A number of bacteria can reduce nitrate (NO3) to nitrite (NO2). Detection of nitrites in urinalysis as an indication of a urinary tract infection is based on this fact. Most Enterobacteriaceae are able to reduce nitrate.
A sample of pure isolate is cultured on a media that contains potassium nitrate (KNO3). The inoculated media is incubated at 35°C for 18-24 hours.
After the incubation period the presence of nitrite is detected by addition of alpha-naphthylene and sulfanilic acid; together these form a red diazonium dye (p-sulfobenzeneazo-alpha-naphthylamine).
A small amount of zinc dust is added to apparently negative reactions. Zinc converts any residual nitrates to nitrites, which will generate a red color. If the color does not change to red after the addition of zinc then the nitrate was broken down to ammonia or other products and not nitrites (false negative). The presence of a red color after addition of zinc indicates a true negative.
Causes of a false positive:
(1) mixed isolate
Causes of a false negative:
(1) defect in media production
To read more or access our algorithms and calculators, please log in or register.
