A patient with dysuria provides a first-morning urine sample; there are many bacteria but nitrite is negative. Which explanation could account for this result?

A patient with dysuria provides a first-morning urine sample; there are many bacteria but nitrite is negative. Which explanation could account for this result?

• A. Insufficient incubation time
• B. Bacteria have nitrate reductase
• C. Patient does not eat green leafy vegetables
• D. White blood cells are not neutrophils

Answer: (C)

Nitrite in urine comes from bacteria that have nitrate reductase converting nitrates (from the diet) into nitrites while the urine is in the bladder. For this test to be positive, there must be enough nitrate substrate in the urine and enough time for the bacteria to reduce it. If a patient does not eat green leafy vegetables, their urinary nitrate load can be low, so even with bacteria present, there isn’t enough nitrate to form detectable nitrite. In that scenario, the nitrite test stays negative despite bacteriuria. Other possibilities are less fitting here: if the bacteria truly possessed nitrate reductase and had access to nitrates in the urine, nitrite would appear positive; insufficient incubation time is unlikely in a first-morning specimen because urine has been in the bladder overnight, providing ample time for conversion. The fact about white blood cells doesn’t influence nitrite production.

Nitrite in urine comes from bacteria that have nitrate reductase converting nitrates (from the diet) into nitrites while the urine is in the bladder. For this test to be positive, there must be enough nitrate substrate in the urine and enough time for the bacteria to reduce it.

If a patient does not eat green leafy vegetables, their urinary nitrate load can be low, so even with bacteria present, there isn’t enough nitrate to form detectable nitrite. In that scenario, the nitrite test stays negative despite bacteriuria.

Other possibilities are less fitting here: if the bacteria truly possessed nitrate reductase and had access to nitrates in the urine, nitrite would appear positive; insufficient incubation time is unlikely in a first-morning specimen because urine has been in the bladder overnight, providing ample time for conversion. The fact about white blood cells doesn’t influence nitrite production.