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Brent E. Turvey
June, 1995

"A-Utopic Determination of Oral Sex in Forensic Science"

Turvey, B., "A-Utopic Determination of Oral Sex in Forensic Science," Knowledge Solutions Library, June, 1995, Electronic Publication, URL:

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According to statistics cited by Schiff[15], there was a general increase in the incidence of reported rape offenses from 1972 through 1976. Bureau of Justice Statistics and the Uniform Crime Reports published by the U.S. Government Printing Office in Washington D.C. since 1976 show that this increase has remained steady for the past two decades, at least in the total number of reported rapes and sexual assault cases. These disturbing statistics in mind, the importance of continually reevaluating Forensic Sexual Assault/Rape protocols within the Law Enforcement and Medical Communities is clear. Determination of oral sex is an important link in those protocols.

Specific laws defining oral sex vary from state to state. For the purposes of this paper, oral sex will be defined as any instance where one party's mouth comes in contact with the genital area, breast area, or anal area of another party. This is an overall definition and should not be taken as an absolute of any kind.

In a behavioral context, Keating[9] addresses the topic of oral sex. He opens up a general discussion that relies upon forensic issues involved in proving that oral sex has taken place. Then he makes a statement and implications about behavior. The issue here is not proving that a victim was forced to engage in oral sex with a suspect or suspects, and not proving the identity of a perpetrator or perpetrators against some known suspects of a sex crime. Those issues are terribly important and are discussed by Keating, but his underlying behavioral assertions are dependent upon the forensic methods used to verify transfer evidence from alleged oral sex. However behavioral his context, Keating's discussion relies on biological detection; when oral sex is at issue, how does Forensic Science address it.

What the forensic examiner is looking for to confirm oral sex is transfer evidence particular to oral sex (i.e.- sperm, semen, saliva, hair, or bite marks). This evidence should be collected from the victim and/or the suspect, along with the appropriate control samples.

Often in a sexual assault or rape protocol, fluid evidence will be collected in the form of swabs taken from the victim to complete a rape kit, or from stains taken from the victim's clothing. In some cases, when permitted, a suspect's clothes may be available to search for stains, or a swab may be taken from a suspect's mouth or penis. Oral, vaginal and anal swabs are the most likely to yield strong positive results for fluid transfer evidence.

Hair may be visually identified in the suspect's mouth[9] and collected immediately, although this is not common.

Bite marks can be important, although they are not as common as sperm, semen, or saliva[9]. Bite marks can be visually identified, but must be photographed and examined by a Forensic Dentist for verification and later comparison. If you have bite marks on the genitals or breasts as verified by a Forensic Dentist, this is confirmatory evidence for oral sex.

Determining whether or not oral sex has taken place begins with the proper collection of evidence. Proper techniques for the collection of fluid, hair and bite mark evidence are outlined in Gaensslen[6], Geberth[8] and Lee[10]. Improper collection of evidence or failure to collect evidence can make a determination of oral sex impossible.


Keating[9] reports that with experience sperm can be microscopically identified with accuracy by their morphological characteristics. Keating further suggests a magnification of 400x to see just the heads of sperm through the objective. There is general agreement on these points, and staining techniques are fairly standard (Keating recommends staining suspected sperm in Haematoxylin and Eosin).

The question of whether or not to confirm the presence of sperm when only heads and no tails are visible remains an issue. For the more conservative examiner, both the heads and tails of sperm should be visually identified to make the call. Some of the more liberal examiners believe that they can determine with certainty the difference between a sperm head with no tail, and something that does a good impersonation of a sperm head with no tail. Somewhere between the conservative and the liberal opinion lies the moderate, who will only call heads if there are a lot of them. At any rate, opinions remain mixed on this point.

If sperm is confirmed on a stained slide made from an oral swab, that's a positive result for oral sex (Brauner[2], Gaensslen et al[7], & McCloskey[12]). Microscopic confirmation of sperm from other swabs (i.e.-anal or vaginal) does not confirm oral sex. Microscopic confirmation of sperm in the victim's or suspect's hair, or on the clothes up around the shoulders and neck is consistent with, but not proof of oral sex.

Unlike an already dried stain on a nonporous surface, there is an urgent time factor involved in the recovery of sperm from a victim's orifices. As reported by Wilson[19], there is no definite time scale for the identification of sperm in a body cavity, but "a number" of days can pass between time of death and time of examination with identifiable sperm being present. In fact Wilson reports a case where sperm is identified after 16 days in the vagina of a dead body. However as Wilson[19] and Sensebaugh[17] note, in a live body in the oral cavity, when saliva, swallowing and normal enzymatic processes are present, the chances for finding sperm decrease far more rapidly. Lee et al[11] also reminds us that increased heat is detrimental to most proteins and enzymes. Estimates for the survival of sperm in a living victim will vary with climate, individual metabolism, bathing habits, and general orifice conditions. The underlying message here is to collect the evidence as soon as possible. It is extremely unlikely that sperm will be found in the oral cavity sixteen days after the fact, whether the host is living or dead. Investigators must be aware of this need for expediency if they hope to have the examiner find sperm when forensic personnel do a rape kit or some other examination of wet evidence for sperm.


Semen is the most common form of body fluid evidence encountered or sought in sexual assault cases[7]. It is the fluid mixture of bodily secretions which contains the sperm Frick et al[5]. There are certain secretions in semen which are considered to be reliable markers for confirming the presence of semen, due either to uniqueness or quantity. These markers are categorized by the results of the tests performed to assay them as either presumptive or conclusive. Using these markers, forensic examiners are able to make reasonable statements about the presence of semen in a sample of evidence.

Acid Phosphatase- The acid phosphatase(AP) test is a well documented presumptive assay for the presence of semen ([2], Brauner et al [3], Baechtel, Brown, Harwood, Mudd, Pilipchuk, Smith, Stone, & Williams [4], McCloskey, et al [12], Schiff [16]).

One quick way to test AP is by a yes/no determination with a color change for positive results (i.e.- Brentamine fast Blue B [9]); swab turns color, AP is present--swab doesn't turn color, AP is not present. The problem here is that there is no regard for amount. What you get is something that has AP in small amounts, but not enough to say it might have come from semen. But with a qualitative test who can tell with certainty? You can get a positive with very little AP present using a yes/no test, so some examiners agree that this alone is not sufficient. However, the quantitative AP test is still an excellent presumptive field test, and therefore extremely useful to the investigator.

There is generally a large amount of the enzyme AP in human semen, and the amount of AP in a sample of evidence can be determined [6]. If the amount assayed in a sample of evidence is large enough (there is no consistent agreement as to how much is enough), then it can be said that semen could be present, that is, a finding consistent with the presence of semen. If you get this result on an oral swab, or from a clothing stain up near the head or shoulders, then you've got a result consistent with oral sex.

Schiff[16] is an example of the most liberal proponent of the AP test. He reports that he will, on the stand, state that the qualitative confirmation of AP "has great merit" for the identification of seminal fluid in the absence of sperm. He also compares this with the use of the polygraph as it relates to skill, saying that his skill and consistency as an examiner add something to the test that is not otherwise present. Let it also be noted that results from a polygraph examination are still not admissible as evidence in a court of law due to widespread skepticism as to the interpretation of those results. Sensebaugh[17], however, takes issue with AP, citing data which show that AP is not unique to semen or prostatic tissue. In fact, Sensebaugh states that Prostatic AP, the AP found in vaginal secretions, and lysosomal AP are genetically identical. Clearly, if Sensebaugh's data is correct, then any qualitative AP test is questionable at best. The qualitative test, like perhaps a polygraph, is a good investigative tool--i.e. it's quick and it points you in a direction.

Not in dispute is the need to get to suspected semen evidence quickly when it's to be collected from the body of a victim (Blake et al [1], [14]). For example, Schiff[16] reports a case of forced fellatio where both AP tests and Spermatozoa tests were negative only two and a half hours after intercourse. This speaks again to the efficiency of the mouth to clean itself and raises again the time issue. Samples should be collected expeditiously if oral sex is to be determined.

Spermine & Choline- The use of these as markers for semen is not typical [17]. However, Gaensslen[6] notes that both are found in semen in high amounts. Choline forms characteristic crystals in iodine solutions (Takatori et al[18]), and spermine forms characteristic crystals in the presence of picric acid [7]. Although, Gaensslen[6] notes that even if you find both in a sample of evidence taken from an oral swab, this is still only presumptive. All that you can say is that semen could be there, and therefore findings are consistent with, but not conclusive for, oral sex.

P30- The test for P30, a protein specific to semen, is an immunological assay using anti-P30 antiserum[17]. P30 has not been reported to be found in any other body fluids or organs[6, 7, & 17]. P30 is easily detectable at even the lowest average levels in the semen of the average male, making it an excellent marker when examining very small amounts of trace evidence. The strongest antiserum reportedly precipitates at seminal plasma titres of 1:128 in Ouchterlony double diffusion gels (antigen detection limit of about 15 g P30/ml)[17]. Aside from being semen specific, of note is that a positive result for P30 is also a strong confirmatory result for semen without the presence of sperm. This becomes critical given the large number of males in the general population who have undergone vasectomy operations, and who subsequently have ejaculate that contain no sperm whatsoever. If you get a confirmatory result for P30 antigen then you have semen. If you get this same result from an oral swab, this is proof positive of oral sex, all without the presence of sperm needing to be confirmed (but sperm confirmed probably sounds better to a jury).

There is only one established confirmatory test for semen, that being the immunoassay for P30. Everything else is presumptive. The Acid Phosphatase tests, the spermine microcrystal test, and the choline microcrystal test are all presumptive--consistent with the presence of semen, but not proof of semen. So unless you're a student of Schiff, you won't make the call for semen in your report or on the stand until you've got P30 backwards and forwards in your Ouchterlony dish (Figure 3).

DNA- It is important to note the continuing development of DNA tests for individualization of semen[18], as well as research on genetic markers for semen in forensic evidence[1]. This research becomes significant when dealing with very small amounts of suspected semen, or when dealing with mixtures. In fact, Takatori et al claim that even mixtures such as vaginal secretions and semen do not get in the way of DNase typing, DNase being their seminal marker. And Blake et al explain about the quantitative tests for PGM, peptidase A, and PGI, all of which have potential for being a genetic marker for semen in one form or another. The research overall is very promising. However, as Gaensslen[6] points out, these markers are not only found in seminal plasma, but in the victim's secretions as well. Therefore these markers must be matched against the victim's to determine their true origin. This is not an easy or trivial undertaking. More substantiating research is definitely requisite.


The most common tests for the identification of saliva are dependent upon the detection of amylase ([6], [7], [9], Pereira et al[13], &[17]). Since no amylase assay can distinguish between salivary amylase and pancreatic amylase, the tests for saliva identification are all presumptive. Therefore, if you go into court and say to a jury that you've confirmed the presence of saliva on a vaginal swab or penile swab because it tested positive for amylase, you are misrepresenting your results. Saliva cannot be confirmed, but amylase can. Confirmation of amylase means that results are consistent with saliva and perhaps then consistent with oral sex.

Determination of amylase activity is typically done with color change reagents, like a Brentamine Blue or Amylopectin Procion Red. Preparation of these yes/no qualitative tests is fairly standard [7], [9]. You prepare a starch impregnated paper, dampen the stain or swab, press it against the paper for a moment and then spray the reagent on the paper. If you get a color change in the spot where you pressed, that's considered positive for amylase activity.

Also reportedly used to determine the presence of saliva is an anti-saliva serum. This immunological assay, however, is dependent again upon the detection of amylase. Therefore this is still only a presumptive test for saliva.

Positive amylase tests confirm amylase activity. Amylase activity is consistent with saliva. Presumed saliva in the right place, according to the laws of your particular state, is consistent with oral sex (i.e. vagina, breast, penis, a stain from the front of underpants).

Why is saliva evidence important if you ultimately can't prove you've got any? Firstly, it's good circumstantial evidence, and it adds to the general body of any official statement you make about whether oral sex or some other form of sexual assault occurred. Secondly, it's a good investigative tool; it can give investigators an avenue to pursue as it can add behavioral information to an individual's profile (which was Keating's overall purpose). Thirdly, secretor status of a suspect can be determined from presumed saliva evidence. This is important for excluding suspects and further focusing investigative search efforts for more likely suspects. Secretor status, however, does not contribute to an examiners official statement about the determination of oral sex.


Determination of oral sex in Forensic Science is dependent on a number of factors, any of which tended improperly could lead to false positive or false negative results. In most cases, getting to the evidence quickly is critical, as with sperm which is "washed away" in the vagina and in the mouth. Once you've arrived at the evidence, proper collection is the next consideration. Careless technique can result in the destruction of evidence, such as making a fold on a stain when bagging clothing evidence from a sexual assault or rape; folding dried sperm stains can shatter the sperm and make them unrecognizable to the examiner. Once the evidence is collected it is tested with a reliable, established forensic exam. After the exam, or exams, yield results, a statement as to the consideration of oral sex must be made. Results from these exams must be interpreted reasonably, or law enforcement and legal community alike may be mislead by the statement. Reasonable interpretation of results does not mean doing anyone's job for them or saying as little as possible to protect yourself; it means saying whether the test confirmed, was consistent with, was inconclusive or was negative for oral sex. For example, if you confirm AP on an oral swab, you can't say you confirmed semen and by extension oral sex. It is reasonable to say that results were consistent with oral sex. Even if a quantitative assay of AP is extraordinarily high or terribly low, it is still reasonable to say that results were consistent with oral sex.

When oral sex is the issue, Forensic Science will speak directly to the evidence it examines. Fortunately, there are strong confirmatory tests for transfer evidence involved in oral sex. There are also useful presumptive assays for oral sex transfer evidence. From a forensic point of view, the ultimate potential of any evidence that has been collected to help establish oral sex is dependent on what kinds of tests can be done on that evidence, i.e. confirmatory or presumptive.

It can rarely be the job of only one individual in the chain of evidence to make a determination as to whether or not oral sex has taken place. A cooperative effort between all individuals in the chain can make accurate determinations possible, and not always then. Cooperation between the investigator, the forensic examiner, the law enforcement community and the medical community are requisite for the best possible results when a sexual assault and/or rape protocol is to be implemented.


1)Blake, E.T. & Sensabaugh, G.F., "Genetic Markers in Human Semen: A Review," Journal of Forensic Sciences, 1976 vol.21, no.4 pp.784-796

2)Brauner, P., "The Evidence Notwithstanding--A case Report on a Rape," Journal of Forensic Sciences, JFSCA, Vol. 37, No.1, Jan. 1992, pp.345-348.

3)Brauner, P.& Gallili, N., "A Condom--the Critical Link in a Rape," Journal of Forensic Sciences, JFSCA, Vol..38, No.5, September 1993, pp.1233-1236.

4)"Forensic Science Symposium On The Analysis of Sexual Assault Evidence", Proceedings, Forensic Science Research and Training Center, Laboratory Division, FBI Academy, Quantico, Virginia, 1983, July 6-8

5)Frick, J., & Luwig, G., Spermatology , 1989

6)Gaensslen, R.E., "Blood, Sweat and Tears...and saliva and semen," Law Enforcement Communications, February 1980, pp.23-30

7)Gaensslen, R.E. & Lee, Henry & Pagliaro, E. & Mills, Robert & Zercie, Kenneth, Physical Evidence In Criminal Investigation, Narcotic Enforcement Officers Association, Westbrook, CT, 1991, pp.140-143

8)Geberth, Vernon J. Practical Homicide Investigation, CRC Press, 1993

9)Keating, S.M. "Oral Sex--a review of it's prevalence and proof," Journal of the Forensic Science Society, 1988; 28; 341-355.

10)Lee, Henry, Crime Scene Investigation, Central Police University, Taoyuan, Taiwan, Republic of China, 1994, pp126-128

11)Lee, Henry & Gaensslen, R.E., Eds., DNA and Other Polymorphisms in Forensic Science, Year Book Medical Publishers, Inc., 1990

12)McCloskey, K.L. & Muscillo, G.C. & Noordewier, B., "Prostatic Acid Phosphatase Activity in the Postcoital Vagina," Journal of Forensic Sciences, 1975, vol.20, p.630-636

13)Pereira, Margaret & Martin, P.D., "Problems Involved in the Grouping of Saliva, Semen and Other Body Fluids," Journal of the Forensic Science Society, 1976, vol.16, p.151

14)Sawazaki, K., & Yasuda, T., & Nadano, D., & Iida, R. & Takeshita, H., & Uchide, K. & Kishi, K., "Deoxyribonuclease I (DNase I) Typing from Semen Stains: Low Enzyme Activity in Vaginal Fluids Does Not Interfere with Seminal DNase I Typing from Mixture Stains," Journal of Forensic Sciences, JFSCA, Vol.38, No.5, Sept. 1993, pp1051-1062

15)Schiff, A.F. "Rape in the United States," Journal of Forensic Sciences, 1978, pp845-851

16)Schiff, A.F. "Reliability of the Acid Phosphatase Test for the Identification of Seminal Fluid," Journal of Forensic Sciences, 1978, vol.23, no. 4, pp.833-844

17)Sensabaugh, G.F. "Isolation and Characterization of a Semen-Specific Protein from Human Seminal Plasma: A potential New Marker for Semen Identification," Journal of Forensic Sciences, vol.23, pp.105-115

18)Takatori, Takehiko & Tanaka, Toshinobu & Tomii, Seibei, "The Determination of Choline in Human Semen by the Enzymatic Method," Forensic Science International, 1981, vol.17 pp.79-84

19)Wilson, E.F., "Sperm's Morphological Survival After 16 Days in the Vagina of a Dead Body," Journal of Forensic Sciences, 1974, vol.19, no.3, pp561-564

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