What types of things can cause a false positive on a Hemastix?

• A. Water
• B. Bleach
• C. Tomato
• D. Rust, tomato, potato, and bleach

Answer: (D)

Hemastix results hinge on a peroxidase-driven color change: the strip is designed to develop color when a peroxidase substrate is oxidized in the presence of hydrogen peroxide, which is how blood’s heme activity is detected. Substances that either provide peroxide-like oxidation or bring in their own peroxidase enzymes can trigger that same color change even without blood, leading to a false positive.

Bleach is a strong oxidizing agent; it can directly oxidize the strip’s colorimet, producing color development without any blood present. Tomato and potato contain natural peroxidase enzymes; in the test environment these enzymes can catalyze the oxidation reaction and mimic a positive result. Rust introduces metal ions and oxidative surfaces that can promote similar oxidation on the strip, again giving a color change not due to blood. These factors explain why rust, tomato, potato, and bleach are capable of causing false positives on a Hemastix.