Phenolphthalein false-positive reactions from legume root nodules

Area of Science:

• Forensic Science
• Biochemistry
• Plant Biology

Background:

• Presumptive blood tests are crucial in forensic analysis for initial evidence screening.
• The Kastle-Meyer test, utilizing phenolphthalein, detects the catalytic activity of hemoglobin.
• Positive results from presumptive tests guide further DNA analysis for individual identification.

Purpose of the Study:

• To investigate the cause of phenolphthalein false-positive reactions observed in forensic samples.
• To identify potential sources of interference with the Kastle-Meyer presumptive blood test.
• To assess the implications of these false positives on forensic casework and DNA profiling.

Main Methods:

• Examined crushed nodules from various legume plants for phenolphthalein reactivity.
• Assessed the color and reaction time of false-positive results compared to true bloodstains.
• Tested purified leghemoglobin from legume nodules for its interaction with phenolphthalein reagent.

Main Results:

• Legume nodules produced phenolphthalein false-positive reactions indistinguishable from true blood.
• Stained materials exhibited persistent false-positive reactivity for years post-exposure to nodules.
• Purified leghemoglobin confirmed its role as the source of phenolphthalein reactivity.

Conclusions:

• Leghemoglobin, found in legume nodules, is structurally and functionally similar to hemoglobin, causing false positives.
• The presence of leghemoglobin can significantly mislead forensic investigators relying on phenolphthalein-based presumptive tests.
• This highlights the need for careful consideration of plant-based evidence and potential cross-reactivity in forensic testing protocols.