Case work guidelines and interpretation of short tandem repeat complex mixture analysis

Interpretation guidelines for short tandem repeat casework analysis are dif ficult to construct. As soon as a set of guidelines are developed, a new ca se evolves that does not fit the painstakingly written document. The casewo rk analysts gather and amend the guidelines again, and again. This article seeks to demonstrate that general guidelines can be set and written such th at it can be used for any detection format. Guidelines published by the Sci entific Working Group for DNA Analysis Methods, a working group of DNA fore nsic experts in the United States, ale used to set the format for the writt en protocol on interpretations. The rule "the interpretation of results in casework is a matter of professional judgment and expertise. Not every situ ation can or should be covered by a preset rule" is stressed. Development o f minimum and maximum threshold values, heterozygote ratios, stochastic lim its, and determination of major and minor components based on validation st ud ies is discussed. The paper travels through setting criteria to evaluate internal lane standards and amplification controls. It continues with esta blishing ranges for interpretation and defining true alleles versus anomali es. Examples of a variety of profiles are given and the potential interpret ation, using signal intensities and genetics. In addition, report writing s trategies and wording routinely used by the Pennsylvania State Police DNA L aboratory System are given.