oct ewj 24 online - Flipbook - Page 80
It’s Written in the Blood The Intricacies of Blood Pattern
Analysis in Forensic Investigations
detecting blood even when diluted to one in 10,000
parts. Despite its sensitivity, the KM test can occasionally yield false positives with substances like rust or
horseradish, necessitating cautious interpretation. If
a stain has the visual appearance of blood and reacts
positively to the KM test it can be assumed to be blood.
Blood Pattern Analysis (BPA) is a cornerstone of
forensic science, providing vital insights into the dynamics of violent crimes. Clare Jarman, a distinguished forensic biologist with over two decades of
experience, delves into the methodologies, applications, and challenges of BPA in criminal investigations
in our recent webinar. This comprehensive overview
synthesises key points from that webinar, detailing
both the strengths and limitations of BPA in forensic
investigations.
Luminol and Leuco Crystal Violet: For non-visible
bloodstains, luminol is a valuable reagent. It reacts
with the haemoglobin in blood to produce, instead of
a colour change, a chemiluminescent glow, revealing
hidden blood patterns in darkened environments.
This allows the visualisation of blood patterns on any
surface regardless of background colour. Luminol is
exceptionally sensitive, detecting blood diluted up to
one in a million and as such can be highly useful in
showing evidence of clean up at a scene, or when
searching for very old or dilute blood. However,
its lack of a fixative can readily cause blood to run,
distorting patterns, particularly on non-porous surfaces. Luminol can also react with other substances
such as vegetation, bleach and metals and as such it can
only be used to indicate that blood may be present,
rather than confirm its presence. In contrast, Leuco
Crystal Violet (LCV), which changes colour to purple
in the presence of haemoglobin, includes a fixative,
preserving the integrity of blood patterns, LCV is
highly sensitive and, similarly to KM displays far fewer
false positive reactions. However, LCV brings with it
some health and safety concerns which makes it more
suited to application in a laboratory setting
Introduction to Blood Pattern Analysis
Blood Pattern Analysis involves the meticulous
examination of the shapes, sizes, and distribution of
bloodstains in an attempt to reconstruct the sequence
of events that occurred as and after blood was shed.
BPA can offer crucial information about the positions
and movements of victims and perpetrators, the type
of weapon used, and a minimum number of blows or
shots delivered. By understanding the behaviour of
blood under different conditions, forensic scientists
can piece together the events of a crime with remarkable accuracy.
Detection and Enhancement Techniques
The initial step in BPA is locating blood at a crime
scene. This often begins with a thorough visual
examination, utilising various light sources to detect
the reddish-brown colour typical of bloodstains. In the
laboratory, light sources and low-power microscopes
can be employed to identify even minute blood stains
on items such as clothing and weapons.
Types of Blood Patterns
Bloodstains are generally categorised into two
primary types: contact stains which occur when a surface wet with blood comes into contact with another
surface and airborne stains resulting from blood
droplets travelling through the air and landing on a
surface.
Chemical Testing for Blood: Once potential bloodstains are identified, chemical tests can be used to confirm their presence. The Kastle-Meyer (KM) test is
commonly used, involving the application of a colourless chemical that turns pink in the presence of
haemoglobin. This test is highly sensitive, capable of
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