What is differential extraction?

Differential extraction is a laboratory process used in forensic DNA testing to separate sperm cells from other biological material in a sample.

The process seeks to answer the evidentiary question: "Does the semen originate from male A."

Read an acquittal case where semen was detected and the differential extraction failed: How DNA Evidence Is Used in Sexual Assault Cases - A Real Case Explained

It is most commonly used in sexual assault cases, where samples may contain a mixture of cellular material from different individuals. The process aims to isolate sperm cells into one fraction and non-sperm cells (such as epithelial or skin cells) into another so that each component can be analysed separately.

This separation allows laboratories to generate DNA profiles that may assist in identifying potential contributors. However, differential extraction is not a perfect process. Complete separation is not always achieved, and DNA from non-sperm cells may remain in the sperm fraction, while sperm DNA may sometimes appear in the non-sperm fraction.

The effectiveness of the separation depends on factors such as the condition of the sample, the number of sperm cells present, the age of the material, and the laboratory methods used. Where samples are degraded, contain low numbers of sperm cells, or include multiple contributors, interpretation becomes more complex.

Importantly, differential extraction is a method of preparing samples for DNA testing. It does not determine when biological material was deposited, whether ejaculation occurred during the alleged event, or what activity led to its presence.