The vast majority of rapes are committed by persons known to the victim: only two percent of assaults are perpetrated by a stranger according to one survey. Therefore the identity of the perpetrator is frequently reported. Biological evidence such as semen, blood, vaginal secretions, saliva, vaginal epithelial cells may be identified and genetically typed by a crime lab. The information derived from the analysis can often help determine whether sexual contact occurred, provide information regarding the circumstances of the incident, and be compared to reference samples collected from patients and suspects. Medical personnel in the United States of America collect evidence for potential rape cases by using rape kits.
DNA profiling is used by crime laboratories for testing biological evidence, most commonly by means of the polymerase chain reaction (PCR), which allows analysis of samples of limited quality and quantity by making millions of copies. An advanced form of PCR testing called short tandem repeats (STR) generates a DNA profile that can be compared to DNA from a suspect or a crime scene. Blood, buccal (inner cheek) swabbings or saliva should also be collected from victims to distinguish their DNA from that of suspects.
Criminals may plant fake DNA samples at crime scenes. In one case Dr. John Schneeberger, who raped one of his sedated patients and left semen on her underwear, surgically inserted a Penrose drain into his arm and filled it with foreign blood and anticoagulants. Police drew what they believed to be Schneeberger's blood and compared DNA on three occasions without a match.
Circumstances and type of rape
Abrasions, bruises and lacerations on the victim help elucidate how a rape was carried out. 8 to 45 percent of victims show evidence of external trauma, most commonly at the mouth, throat, wrists, arms, breasts and thighs: trauma to these sites comprise approximately two thirds of injuries, while trauma to the vagina and perineum account for approximately 20 percent.
Recent coitus can be determined by performing a vaginal wet-mount microscopy examination (or oral/anal if indicated) for detection of motile sperm, which are seen on the slide if less than three hours have elapsed since ejaculation. However only one-third of sexual assaults result in ejaculation into a body orifice. Further, the alleged assailant may have had a vasectomy or have experienced sexual dysfunction (roughly 50 percent of assailants suffer from impotence or ejaculatory dysfunction). In addition, acid phosphatase levels in high concentrations is a good indicator of recent coitus. Acid phosphatase is found in prostatic secretions and activity decreases with time and is usually absent after 24 hours. Prostate-specific antigen (PSA) may be detected within a 48-hour period. The seminal fluid of vasectomized men also contains a significant PSA level. Nonmotile sperm may be detected even beyond 72 hours after intercourse depending on staining techniques.
It was held by the Chandhigarh High Court [India] as reported in 2006 Volume No.  Name of Legal Monthly Publication -Acquittal Page No. 199 – Chandhigarh High Court – Dilip Rao sahib Deshmukh, J. – Bablu @ Uday Vs. State of Chandhigarh – Criminal Appeal No. 412 of 2006 – Decided on 27.7.2006 – Where in a rape case doctor’s found hymen intact, merely because doctors stated that partial penetration was possible, no such inference could be drawn in favor of the prosecution and the conviction in the rape case was set aside by the High Court.
- Abbey, A., BeShears, R., Clinton-Sherrod, A. M., & McAuslan, P. (2004). Psychology of Women Quarterly, 28, 323-332."Similarities and differences in women's sexual assault experiences based on tactics used by the perpetrator". Accessed 10 December 2007.
- A National Protocol for Sexual Assault Medical Forensic Examinations National Criminal Justice Reference Service (NCJRS). September 2004
- (5 January 2015) New hope for rape kit testing advocates BBC News Magazine, Retrieved 5 January 2015
- Emergency Management of the Adult Female Rape Victim. American Family Physician, June, 1991. by Diane K. Beebe