Which mechanism is used to detect blood on a urine dipstick, and what can falsely elevate the result?

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Multiple Choice

Which mechanism is used to detect blood on a urine dipstick, and what can falsely elevate the result?

Urine dipstick detects blood through the peroxidase activity of hemoglobin. Hemoglobin in red blood cells acts like a peroxidase enzyme and catalyzes the oxidation of a color-changing substrate on the strip in the presence of hydrogen peroxide. This chemical reaction produces a visible color change, so any blood present in the urine drives the test to positive. False positives can occur when substances with oxidizing power or peroxidase-like activity activate the same color-changing reaction, even without actual blood. For example, contamination from menstrual blood can give a positive reading because it introduces blood into the sample, while household bleach, a strong oxidizer, can directly oxidize the strip’s reagent and mimic a positive result. Other proposed mechanisms—such as an immunoassay for red blood cell antigens, turbidity from red blood cells, or leukocyte esterase interference—do not describe how the standard dipstick detects blood and therefore aren’t the correct basis for this test. (Ascorbic acid, by contrast, can cause a false negative by reducing the color change, not a false positive.)

Urine dipstick detects blood through the peroxidase activity of hemoglobin. Hemoglobin in red blood cells acts like a peroxidase enzyme and catalyzes the oxidation of a color-changing substrate on the strip in the presence of hydrogen peroxide. This chemical reaction produces a visible color change, so any blood present in the urine drives the test to positive.

False positives can occur when substances with oxidizing power or peroxidase-like activity activate the same color-changing reaction, even without actual blood. For example, contamination from menstrual blood can give a positive reading because it introduces blood into the sample, while household bleach, a strong oxidizer, can directly oxidize the strip’s reagent and mimic a positive result.

Other proposed mechanisms—such as an immunoassay for red blood cell antigens, turbidity from red blood cells, or leukocyte esterase interference—do not describe how the standard dipstick detects blood and therefore aren’t the correct basis for this test. (Ascorbic acid, by contrast, can cause a false negative by reducing the color change, not a false positive.)

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