Forensic Evidence Collection and Interpretation
Forensic evidence is infrequently found in CSA cases due to reasons discussed earlier (delayed disclosure, etc.). However, in selected cases the medical expert in child abuse may be asked for a competent forensic work-up in an acute assault which is usually done at the same time as the physical examination.
This requires coordinating specific details of collection, labeling and packaging with the forensic laboratory. Handling of the specimens must be documented with scrutiny to maintain the chain of evidence. Pregnancy and sexually transmitted diseases have forensic significance but are usually not classified in this category.
The decision to perform a forensic exam mainly depends on the time of presentation after the assault. The recommendation of a 72-hour time frame is based on experience in adult victims that semen, seminal fluid and other products are rarely recoverable after this time. In spite of certain limitations, exceptions and little research in abused children this serves as a reasonable compromise.
A recent study by Christian et al.  however did not find any positive swabs for sperm or semen later than 9 h post-contact in 273 children. No forensic evidence was collected after 24 h except for the clothing or linens of the children. Christian et al. concluded that guidelines for forensic evaluation of adult victims may not be appropriate for children.
Sperm may be present on a wet mound for only 0.5 h, in the adult vagina for 12–20 h, rarely up to 48–72 h, in the cervix up to 5 days. Survival time of sperm in the prepubertal vagina is significantly shorter. Dead or immotile sperm is detectable longer. Dry specimens however are quite stable and sperm in stains has been detected up to 12 months [Finkel and De Jong, 2001].
The Wood’s lamp or ultraviolet light has traditionally been recommended to identify semen on a victim’s skin. Recent research has shown significant limitations and shortcomings: semen and urine were not distinguishable, urine fluoresces considerably longer, and a number of child care products will equally fluoresce undistinguishable from semen [Gabby et al., 1992; Santucci et al., 1999]. Keeping these limitations in mind the Wood’s lamp may serve as a screening aid where to take swabs from a victim’s body.
The marked elevation of acid phosphatase in the vagina indicates sexual contact within 24–48 h, but it may also turn to normal within 3 h in some cases. p30, a semen glycoprotein of prostatic origin, and MHS-5, an antibody against a vesicle protein, are male-specific and improve sensitivity markedly. Negative results of all tests do not exclude sexual abuse however.
Semen, blood, saliva, body hair and other materials found on the victim may help to prove the identity of the offender by several methods.
They include ABO blood group antigens, subtypes of the enzyme phosphoglucomutase, the enzyme peptidase A and most sensitive DNA profiling. Bite mark identification by a forensic odontologist may also determine the offending person by analysis of good-quality photographs taken with a ruler. Swabs from acute bite marks may be analyzed for genetic markers [Finkel and Giardino, 2002; Jenny et al., 1989].
Revision date: June 22, 2011
Last revised: by Dave R. Roger, M.D.