Quality Assurance Standards for Forensic DNA Testing Laboratories and Technical Working Group on DNA Analysis Methods (TWGDAM) Guidelines: Validation With Nonprobative Evidence

The DNA Advisory Board has issued "Quality Assurance Standards for Forensic Testing Laboratories" and the Technical Working Group on DNA Analysis Methods (TWGDAM) has made recommendations for a laboratory doing forensic DNA testing. Both of these documents (Standard 8.1.3 and Guideline 4.1.5.8) advise that validation of any new DNA test system include testing with nonprobative evidence samples. The purpose is to provide experience with and an understanding of how forensic samples perform with a new test system. Different definitions of nonprobative evidence have been used by crime laboratories, including adjudicated cases (convictions) or cases where the defendant confessed to the crime. When laboratories are training analysts and/or validating new DNA test systems, discretion should be used in selecting samples from "nonprobative evidence" samples. To prevent the unfortunate and unnecessary destruction of evidence that may be important in a postconviction relief proceeding, the following criteria are provided for the selection of test samples from actual forensic cases:

  • Only select samples where sufficient original and untested evidence samples will remain to do a minimum of two to three DNA tests using currently available DNA technology. For example, if four vaginal swabs remain after successfully using one for RFLP testing previously, then one to two swabs could be used for validation studies because the one untested swab would be sufficient for several PCR tests. Similarly, a portion of a large bloodstain could be used for validation studies without jeopardizing any future testing of the stain.
  • Only test samples that are less informative regarding the crime than other samples. For example, if several samples containing sperm have been associated with a sexual assault, select the sample for validation studies that would be duplicative and less informative than another sample, such as a panty crotch or a semen stain on a blouse, rather than the vaginal swab. Caution is advised because making these determinations requires specific information regarding the crime, such as whether the perpetrator ejaculated in the vagina, whether there were previous consensual partners, which may cause mixed DNA results from the vaginal swab and make the swab the less informative sample, or whether there were multiple assailants, in which case every semen stain may be probative.
  • Use DNA isolated from previous validation studies whenever possible rather than consuming additional evidence in a case.

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