A Urinalysis (commonly abbreviated UA) is an examination of a person’s urine. Most clinicians don’t realize all the subtleties to this simple exam. There are two ways of performing a UA:
Urine Dipstick — This is a simple test in which a dipstick that can test for a variety of chemicals is inserted into the urine specimen. Each of the results is read at a fixed time between 30 secs. and 2 min. The following findings can be identified:
- ph. Tells us how acid the urine is. Low = acid. High pH occurs with vegetarian diets, but also with the bacteria Proteus that can sometimes cause kidney stones.
- Specific Gravity. A low specific gravity (<1.005) is a dilute urine, so some of the tests may not be accurate (i.e. the urine is watered-down). High specific gravity can indicate Dehydration.
- Glucose (sugar). If positive, we suspect diabetes, which must be confirmed by obtaining a blood glucose level.
- Ketones. Occur in starvation. If maximally high in a patient with Type-1 Diabetes, we worry about the serious complication ketoacidosis (see topic Diabetes).
- Protein. Can occur for a variety of reasons, some of which have no importance. But can also be a sign of various kidney diseases, so often we may do more tests.
- Blood. Can occur with infection, with a kidney stone, or as an early sign of cancer. But can also occur irrelevantly from a simple sore, or from a menstrual period.
- Nitrites. If positive, means there are a lot of bacteria in the urine, so we might diagnose infection (UTI). But if normal, we cannot rule out an infection (the test is not sensitive, so there are frequent false negatives). See topic Painful Urination in Women.
- Leukocyte Esterase. An enzyme in white blood cells (WBCs). If positive, it means there are WBCs in the urine. If a person has symptoms of a urinary infection, the presence of WBCs helps confirm the diagnosis, which we make even without a microscope. A negative leukocyte esterase rules out a UTI (see below for false positives and negatives).
The Dipstick is an easy exam that a medical assistant can do right in the office, with immediate results. If the UA gets sent out to a main laboratory, it might be ordered “reflex to microscopic.” That means if certain things are abnormal (blood, leukocyte esterase, nitrites), a lab tech examines it under a microscope. Otherwise, just the dipstick gets reported.
Complete UA — this includes a Urine Dipstick (see above), plus a Microscopic exam, i.e. looking at the urine under a microscope. The specimen is first prepared by spinning it a few minutes in a centrifuge, to separate the water from solid matter called sediment, which contains the cells we’re interested in. It’s the sediment we examine by microscope.
In sediment, we can see white blood cells (WBC), red blood cells (RBC), and bacteria, but their presence we knew from the dipstick. We can also see crystals; anybody may have a few, lots suggest a kidney stone. If we’re worry about certain kidney rare diseases, we might see RBC casts, though nephrologists (kidney specialists) are better at finding these than lab techs are.
Importantly for women, we can easily see vaginal epithelial cells by microscopic. If there are lots, then maybe everything else we see got washed in from the vagina. So the presence of WBCs indicated by the dipstick were really from the vagina, thus false-positive, meaning there’s no UTI after all. See suggestion for patients Collecting a Clean Catch Urine Specimen.
No WBCs (or leukocyte esterase) in the urine means no UTI. However, there are reasons we might get a false-negative UA:
- Very dilute urine, specific gravity <1.005, suggests possible false-negative dipstick
- Lab tech doesn’t extract sediment well, includes some water, watering down the microscopic specimen, producing false-negative (we can never know this)
- Patient with infection had recently urinated, so not enough WBCs accumulated yet
- Staff who did the UA dipstick didn’t wait the full 2 minutes (won’t happen in dipsticks done by formal laboratories, where they use timers)
- Expired dipstick, or dipstick opened way too often letting moisture in, can cause false-negatives (happens in office or clinic settings; formal labs have quality control)
- Nitrite test on dipstick only shows positive if there are >100,000 colonies of bacteria in urine, but most UTIs have fewer
See also our topic Urine Culture and Sensitivities.