology

21/09/2015

13

• Issue reproducible in STRmix™ v2.06 even if a more

thorough sampling of profile carried out

• Improved modelling in STRmix™ v2.3.06 → results align with

intuitive expectations

STRmix™ v2.3.06 results

Contrib 1: 1%Contrib 2: 33%Contrib 3: 66%

STRmix™ v2.3.06 results cont.

Page 14: NSW Forensic & Analytical Science Service€¦ · • Forensic Biology & DNA • Forensic Toxicology • Drug Toxicology • Illicit Drugs • Chemical Criminalistics Forensic biology

21/09/2015

14

Solutions

• Up-skilling of non-reporting biologists

• No further interpretation of complex mixtures in a

case if ‘simpler’ results available to interpret

“Due to the low level and/or complexity of the mixture,

further interpretation has not been carried out.”

Summary

• Expert systems such as STRmix™ enable interpretation of weak &

complex results that would previously have been reported as

‘inconclusive’

• Validation of STRmix™ by NSW FASS and other Australasian

laboratories has demonstrated its effectiveness

• Objective system but does rely on subjective decisions by analyst

e.g. number of contributors, removal of artefacts etc.

• Results need careful review → expert system for use by experts

[email protected]