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 EXPLANATION:
    Whenever red blood cells are observed microscopically yet negative on the chemical test, ascorbic acid (vitamin C) interference should be suspected. Ascorbic acid is a strong reducing agent that will react with the peroxide impregnated on the reagent pad. This uses up the peroxide (H₂O₂) needed to oxidize the chromogen that is vital to the test. Normally, the pseudoperoxidase activity * of the heme component catalyzes the reaction between the peroxide and the chromagen, which produces a color change. The following reaction takes place:
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