Forensic Medicine and Toxicology
Forensic Laboratory Investigation
Forensic laboratory science for MBBS: Locard's principle, chain of custody, identification of blood, semen and saliva, DNA profiling and exhibit dispatch, mapped to NMC codes FM6.1 to FM6.3.
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Forensic Laboratory Investigation
Forensic laboratory science for MBBS: Locard's principle, chain of custody, identification of blood, semen and saliva, DNA profiling and exhibit dispatch, mapped to NMC codes FM6.1 to FM6.3.
This chapter covers the laboratory side of the investigation, from the scene to the courtroom report. It explains the principles of trace evidence and Locard's principle, the identification of blood, semen, saliva and hair, the role of DNA profiling, and the correct collection, preservation and dispatch of exhibits to the Forensic Science Laboratory.
High-yield: Forensic Laboratory Investigation
- Locard's exchange principle states that every contact leaves a trace, so a criminal both brings and takes away material at a scene.
- The chain of custody is the documented, unbroken record of who handled an exhibit from collection to court, and any break can render evidence inadmissible.
- Blood is first confirmed by presumptive tests such as the benzidine or Kastle-Meyer test, then confirmed and typed.
- The precipitin (ring) test and modern anti-human antibody tests establish whether a bloodstain is of human origin.
- The Barberio test identifies semen by spermine crystals, and the acid phosphatase and microscopy for spermatozoa confirm seminal fluid.
- Saliva can be identified by testing for amylase, and about 80% of people are secretors who express their ABO blood group antigens in body fluids.
- DNA profiling using short tandem repeats is the gold standard for individual identification and parentage testing.
- Samples for DNA should be kept cool and dry to prevent bacterial degradation, and contamination must be strictly avoided.
- Hair examination can indicate species, body region and whether the root was plucked or shed, but hair alone rarely gives a positive identity without DNA.
- Fibres, glass, paint and soil are trace evidence that can link a suspect to a scene through comparison.
- Exhibits must be individually packed, sealed, labelled and dispatched to the Forensic Science Laboratory with a forwarding note.
- A control sample, such as blood-free cloth from the same garment, is sent alongside stained exhibits for comparison.
- The laboratory report is interpreted with an understanding of its limits; a match gives a probability, not absolute certainty except with a full DNA profile.
- Species origin of a stain is settled before further typing, since animal blood can mimic human at the presumptive stage.
- Proper labelling records the case number, the nature of the exhibit, the date, the site of collection and the collector's signature.
Identifying common biological stains
- **Blood:** Presumptive benzidine or Kastle-Meyer test, then precipitin test for human origin and ABO or DNA typing.
- **Semen:** Acid phosphatase and Barberio test, with microscopy for spermatozoa as confirmation.
- **Saliva:** Detected by amylase activity; secretor status reveals ABO group in about 80% of people.
- **Individual identity:** DNA short tandem repeat profiling is the definitive method.
NMC competencies in this chapter
- **FM6.1:** Specimens, biological tissues, identification tests and Locard's principle
- **FM6.2:** Collection, preservation, labelling, dispatch and report interpretation
- **FM6.3:** Communication with the FSL and interpretation of biological or trace evidence
Frequently Asked Questions
What is Locard's exchange principle?
It holds that every contact leaves a trace: an offender carries material away from a scene and leaves material behind, which is the basis of trace evidence analysis.
How is a bloodstain confirmed to be human?
A presumptive test such as benzidine first detects blood, then the precipitin test or an anti-human antibody test confirms human origin before typing or DNA analysis.
Why does the chain of custody matter?
It is the unbroken documented record of everyone who handled an exhibit. A break casts doubt on integrity and can make the evidence inadmissible in court.
Why is a control sample sent with a stained exhibit?
An unstained portion of the same material lets the laboratory exclude contamination from the fabric itself and compare it against the stained area.
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