Let us call the people involved in this case Jane Doe and John Smith. Who they are is not important to understanding the scientific issues. In the 1980s Jane Doe reported a sexual assault in a suburban town adjacent to New York City. The clinical examination she underwent at the time included taking a vaginal swab, among other items, into evidence. Doe was a type B secretor, so she would produce blood group substances B and H. When the swab was analyzed, it showed the presence of blood group substances B and H, as well as semen.
The suspect, Smith, was later apprehended as a result of the police investigation, and Doe identified him as the man who assaulted her. Smith was tested and found to be a type A secretor, so his ABO blood typing was expected to produce blood group substances A and H. No blood group substance A was found in the evidence, however.
What did this finding mean? Did it mean that Smith had to be excluded as a source of the semen? That is almost right. Smith was excluded on the face of things, but there were still a few matters the analyst had to consider.
One such matter is variation in the ratios of blood group substances. People can produce different quantities of these blood group substances in their body fluids. For instance, the relative proportions of blood group components may be different in the same person’s blood and saliva. When a person is tested to find out his or her secretor status, saliva is used. In this case the analyst had to consider the possibility that the A and H quantities could be present in different ratios in the saliva and the semen of the same type A secretor man. It was possible that an A secretor man could have more A in saliva (enough that the lab detected it when the saliva was tested) and less in semen (so that it was not detected in the semen found in the evidence specimen). Under those circumstances the man would still be included as a possible source.
But there is no way of knowing about the A and H levels in saliva versus in semen in any given case. When a person is tested for secretor status, what the lab tests is saliva because this fluid is relatively easy to obtain and getting a saliva specimen from a person is relatively nonintrusive. Forensic scientists are almost never able to test someone’s actual semen, because it is too intrusive for any court to order someone to produce such a specimen. As a result it was generally not possible to sort out these possibilities through ABO blood type testing.
A second matter affecting interpretation of lab results is what the analyst does not know. For instance, if the complainant had another sexual partner besides the person who assaulted her, the analyst would not know this unless she told the police investigators about it. Even then, the analyst would have no way of knowing whether the information was correct and true.
As a result of these uncertainties it was common, when ABO blood typing results represented the best evidence available, for forensic biologists to state conclusions about ABO blood type results conditionally. In this case the scientist might say that the man is excluded prima facie (on the face of it) by the blood group results, but that some conditions would have to be met before the exclusion could be considered absolute.
