Lab Values Indicating a Urinary Tract Infection


Urinary tract infections are caused by bacteria that enter the urethra and cause inflammation. To diagnose and treat the infection, physicians will sometimes order an analysis of the patient’s urine. The analysis is done in two steps, chemical analysis and microscopic analysis. When the lab identifies the signs of an infection, a bacterial culture of the urine is done to identify the bacteria causing the infection. Physicians sometimes adjust antibiotic treatment based on the culture results.

Urinary Tract Infections

An infection of the urinary tract (UTI) occurs when bacteria from outside the body enter the urinary tract. The bacteria travel up the urinary tract to the bladder, a condition called cystitis. If the bacteria reach the kidneys, they cause nephritis. The patient with UTI has an increase in their frequency of urination, burning when they urinate, pelvic pressure, blood in the urine, and, in complicated cases, fever and abdominal pain.

Urine Samples

To diagnose a UTI, the physician will order a urinalysis (analysis of urine) test. The patient is given a urine cup and two non-alcohol wipes. The patient is then instructed to take the wipes and clean the tip of their urethra. The patient will then begin to urinate outside the cup, then urinate inside the cup mid-stream. Because of the cleaning and the collection procedure, this urine sample is called a “clean catch” urine sample.

Chemical Analysis

Urine samples must arrive at the laboratory within an hour from collection. Once at the lab, the lab tech will use a plastic strip coated with chemical pads. Each pad corresponds to a different chemical reaction. Chemicals in the urine will cause a color change of the pads according to the concentration of chemicals in the urine. In a person with UTI, the chemical analysis will detect nitrites, protein, white blood cells and blood in the urine. Nitrites are byproducts of bacterial metabolism. Protein is detected because the bacteria are made from it. The blood is present in the urine as a result of the inflammation caused by the bacteria.

Microscopic Analysis

After the chemical analysis, the sample of urine is spun down using a centrifuge. This forces all the cells and other debris to form a pellet at the bottom of the urine tube. The urine is then decanted and the pellet is placed on a glass slide to be viewed under the microscope. In a UTI urine sample, the lab tech will see bacteria, red blood cells, white blood cells and strands of protein through the microscope.

Bacterial Culture

Because different bacteria require different antibiotic treatments, the urine sample will be placed on a petri dish with nutrients to grow and identify the bacteria in the sample. Physicians will usually use broad-spectrum antibiotics to cover all possible bacteria that cause a UTI. Once the lab identifies the bacteria in the sample, the report helps the physician use a more adequate antibiotic for the specific bacteria type if necessary.

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