Highlights
- •Analysis of mixture samples from the 22nd GHEP-ISFG intercomparison exercise.
- •Autosomal, Y and X STRs, mtDNA results analyzed.
- •Drop-out and drop-in alleles were the main cause of errors for STRs.
- •Nomenclature, contamination and sample mix-up issues detected in mtDNA analyses.
Abstract
Since 1992, the Spanish and Portuguese-Speaking Working Group of the ISFG (GHEP-ISFG)
has been organizing annual Intercomparison Exercises (IEs) coordinated by the Quality
Service at the National Institute of Toxicology and Forensic Sciences (INTCF) from
Madrid, aiming to provide proficiency tests for forensic DNA laboratories. Each annual
exercise comprises a Basic (recently accredited under ISO/IEC 17043: 2010) and an
Advanced Level, both including a kinship and a forensic module. Here, we show the
results for both autosomal and sex-chromosomal STRs, and for mitochondrial DNA (mtDNA)
in two samples included in the forensic modules, namely a mixture 2:1 (v/v) saliva/blood
(M4) and a mixture 4:1 (v/v) saliva/semen (M8) out of the five items provided in the
2014 GHEP-ISFG IE. Discrepancies, other than typos or nomenclature errors (over the
total allele calls), represented 6.5% (M4) and 4.7% (M8) for autosomal STRs, 15.4%
(M4) and 7.8% (M8) for X-STRs, and 1.2% (M4) and 0.0% (M8) for Y-STRs. Drop-out and
drop-in alleles were the main cause of errors, with laboratories using different criteria
regarding inclusion of minor peaks and stutter bands. Commonly used commercial kits
yielded different results for a micro-variant detected at locus D12S391. In addition,
the analysis of electropherograms revealed that the proportions of the contributors
detected in the mixtures varied among the participants. In regards to mtDNA analysis,
besides important discrepancies in reporting heteroplasmies, there was no agreement
for the results of sample M4. Thus, while some laboratories documented a single control
region haplotype, a few reported unexpected profiles (suggesting contamination problems).
For M8, most laboratories detected only the haplotype corresponding to the saliva.
Although the GHEP-ISFG has already a large experience in IEs, the present multi-centric
study revealed challenges that still exist related to DNA mixtures interpretation.
Overall, the results emphasize the need for further research and training actions
in order to improve the analysis of mixtures among the forensic practitioners.
Keywords
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Article info
Publication history
Published online: July 19, 2016
Accepted:
July 17,
2016
Received in revised form:
July 14,
2016
Received:
April 21,
2016
Identification
Copyright
© 2016 Elsevier Ireland Ltd. All rights reserved.
