An inter-laboratory comparison study on transfer, persistence and recovery of DNA from cable ties

Highlights

• DNA transfer and recovery from cable ties was studied by four laboratories.

• Impact of laboratory analysis and interpretation methods on results was examined.

• Amount of DNA recovered and DNA profile composition differed between laboratories.

• Inter laboratory differences may impact on weight of evidence at activity level.

Abstract

To address questions on the activity that led to the deposition of biological traces in a particular case, general information on the probabilities of transfer, persistence and recovery of cellular material in relevant scenarios is necessary. These figures may be derived from experimental data described in forensic literature when conditions relevant to the case were included. The experimental methodology regarding sampling, DNA extraction, DNA typing and profile interpretation that were used to generate these published data may differ from those applied in the case and thus the applicability of the literature data may be questioned. To assess the level of variability that different laboratories obtain when similar exhibits are analysed, we performed an inter-laboratory study between four partner laboratories. Five sets of 20 cable ties bound by different volunteers were distributed to the participating laboratories and sampled and processed according to the in-house protocols. Differences were found for the amount of retrieved DNA, as well as for the reportability and composition of the DNA profiles. These differences also resulted in different probabilities of transfer, persistence and recovery for each laboratory. Nevertheless, when applied to a case example, these differences resulted in similar assignments of weight of evidence given activity-level propositions.

Introduction

Increased sensitivity of DNA analysis methods has led to the ability to obtain full DNA profiles from merely a few cells. Often evidentiary items carrying such little cell material are submitted for DNA profiling. Such minute quantities of DNA are commonly referred to as ‘trace DNA’ [1].

With a decreasing amount of DNA, and uncertainty surrounding its cellular source, the number of potential scenarios that may have led to the deposition of the DNA increases. As a result, there is an increasing need for forensic scientists to address questions on how and when the cell material could have been deposited in light of scenarios put forward during legal proceedings. Consequently, information on the transfer, persistence, recovery and prevalence of trace DNA is required.

A methodology for evaluative reporting (outlined in the guidelines for evaluative reporting of ENFSI [2]) provided by Bayesian probabilistic theory and the hierarchy of propositions [3], allows the scientist to provide balanced, transparent and logically correct statements on the probability of DNA typing results given ‘activity level’ propositions.

Within the framework for evaluative reporting, the probability of recovering a person’s DNA (or cellular material) under mutually exclusive propositions is assigned. To make an accurate assessment of these probabilities, relevant context information is gathered from the criminal case as well as data on the probabilities of transfer, persistence, recovery and prevalence of cellular material for relevant items or surfaces. One approach to obtain data on these probabilities is to perform laboratory experiments that closely align with the case circumstances. Due to restrictions in time and resources, these case-specific experiments often have small sample sizes, while the application of the data is limited to one or few cases. A second approach would be to assess case-specific probabilities based on data from published research papers in which the data are generally based on more generic experiments.

There is a body of scientific work that examines the circumstances that influence the transfer, persistence and recovery, or survey the prevalence of trace DNA (see [4], [5] for an overview of relevant literature). Since these studies focus on the factors that in general influence transfer, persistence and recovery they do not directly assign probabilities to specific DNA transfer events encountered in criminal casework. Some studies (e.g. [6], [7], [8]) do assign probabilities for obtaining a particular test result that they obtained through the experimental study. When these probabilities are to be used in casework, the data on which they are based need to have been interpreted according to the in-house casework protocols [9] that may differ among laboratories [10]. Not only the interpretation of data, but also the trace recovery strategies, DNA extraction methods, DNA analysis systems (number of PCR cycles, type of DNA profiling kit, injection settings) and overall equipment are individual to laboratories, which may affect DNA typing results as discussed in van Oorschot et al. [1]. For example, different swab types for trace recovery can present significant different collection and extraction efficiencies [11]. The probabilities assigned to DNA transfer, persistence and recovery events based on experimental data are therefore dependent on the laboratory conducting the study.

The aim of this study is to investigate the influence of inter-laboratory differences in the data obtained from an experiment on transfer, persistence and recovery of DNA, on case evaluation at activity level. Knowledge on the concordance of results obtained through varying laboratory protocols will assist a scientist in making an informed decision on the applicability of studies from different laboratories in case-work.