Limitations of Blood Preliminary tests
Preliminary tests for blood may produce false positive or false negative results. False positives occur when a test indicates the presence of blood when it is not actually present, while false negatives occur when a test indicates the absence of blood when it is actually present.
FALSE-POSITIVE REACTIONS
The preliminary or presumptive tests are not specific to blood, which means substances other than blood can also give a positive reaction to these tests, leading to false positive results. Some of the substances that may cause a false-positive reaction are discussed below:
Chemical oxidants and Catalysts
The presence of certain oxidizing agents and chemicals capable of catalyzing an oxidation reaction can result in a false-positive reaction. Potassium dichromate (K2Cr2O7), Potassium permanganate (KMnO4), salts of certain metals like copper and nickel, household cleaning agents like bleaches that contain hypochlorite ions, hair coloring products that contain hydrogen peroxide, other compounds of copper, cobalt and iron work as oxidants.
Plant peroxidases
Many plants such as apple, potato, tomato, cucumber, horseradish, etc. contain peroxidases capable of carrying out the oxidation of reagents used in preliminary tests, thus, giving false positive results.
Sources of Animal origin
Similar to plant peroxidases, tissues of animal origin such as brain tissue, bone marrow, and lung tissue along with saliva, mucus, and spinal fluid contain various peroxidase enzymes capable of carrying out an oxidation reaction.
✏️ Common False Positive Reactions of Blood Preliminary Tests
False positive for Benzidine: Benzidine gives false positive results for horseradish, garlic, grapes, ginger, garlic, coriander, onion, tomato, cabbage, beet root, spinach, guava, banana, cucumber, sugarcane, rust, ferrous sulphate along with things like recycled paper and shoe polish.
False positive for Phenolphthalein: It gives false positive results with horseradish, potato, turmeric, cobalt, manganese, iron, lead, saliva and pus.
False positive for Luminol: Luminol gives false positive results with brass, bronze, copper salts, and other copper containing alloys.
FALSE-NEGATIVE REACTIONS
Although it’s not common, a sample containing strong reductants such as certain metal ions including lithium and zinc may inhibit the oxidation reaction of blood, leading to false-negative results.