Introduction to Blood: Components, detection, & handling

Introduction to Blood: Components, detection, & handling January 9, 2019

Red blood cells (erythrocytes) • Contain hemoglobin • Transport O2 • White blood cells (leukocytes) • Produce antibodies, attack and digest pathogens What is blood made of? • Platelets • Repair broken blood vessels and form clots • Plasma • Water & dissolved chemicals (sugars, wastes, ions, etc)

Nearly any test has some chance of providing the wrong result. Medical tests are often discussed in terms of rates of false positives and false negatives. • at-home pregnancy test • very low rate of false positive • a positive result means you are almost certainly pregnant • high rate of false negative if taken early • if you get a negative result you should retest in a few days. • A mammogram • low rate of false negative • Abnormal breast tissue is easily identified • high rate of false positives • only some abnormalities are cancerous. • Do presumptive tests have high false negatives? Or high false positives? • … first, a definition • A presumptive test is a test that lets you know that a certain substance (i.e. blood) may be present. • A negative result = NO blood present • A positive result = blood may be present, or a different substance may be causing the positive result • If investigators get a positive result with a presumptive test, they will usually pretty confident that blood is present – but they will need to do more tests to prove it in court. Identifying Blood Stains

Catalytic Color Tests • The hemoglobin in blood acts like a catalystfor some • oxidation reactions. • Many versions of these tests but all involve a color change when blood is present • One common test uses Kastle-Myer reagent. For ease of field use, this reagent can be put on strips of paper (Hemastix) that can be moistened with distilled water then toughed to a stain. If the stain has blood, the strip will turn green. Catalyst – speeds up chemical reactions without being used up itself Oxidation– loss of electrons (e.g rusting of metal) Identifying blood stains

Catalytic Color Tests • Causes of misleading results • False negatives: a stain that has been cleaned with bleach will often not react • False positives: some vegetables, including potatoes and horseradish • Are catalytic color tests presumptive? • Why or why not? Identifying blood stains

Luminol & Bluestar • Uses chemiluminescence – the emission of light from a chemical reaction – to identify blood • Sprayed over an area with suspected stains; blood will light up; investigators will photograph • Very sensitive – can detect • blood that has been • washed away / diluted to • 1 part per 5,000,000 • Luminol is visible only in a • fairly dark room; Bluestar can • be seen under normal lighting Identifying blood stains

Luminol • Very sensitive, so few false negatives • False positives: bleach, metals, plaster Identifying blood stains Is luminol a presumptive test? Why or why not?

Microcrystalline tests • Formation of crystals from the heme groups of hemoglobin • Confirmatory test – very low incidence of false positives • Does cause some false negatives – can be a difficult procedure to master Identifying blood stains If a crystal test is confirmatory, why not just do that immediately?

Partner 1 • Name the four components of blood. • All blood identification tests detect which component of blood? • Partner 2 • Describe two tests for blood identification and explain whether or not each is presumptive or confirmatory. Quick Review

Collecting from a liquid stain/pool of blood • Dip a cotton swab into each stain • Allow swabs to air-dry • Collecting dried blood • When possible, collect the entire stained object (e.g. piece of clothing) • If not possible to collect entire object then investigators may cut out the stain (e.g. from car upholstery) OR may moisten the stain with distilled water and then collect on cotton swabs Collection & Handling of Blood Remember! All biological materials should be packaged in paper bags / envelops, NOT plastic PPE (personal safety equipment) is essential during collection! Other bodily fluids (e.g. semen) are collected and packaged in a similar fashion

Introduction to Blood: Components, detection, & handling