9 - semen identification in a forensic sample - part 3

ABO typing of semen in secretors is conventionally conducted using the technique of absorption-inhibition. The principle behind the technique is simple, and can be illustrated with reference to an A secretor. If anti-A is added to a sample (semen, saliva or a semen stain extract), the antibody will complex with the antigen in the sample. If a suspension of A cells is now added, there will be no agglutination since there is no free antibody in solution. A matrix can be set up to cover A, B, and H. There are some important factors to remember when conducting the test:

• The indicator cells are best prepared as a weak suspension in a saline-albumin solution.
• The antisera should be titrated against the indicator cells and used at the weakest dilution that will give a reliable result.
• The titration is conducted using serial dilutions.
• Negative results in inhibitions may be due to small sample size or a weak expression of the Se gene.
• Negative results should be reported as “No ABH activity detected.”
• Absolute definition of secretor status requires Lewis typing of RBCs and confirmation of the presence of Lewis b substance.

Some laboratories do not perform inhibitions but go straight to absorption-elution. The principle behind this approach is that it will detect ABH activity in all cases, not just secretors. However, certain issues must be noted:

• Invoking conclusions about secretor status from absorbtion-elution results is not reliable since the difference is quantitative.
• High levels of antigen can result in false negative in absorption-elution as the Ab-Ag complex dissolves in the excess Ag.
• It is always best to prepare at least a 1:10 dilution of extract in absorption-elution to try to overcome these problems.