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Short Tandem Repeat (STR) Interpretation Guidelines by SWGDAM (Forensic Science Communications, July 2000)

Short Tandem Repeat (STR) Interpretation Guidelines by SWGDAM (Forensic Science Communications, July 2000)

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July 2000 - Volume 2 - Number 3

Short Tandem Repeat (STR) Interpretation Guidelines


Scientific Working Group on DNA Analysis Methods (SWGDAM)


Introduction | Preliminary Evaluation of Data | Designation |
Interpretation of Results | 4. Conclusions | Statistical Interpretation
References/Suggested Readings

Introduction

The interpretation of results in casework is a matter of professional judgment and expertise. Not every situation can or should be covered by a preset rule. It is important that the laboratory develops and implements written guidelines for interpretation of analytical results. This document provides a framework for the laboratory to develop short tandem repeat (STR) interpretation guidelines. The laboratory's interpretation guidelines should be based upon validation studies, data from the literature, instrumentation used, and/or casework experience.

1. Preliminary Evaluation of Data

1.1. The laboratory should develop criteria to determine whether the results are of sufficient intensity/quality for interpretation purposes using methods appropriate for the detection platform. These criteria should be determined by evaluating data generated by the laboratory.

1.1.1. When quantitative results (e.g., peak amplitude) are used to evaluate STR profiles, the results should be examined to determine if they meet the laboratory's defined analytical and interpretational threshold(s).

1.1.1.1. The analytical threshold(s) is defined as the minimum and maximum intensity thresholds that are determined to assign alleles.

1.1.1.2. The interpretational threshold should be defined empirically.

1.1.2. When quantitative results are not used, the laboratory should establish criteria to interpret alleles based on visual inspection of gel images.

1.2. The laboratory should develop criteria to evaluate internal lane size standards and/or allelic ladders.

1.3. Controls are required to assess analytical procedures.

1.3.1. The laboratory should establish criteria for evaluation of the following controls, including but not limited to: reagent blank, amplification blank, and positive control.

1.3.2. The laboratory should develop criteria for the interpretation and documentation of results in the event that the controls do not perform as expected.

1.4. A laboratory using STR multiplexes that contain redundant loci should establish criteria regarding the concordance of such data.

2. Designation

2.1. The laboratory should establish criteria to assign allele designations to appropriate peaks or bands.

2.1.1. Locus Designation: The laboratory should establish criteria to address locus assignment for alleles.

2.1.2. Allele Designation: The laboratory should designate alleles in accordance with Combined DNA Index System (CODIS) recommendations.

2.1.2.1. Whenever possible, allele designation should be based operationally on the number of repeat sequences contained within the allele and by comparison to an allelic ladder.

2.1.2.2. The designation of alleles containing an incomplete repeat motif (i.e., an off-ladder allele falling within the range spanned by the ladder alleles) should include the number of complete repeats and, separated by a decimal point, the number of base pairs in the incomplete repeat (e.g., FGA 18.2 allele).

2.1.2.3. If an allele falls above the largest or below the smallest allele of the allelic ladder, the allele should be designated as either greater than (>) or less than (<) the respective ladder allele, or when appropriate interpolation can be used.

2.2. Artifacts can occur and should be noted. These may include, but are not limited to, the following: pull-up, stutter, and nontemplate nucleotide addition. The laboratory should establish guidelines based on empirical data (obtained internally or externally) to address the interpretation of these and other artifacts.

3. Interpretation of Results

3.1. The laboratory should define conditions in which the data would lead to the conclusion that the source of the DNA is either from a single person or more than one person. This may be accomplished by an examination of the number of alleles at each locus, peak height ratios, and/or band intensities.

3.1.1. Single Contributor: A sample may be considered to be from a single contributor when the observed number of alleles at each locus and the signal intensity ratios of alleles at a locus are consistent with a profile from a single contributor. All loci should be evaluated in making this determination.

3.1.2. Mixtures With Major/Minor Contributors: A sample may be considered to consist of a mixture of major and minor contributors if there is a distinct contrast in signal intensities among the alleles. The difference is evaluated on a case-by-case context. All loci should be evaluated in making this determination.

3.1.3. Mixtures With a Known Contributor(s): In some cases, when one of the contributors (e.g., the victim) is known, the genetic profile of the unknown contributor may be inferred. Depending on the profiles in the specific instance, this can be accomplished by subtracting the contribution of the known donor from the mixed profile.

3.1.4. Mixtures With Indistinguishable Contributors: When major or minor contributors cannot be distinguished because of similarity in signal intensities or the presence of shared or masked alleles, individuals may still be included or excluded as possible contributors.

3.2. The laboratory should have guidelines for interpretation of partial profiles (i.e., profiles with fewer loci than tested) that may arise from degraded or limited quantity DNA or from the presence of polymerase chain reaction (PCR) inhibitors.

3.3. The laboratory should establish guidelines to interpret profiles that exhibit potential stochastic effects (e.g., allele dropout and/or substantial imbalance of alleles).

4. Conclusions

4.1. The laboratory should prepare guidelines for formulating conclusions resulting from comparisons of single source samples and mixtures with known reference samples.

4.1.1. General categories of conclusions include, but are not limited to: inclusion or match, exclusion or nonmatch, inconclusive or uninterpretable, and no results.

5. Statistical Interpretation

5.1. The source of the population database(s) used should be documented. Relevant population(s) for which the frequency will be calculated should be identified.

5.2. The formulas used in calculating the frequency of a DNA profile should be defined for the following:

5.2.1. Heterozygote profiles

5.2.2. Homozygote profiles

5.2.3. Composite profiles (i.e., multiple locus profiles)

5.2.4. Minimum allele frequencies

5.2.5. Mixture calculations

5.2.6. Biological relationships, where appropriate

5.3. When used, criteria for the declaration of source attribution should be documented.

6. References/Suggested Readings

Committee on DNA Forensic Science, National Research Council. An Update: The Evaluation of Forensic DNA Evidence. National Academy Press, Washington, DC, 1996.

DNA Advisory Board. Quality assurance standards for convicted offender DNA databasing laboratories (approved April 1999), Forensic Science Communications (July 2000) 2. Available at www.fbi.gov/programs/lab/fsc/backissu/july2000/codispre.htm

DNA Advisory Board. Quality assurance standards for forensic DNA testing laboratories (approved October 1998), Forensic Science Communications (July 2000) 2. Available at www.fbi.gov/programs/lab/fsc/backissu/july2000/codispre.htm

DNA Commission, ISFH. DNA recommendations: 1994 report concerning further recommendations regarding PCR-based polymorphisms in STR (short tandem repeat) systems, Forensic Science International (1994) 69:103–104.

Federal Bureau of Investigation. National DNA Index System (NDIS) Procedures Manual. U.S. Department of Justice, Washington, DC, February 1999 (revised).