Blood evidence analysis has rapidly evolved and progressed over the last 20 years and nowadays, there exist a number of various techniques to collect, store and determine blood. Forensic scientists may opt for different ways to analyze the most valuable and helpful evidence of a crime in order to increase the chances to solve a case. The quality and reliability of a certain presumptive test usually depends on the reagents used in analysis and therefore, the right choice of a certain element is critical for determining associations and connections between the victim, perpetrator and different factors of a crime scene. As for the phenolphthalein test, its technology is based on the catalytic activity including the heme-group component of hemoglobin (Kovacs & Petersen, 2014, p.481). It allows to screen potential bloodstains immediately at the scene of a crime.
Presumptive blood analysis enables one to indicate body fluids and hemoglobin possibly present in the substance. One of the tests utilizing phenolphthalein is Kastle-Meyer one and when positive for blood, the swab used to collect the blood sample turns bright pink color. It is very important to correctly count the right proportions of active reagents, because some of them can damage essential material or even present false results of the forensic analysis. Alenazy, Babu & Refaat (2015) provides a detailed comparison of two tests used in forensic analysis, one of which utilizing phenolphthalein and another - leucomalachite green. The first one provided a full DNA profile from a bloodstain after interval from 1 hour to 1 week, whereas blood samples treated with leucomalachite green were significantly degraded and provided only poor partial DNA profiles. Thus, phenolphthalein has been recommended as one of the safest presumptive test reagents to detect blood evidence from crimes and that might further undergo DNA analysis (Alenazy, Babu & Refaat, 2015, p. 103). The level of degradation of DNA with phenolphthalein is comparatively less and is not a serious obstacle for successful blood analysis. Besides, this test can give conclusive results whether a substance is actually blood and nothing else, because its mechanism is based on the analysis of hemoglobin and it might be only required to conduct additional tests (Ouchterlony Test) to conclude whether the blood is human or belongs to other species.
References
Petersen, D.; Kovacs, F. (2014). Phenolphthalein False-Positive Reactions from Legume Root
Refaat, A.; Babu, S.; Alenazy, N. (2015). Comparison of the effects of two presumptive test
reagents on the ability to obtain STR profiles from minute bloodstains. Egyptian Journal of Forensic Sciences, 5, 103-108.
