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Estimation of the Success of Male STR Analysis based on the Number of Visible Sperm Detected Under the Microscope

A chance to obtain an autosomal STR complete profile of a rapist from vaginal swabs of a victim has been evaluated in this study. Two-hundred vaginal swabs were retrospectively examined. The samples were tested for acid phosphatase (AP) activity before being equally separated into two groups. Those in the first group were used for sperm count by which both estimation and total count of the sperm were conducted. The samples were then graded into six levels (0–5) based on the sperm count. Meanwhile, the samples in the second group were subjected to sperm estimation count and STR profile analysis. Samples graded level 5 were 100% AP positive. The number of detected alleles obviously depended on the sperm grade. The percentage of the full profile was found highest (85%) in the grade 5 group and gradually declined with the decrease in sperm grade. Remarkably, the chance to get the full STR profile of the perpetrator fell below 50% when there were fewer than 60 sperm cells in the sample based on the estimation count. This study could help guide the decision made by a forensic DNA examiner concerning evidence gathered from a rape case.