Examination for the Presence of Semen in Medicolegal Cases

In the context of alleged rape or sexual assault, forensic analysis often includes the examination of biological material such as samples obtained from the vagina, stains on clothing, skin, hair, or other body parts to detect the presence of semen.

Collection of Samples

• Vagina: Samples are collected either through direct aspiration or by using saline lavage.
• Clothing: Semen stains on clothing, when exposed to ultraviolet light, emit a green-white fluorescence. For analysis, a small piece (approximately 1 cm²) from the stained area is soaked in 1-2 ml of physiological saline for one hour. A similar piece of unstained clothing is soaked as a control.

Laboratory Procedures

Microscopic Examination for Sperms

The presence of motile sperms in vaginal fluid suggests that the intercourse occurred within the last 8 hours. Smears prepared from the collected samples are stained and examined microscopically for the presence of sperms.

Acid Phosphatase Test

Acid phosphatase levels are measured in vaginal or clothing samples. Due to the high concentration of acid phosphatase in semen, a level of ≥50 U/sample is indicative of recent sexual activity.

Determination of Blood Group Substances

When semen is confirmed in a sample, it is possible to test for blood group substances. In ‘secretor’ individuals (comprising about 80% of the population), blood group antigens are present in bodily fluids, including semen.

Florence Test

This test detects choline, which is found in high concentrations in semen. The procedure involves adding an equal volume of reagent (iodine 2.54 g, potassium iodide 1.65 g, distilled water 30 ml) to the sample. A positive result is indicated by the formation of rhombic or needle-like crystals of choline periodide. However, false positives can occur due to the presence of high choline levels in some other body fluids.