Diagnosing Urinary Tract Infections

The urinary tract system-consisting of two kidneys, two ureters, a bladder and a urethra-is prone to urinary tract infections (UTIs). These infections result in more than 7 million visits to doctor’s offices annually, according to the American Urological Association Foundation. This common and significant diagnosis in routine clinical practice demands a high level of diagnostic accuracy.

“In straightforward cases, a diagnosis can be made based on symptoms alone, without laboratory testing,” said Nathan C. Walk, MD, pathologist/dermatopathologist and medical director of chemistry at Middlesex Hospital in Middletown, Conn.

In questionable cases, or if there is suspicion of a more serious problem, physicians use laboratory testing to back up their clinical judgment. “Most physicians prefer to treat urinary tract infections based on the results of a culture, not just symptoms,” according to Walk.

Laboratory Tests
Tests available to diagnose urinary tract infections include indirect and direct methods. “Indirect testing uses a reagent strip dipped into a urine sample in the clinician’s office,” Walk explained. “This strip tests for the presence of nitrite and leukocyte esterase, indirect indicators of bacteria and white blood cells in the urine.”

If there is ambiguity following indirect testing, then the clinician will turn to direct methods. “That is, directly testing for inflammatory cells and bacteria,” Walk explained. “Methods include urinalysis and urine culture.”

In urinalysis, a clean catch urine sample is important to minimize contamination. “The testing is automated in this day and age, with a considerable amount of data being gleaned, including urine pH, appearance, specific gravity, noting the presence of glucose, ketones, and bilirubin, and specifically quantifying the presence of white blood cells, red blood cells and bacteria,” Walk explained.

The urinalysis is a helpful screening and diagnostic tool, but urine culture is the gold standard, according to Walk. “Most UTIs are detected by urinalysis and confirmed with a urine culture,” he told ADVANCE.

“A urine culture is the most reliable diagnostic test for a UTI.”

With a urine culture, a laboratory professional streaks urine on an agar plate and then incubates the sample for at least 24 hours. Next, the bacteria that grow on the agar are counted. Depending on the urine collection method, clean catch voided versus sterile catheter, there are different thresholds for what constitutes a positive culture.

Additionally, the number of bacteria species that grow is important. “With a true infection, there is typically one dominant organism, as compared with multiple organisms found in a healthy patient’s normal flora,” Walk shared.

Walk stresses the importance of considering the sensitivity and specificity of each test, and evaluating these in the context of the patient’s clinical presentation. “A problem we see with bacterial cultures is a tendency for over-diagnosis,” according to Walk.

Follow-up Tests
The physician may order a blood culture if there is reason to believe that the UTI has spread into the bloodstream. Similarly, testing may be done for sexually transmitted diseases if the patient’s symptoms suggest the presence of chlamydia or gonorrhea.

For a blood culture, a laboratory professional will test two blood samples for the presence of bacteria. The samples are incubated at body temperature and checked regularly for bacteria growth. The same microorganism will be found in the blood and urine cultures when a person is septic from a urinary tract infection.

For patients with chronic UTIs, the physician may order other lab tests such as glucose, BUN and creatinine. If a patient has recurrent or chronic UTIs, suspicion rises for a structural problem, and one or more imaging tests may be ordered: kidney and bladder ultrasound, voiding cystourethrogram (VCUG), nuclear scans, intravenous pyelogram (IVP), or computed tomography (CT).

A useful but more invasive procedure is cystoscopy, which involves the use of a scope to directly examine the urinary tract. “These testing modalities are reserved for recurrent infections or complicated cases because of the cost and sometimes invasive nature of the procedures involved,” said Walk.

Establishing a diagnosis of UTI requires a good clinical history and physical examination of the patient, combined with confirmatory laboratory testing. “Our job in the lab is to support or refute a clinical impression,” explained Walk.

Rebecca Mayer Knutsen is on staff at ADVANCE. Contact: rknutsen@advanceweb.com

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