The nitrite test on the urine reagent strip detects the presence of nitrate-reducing bacteria (ie, gram-negative bacteria). Bladder infections are usually caused by gram-negative bacteria. These bacteria reduce nitrate to nitrite when urine remains in the bladder at least three hours. Gram-positive bacteria do not produce a positive nitrite reaction because they lack the enzyme nitrate reductase that converts urinary nitrate to nitrite.
Nitrite is not present in urine under normal circumstances.