Urinalysis - The Carter Center

for urine culture unless the specimen has been improperly collected or
stored after collection which allow bacterial growth.
A negative result should never be interpreted as indicating absence of
bacteriuria because:
a. If an overnight sample were not used, there may have been
insufficient time for the conversion of nitrate to nitrite to occur.
Urine that has been left in the collection vessel for several hours
may be falsely positive.
b. Some amounts are caused by organisms that do not convert
nitrate to nitrite (such as enterococci, acinetobacter spp and some
pseudomonas species.).
c. The patient was in a vegetable free diet, which is the important
source for nitrate.
d. Administration chemotherapeutic agents should be discontinued
three days before the test, because antibiotic therapy may alter
bacterial metabolism so as to render nitrite detection invalid.
e. High doses of ascorbic acid.
f. Presence of urobilinogen
g. Low pH (< 6)
Even though the test depends upon the conversion of nitrate to nitrite by
certain bacterial action in the urine, not all bacteria in the bladder
convert nitrate to nitrite. Known nitrate reducing bacteria at significant
levels produce false negative results by reducing nitrate to ammonia
nitrite, and nitrous oxide, hydroxylamine, and nitrogen and will therefore
give a negative nitrite test.
Principle
The Reagent Strip ( Multistix with an acid pH) contains para-arsanilic
acid which reacts with nitrite, to give a diazonium salt, which by looping
with a benzoquinoline forms pink azo dye.
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