Detection of Blood Group from Saliva
A Forensic Perspective on Using Salivary Biomarkers for Identification
In the realm of forensic science, the ability to determine blood group from biological evidence plays a critical role in personal identification and crime scene analysis. While blood is the most commonly tested fluid for blood grouping, saliva also holds diagnostic and evidentiary value. This article explores the detection of blood group antigens from saliva, focusing on its scientific basis, methodology, forensic significance, and limitations.
Scientific Basis: Secretors vs. Non-Secretors
The presence of blood group antigens in saliva depends on an individual's secretor status. Secretors possess the FUT2 gene, enabling the secretion of water-soluble ABO blood group antigens in bodily fluids like saliva, semen, and gastric juice. Approximately 80% of the human population are secretors, making saliva a viable alternative biological sample for blood grouping.
The determination of secretor status itself can be important in forensic investigations, especially when comparing biological fluids from different sources.
Methodology: Testing Blood Group from Saliva
The Absorption-Inhibition Method is the most widely used technique for determining blood group from saliva.
Steps Involved:
- Collection of Saliva Sample: The sample is collected in a sterile container, often through expectoration or with a swab.
- Boiling/Centrifugation: To remove mucins and bacteria, the sample is boiled or centrifuged.
- Addition of Specific Antisera: Known antisera (anti-A, anti-B, and anti-H) are added to the saliva sample.
- Incubation and Absorption: If corresponding antigens are present in the saliva, they bind to the antisera and neutralize them.
- Addition of Indicator Cells (Known RBCs): Red blood cells with known antigens are added. If agglutination occurs, the specific antigen was not present in the saliva (i.e., the antisera was not neutralized). If no agglutination occurs, the antigen was present.
Interpretation:
This indirect test hinges on inhibition of agglutination, hence the name "Absorption-Inhibition".
Forensic Applications
- Linking Suspects to Crime Scenes: Saliva left on cigarette butts, cups, or bite marks can be tested for blood group.
- Victim Identification: Saliva from personal items can assist in postmortem identification.
- Sexual Assault Cases: Mixed samples may include saliva containing blood group antigens useful for narrowing down suspects.
Advantages
- Non-invasive sample collection (compared to blood).
- Useful in cases with limited biological evidence.
- Can be conducted with small sample volumes.
- Cost-effective and simple methodology.
Limitations
- Only secretors will yield detectable ABO antigens in saliva.
- Sensitivity may be lower than blood-based tests.
- Degraded or contaminated samples may give inconclusive results.
- Cannot determine Rh factor from saliva.
Conclusion
Detection of blood group from saliva exemplifies how forensic science adapts to analyze alternate biological samples when traditional evidence is unavailable. Despite limitations, it remains a vital tool, especially in preliminary screening and in cases where blood is absent. As molecular techniques evolve, the specificity and reliability of saliva-based testing are expected to improve, further cementing its place in forensic investigations.
