Why will a false-positive catalase test result if the organisms are tested on medium containing blood?

Catalase is an enzyme produced by many bacteria that decomposes hydrogen peroxide (H2O2) into water and oxygen. A catalase test is used to differentiate between bacteria that produce catalase (catalase-positive) and those that do not (catalase-negative).

Catalase activity can be detected by adding a drop of hydrogen peroxide to a bacterial culture. If the organism is catalase-positive, the hydrogen peroxide will be decomposed, and oxygen gas will be produced, causing bubbles to form. If the organism is catalase-negative, no bubbles will be produced.

Blood contains hydrogen peroxide, so if a catalase test is performed on a culture containing blood, a false-positive result may be obtained. This is because the hydrogen peroxide in the blood will be decomposed by the catalase in the blood, even if the bacteria themselves do not produce catalase.

To avoid false-positive results, catalase tests should be performed on cultures that do not contain blood.