Biological stain collection – Absorbing paper is superior to cotton swabs

Biological evidence at crime scenes often contains very small amounts of DNA. Therefore, it is important to use the most effective sampling devices and procedures for stain collection. Currently, cotton swabs moistened with water are widely used, also in our laboratory. However, several studies have shown that other methods may be more efficient. In this study, we compared the DNA sampling efficiency of cotton swabs (Puritan) and pieces of absorbing paper (Kimtech) moistened with two liquids, water and ethanol. An initial experiment with blood stains deposited on glass slides showed that DNA yields were highest for samples collected with absorbing paper and ethanol. To reflect casework conditions, we tested cotton swabs with water versus absorbing paper with ethanol on a range of used items and clothing from four surface classes: leather, plastic, natural and synthetic fabrics. We found that DNA yields were higher when using absorbing paper and ethanol than with cotton swabs and water. These findings were significant for all surface classes except synthetic fabrics for which there was a trend in the same direction though. These results suggest that pieces of absorbing paper moistened with ethanol can improve the efficiency of stain collection, especially when stains are expected to contain low amounts of DNA. However, user-friendliness could still be improved and contamination risk reduced if an easy-to-handle collection device based on absorbing paper was developed.