This includes examination of material obtained from vagina, stains from clothing, skin, hair, or other body parts for semen. This is carried out in cases of alleged rape or sexual assault.
Collection of Sample
- Vagina: Direct aspiration or saline lavage
- Clothing: When scanned with ultraviolet light, semen produces green white fluorescence. A small piece (1 m2) of clothing from stained portion is soaked in 1-2 ml of physiologic saline for 1 hour. A similar piece of clothing distant from the stain is also soaked in saline as a control.
1. MICROSCOPIC EXAMINATION FOR SPERMS
Presence of motile sperms in vaginal fluid indicates interval of < 8 hours. Smears prepared from collected samples are stained and examined for the presence of sperms.
2. ACID PHOSPHATASE
Acid phosphatase is determined on vaginal or clothing samples. Due to the high level of acid phosphatase in semen, its presence indicates recent sexual intercourse. Level of ≥50 U/sample is considered as positive evidence of semen.
3. DETERMINATION OF BLOOD GROUP SUBSTANCES
When semen is positively identified in vaginal fluid or other sample, test can be carried out for the presence of blood group substances in the same sample. The ‘secretor’ individuals (80% individuals are secretors) will secrete the blood group substances in body fluids, including semen.
4. FLORENCE TEST
This test detects the presence of choline found in high concentration in semen. To several drops of sample, add equal volume of reagent (iodine 2.54 g, potassium iodide 1.65 g, distilled water 30 ml); in positive test rhombic or needle-like crystals of periodide of choline form. False-positive tests can occur due to high choline content of some other body fluids.