Lessons from a study of DNA contaminations from police services and forensic laboratories in Switzerland

Abstract

In Switzerland, the DNA profiles of police officers collecting crime scene traces as well as forensic genetic laboratories employees are stored in the staff index of the national DNA database to detect potential contaminations. Our study aimed at making a national inventory of contaminations to better understand their origin and to make recommendations in order to decrease their occurrence. For this purpose, a retrospective questionnaire was sent to both police services and forensic genetic laboratories for each case where there was a contamination. Between 2011 and 2015, a total of 709 contaminations were detected. This represents a mean of 11.5 (9.6-13.4) contaminations per year per 1'000 profiles sent to the Swiss DNA database. Feedbacks were obtained from the police, the laboratory or both for 552/709 (78%) of the contaminations. Approximately 86% of these contaminations originated from police officers whereas only 11% were from genetic laboratories employees and 3% were associated to other sources (e.g. positive controls, stain-stain contaminations). Interestingly, a direct contact between the stain and the contaminant person occurred in only 51% of the laboratory contaminations whereas this number increased to 91% for police collaborators. The high level of indirect DNA transfer in laboratories might be explained by the presence of "DNA reservoirs" suggesting that cleaning procedures should be improved. At the police level, most contaminations originated from the person who collected the trace and likely occurred directly at the crime scene. Improving sampling practices could be beneficial to reduce these contaminations.

Positive impact of DNA contamination minimization procedures taken within the laboratory

“…As expected most of these (i.e. 247/260) were described as touch DNA and the concentration of DNA recovered was generally very low (often below 20pg/ul) [5]. to the Swiss database [5].…”

“…Several procedures have recently been proposed to reduce the risk of DNA contamination incidents on both the crime scene and in the laboratory [1,[4][5][6][7][8][9]. Among these procedures, several are particularly adapted to the laboratory environment.…”

“…Both the profiles of one contributor (single or major contributor) and mixtures of two contributors can be sent to the Swiss national DNA database if at least six, respectively eight, loci have been validated. In contrast, mixtures of more than two contributors and minor components of mixtures cannot be sent to the database [5]. For improvement purposes, the laboratory records all the contamination incidents of trace samples analyzed within the laboratory.…”

“…For improvement purposes, the laboratory records all the contamination incidents of trace samples analyzed within the laboratory. These contamination incidents are generally detected through the use of the staff index of the Swiss database, through controls in a local laboratory staff database or after controls requested by the police [5]. Potential matches with staff profiles are reported by the Swiss database when at least 5 loci, respectively 7 loci, are shared in cases of profiles appearing as single source or as mixtures of two persons.…”

Biological traces at the crime scene

“…Sve osobe prisutne na mjestu događaja moraju nositi propisanu zaštitnu opremu; jednokratne rukavice koje valja mijenjati prilikom uzimanja različitih tragova. Isto tako, potrebno je nositi masku za lice, kapu, kombinezon i nazuvke za obuću [50][51][52][53] . Predmeti i tragovi oštećene osobe ne smiju doći u kontakt s predmetima i tragovima osumnjičene osobe i mjesta događaja.…”