Serological Evidence in Sexual Assault Investigations

Collecting Physical Evidence

Many of the items used for evidence collection are available in sexual assault kits. However, these commercial kits vary widely, and basic minimums should be considered.

Vaginal, oral and/or anal swabs should be taken from the victim using sterile cotton swabs. These swabs should then be air-dried, appropriately labeled, initialed by the examiner and packaged separately. In most assaults involving vaginal penetration, two to four vaginal swabs and two cervical swabs are adequate for analysis. In cases of oral or anal sodomy, oral or anal swabs should be obtained from the victim. Two clean swabs taken from the same package as the unstained control swabs should be submitted to show that any useful serology results obtained during analysis were due to body fluids and not any contaminant initially on the swabs.

Smear slides, unfixed and unstained, are sometimes useful for demonstrating the presence of sperm cells (spermatozoa). Vaginal, oral and/or anal smear slides should be obtained from the victim using the same swabs mentioned above. The smear slides should be appropriately labeled and should indicate which individual swab was used to create which microscope smear slide. Examining physicians in some jurisdictions prepare and microscopically examine smear slides to determine the presence of motile sperm cells indicative of recent sexual activity. In such cases, examining physicians may be required to testify in court proceedings regarding their observations. In any event, stained and fixed smear slides are useless for further serological analysis and should not be submitted to crime laboratories.

Pubic combings should be taken from the victim to identify any foreign hairs or fibers that may have been transferred during the assault. The physician should comb the pubic area and submit the comb and any resultant debris in an appropriately marked, sealed envelope. Head hair combings should be obtained from the victim in cases where other evidence is insufficient to show interpersonal contact. Pubic and head hair combings should also be obtained from the suspect if appropriate to the investigation.

Any obvious debris (soil, fibers, hair, grass, etc.) observed during the examination of the victim should be collected and submitted in a separate envelope describing the location of the debris. The examining physician should also scrape all residue from under the fingernails of each hand of the victim and place the residue in a specimen envelope or clip the fingernails and place the clippings in separately labeled envelopes.

Using a sterile pad that has been moistened lightly with distilled water,the physician should swab the vulva and the inner portion of the victim’s thighs adjacent to the vaginal area. The genital swabbing pad should then be air-dried and submitted for laboratory analysis in an appropriately labeled specimen envelope.

The physician should swab any dried secretions observed during the examination of the victim, i.e., saliva around bite marks, using a sterile pad that has been moistened lightly with distilled water. The pad should also be air-dried and submitted for analysis. In cases where dried blood or encrusted semen is observed, the material should be scraped from the body into a specimen envelope and submitted for analysis. Encrusted matter should never be re-hydrated, since it dilutes the sample. The location of each sample should be noted on a body diagram. Pubic or head hair containing encrusted semen should be carefully clipped and placed in a labeled specimen envelope.

In the event of oral ejaculation, gagging, swallowing or regurgitation during the assault may force air carrying semen through the nasal passages. The victim should blow her nose, very hard, several times into the center of filter paper. The resultant nasal mucous sample should be allowed to air dry and then submitted for analysis.


Head hair and pubic hair standard samples should be obtained from the victim and any suspects developed from the sexual assault investigation. The hair samples should be pulled with the bulb intact, not clipped. Head hair samples should be taken from four separate areas of the scalp. Twenty-five full-length hairs are generally considered adequate to represent an individual’s hair characteristics.

Liquid blood samples should also be obtained from the victim, any consensual sexual partners from at least 72 hours prior to the assault, and any developed suspects. Known blood and saliva samples from a suspect in a sexual assault case must usually be obtained through a court order issued by a judge or local magistrate. Blood samples from each individual should be collected in both red-topped and purple-topped blood collection tubes. Red-topped tubes are used for traditional serological analysis, such as ABO grouping, secretor status and enzyme electrophoresis. A red-topped tube indicates that the collected blood is exposed to no preservatives or blood anticoagulants. Purple-topped tubes are used for DNA profiling only. These tubes contain a chemical chelator(EDTA) that inhibits the action of enzymes that would normally act to breakdown the DNA molecules in the blood samples. In the event that toxicology examinations will be requested, an additional blood sample taken in a grey-topped tube (containing sodium fluoride) and a 10 cc. urine sample should also be collected. All of the collected blood and urine should be refrigerated, not frozen, and submitted for analysis as soon as possible.

Dried saliva samples should also be obtained from the victim, from consensual sexual partners from at least 72 hours prior to the assault, and from any developed suspects. The donor should expectorate on filter paper to produce a stain approximately 1 1/2 inches in diameter. Saliva should be clean and undiluted. Prior to giving the sample, the donor must have abstained from eating (food, gum, chewing tobacco), drinking and smoking for about 30 minutes. The stain should be circled in pencil before the drying is complete. When the samples have air-dried completely, they should be placed in a specimen envelope that has been dated and initialed.