Urine nitrites: what a positive dipstick says about bacteria

Vegetables and other foods supply nitrate, which passes into urine. Some bacteria carry an enzyme, nitrate reductase, that converts nitrate into nitrite. The dipstick pad reacts with nitrite and changes colour. GPnotebook states that nitrite is not found in normal urine and that gram-negative bacteria reduce dietary nitrate to nitrites, so a positive test points to bacterial presence.

The test therefore detects a chemical product of bacteria rather than bacteria themselves, or the white cells that accompany infection. It is a quick, near-patient test, with results in about a minute.

The NHS describes urinary tract infections as affecting parts of the urinary tract including the bladder, urethra and kidneys, with symptoms such as burning when passing urine, frequency and urgency, cloudy or bloody urine, lower tummy or back pain and fever. A GP may do a urine test, though this is not always needed, and treat with antibiotics.

Lab Tests Online UK lists nitrite among the dipstick chemical tests that give clues about infection, along with leukocyte esterase and blood. In practice, nitrite is rarely used alone: it is interpreted with leukocyte esterase, which signals white cells, and with the symptoms.

GPnotebook says a positive urinary nitrite test should always be followed by collecting a mid-stream urine sample for microscopy, culture and sensitivity, and gives a figure of 87 per cent sensitivity and 67 per cent specificity for UTI when combined with a leucocyte esterase test. Those values mean the combination is useful but not perfect: some infections are missed and some positives are not infections.

Nitrite testing detects bacteria with enzymes that convert urinary nitrates to nitrites, which is associated with members of the Enterobacterales family, whereas other pathogens such as Staphylococcus, Pseudomonas and Enterococcus do not produce nitrate reductase. If the bladder incubation time is insufficient, bacteria will not have time to produce nitrite, so a negative result is common in people who pass urine frequently, such as children and those with severe urgency.

Use a fresh mid-stream sample. Lab Tests Online UK describes cleaning the genital area first, letting some urine pass into the toilet and then collecting the middle of the stream. The first morning sample is most valuable because it is most concentrated and urine has been in the bladder longest, which gives bacteria the time to make nitrite. Tell the clinician about any antibiotics you have already taken, as they can affect the result.

With typical symptoms, a GP may prescribe antibiotics straight away and send a culture to confirm the organism and the best antibiotic. The NHS says it is important to take the whole course even if you feel better, to drink enough fluids so you pass pale urine regularly, to use paracetamol for pain and to avoid bladder irritants such as fruit juice, coffee and alcohol.

The NHS says to ask for an urgent GP appointment or NHS 111 if you are aged 65 or older, a man, pregnant, have diabetes, a catheter or a weakened immune system, have blood in your urine, have back or lower tummy pain, or your symptoms get worse quickly or do not improve within 48 hours. Call 999 for confusion, drowsiness or difficulty speaking. If infections recur, two in six months or three in twelve, you need GP review.