A forensic DNA laboratory should develop its own STR interpretation guidelines based upon their own validation studies and results reported in the literature (SWGDAM 2000). Practical experience with instrumentation and results from performing casework are also important factors in developing an interpretation strategy.
■ Conduct necessary validation studies (see Chapter 16) and gain experience in your lab
■ Utilize analyst's experience
■ Use literature references as a resource in understanding if an 'off-ladder' allele has been observed before. As a useful resource, we have included a list of all known alleles for the 13 CODIS STR loci in Appendix I. A more up-to-date list can be found in the STRBase STR fact sheets for each STR locus or the non-published variant allele page (http://www.cstl.nist.gov/biotech/STRBase/var_tab.htm).
Validation studies will define observed stutter ratios for each locus, establish minimum peak heights, and define heterozygous peak ratios within a locus. When in doubt on a sample's correct result, the sample should be re-tested. This may be as simple as re-injecting it on the ABI 310 or putting another aliquot of the sample on the next gel. Even if sample re-testing involves re-extracting and/or re-amplifying the 'problem' sample, it is worthwhile in order to obtain an accurate result.
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