Investigating the detection limits of scent-detection dogs to residual blood odour on clothing

Abstract

Blood-detection dogs are trained to locate blood evidence and search for potential crime scenes in cases where a cadaver may not be present. The locations of crime scenes are often ambiguous and evidence may not always be obvious during initial processing. In cases of foul play, a criminal may attempt to clean biological evidence from a crime scene; however, trace evidence that appears invisible to the naked eye may still be detectable. For example, it has been reported anecdotally that blood-detection dogs are capable of detecting blood on clothing that has been washed up to five times, or on surfaces which have been scrubbed clean. This study aimed to investigate the baseline detection limits of blood-detection dogs and cadaver-detection dogs to latent blood evidence on washed clothing and to compare the dogs’ responses to current presumptive chemical and analytical techniques. Blood was deposited onto cotton swatches and washed up to five times with a standard household washing machine. Following washing, the cotton swatches were allowed to dry and presented to blood-detection and cadaver-detection dogs during law enforcement training. Replicates of these samples were tested with luminol spray and analysed using headspace solid phase microextraction – comprehensive two-dimensional gas chromatography – time-of-flight mass spectrometry (HS-SPME-GC×GC-TOFMS). Results indicated that the olfactory system of blood-detection and cadaver-detection dogs is a viable complementary technique to presumptive chemical tests and more sensitive than current scientific instrumentation, with some of the dogs able to detect blood after five washes but HS-SPME-GC×GC-TOFMS only able to detect blood after two washes or less. This limit of detection could likely be lowered for the dogs with further and more consistent training. Luminol was similarly able to detect blood washed up to five times, which indicates that the scenting abilities of these dogs can provide investigators with valuable information that may be overlooked during preliminary searches in cases when chemical testing is unsuitable. This study highlights the importance of training blood-detection and cadaver-detection dogs for increased sensitivity to blood so that evidence collected at a scene can be further analysed for greater evidentiary value.