Urinary Tract Infection - Diagnosis

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Escherichia coli is gram negative and the most common cause of UTIs. If doctors suspect that bacteria other than E. coli are causing a UTI, a Gram stain is useful for identifying other species.

Imaging Techniques

Because of the expense and the limited accuracy of imaging procedures, these techniques are used only for the following:

• Serious and recurrent cases of pyelonephritis
• When structural abnormalities are suspected
• If infections do not respond to treatment
• If a doctor suspects obstruction or an abscess
• After a first urinary tract infection in children aged 2 to 24 months to detect possible obstruction or vesicoureteral reflux. Tests include ultrasound and a voiding cystourethrogram and possibly scans. Some evidence suggests that ultrasound is probably not necessary, but at this time it is recommended by major medical groups.

Ultrasound. Ultrasound is a noninvasive, risk-free imaging test that can be used to screen for hydronephrosis (obstructions of the flow of urine), kidney stones that predispose to infection, and kidney abscesses. In men, ultrasound can detect enlargement or abscesses of the prostate and, when combined with x-rays, is an accurate method for detecting incomplete emptying of the bladder, a common cause of UTI in men over age 50. In children with urinary tract infections, it also can be used to detect vesicoureteral reflux, the defect of the valve-like mechanism between the ureter and bladder. Ultrasounds are not as accurate as voiding cystourethrograms.

Nuclear Scans. Imaging techniques called nuclear scans may be useful in certain complicated cases, such as detecting kidney scarring after pyelonephritis in children. They produce better images and expose the patient to far less radiation than x-rays. One such scan called dimercaptosuccinic acid (DMSA) scintigraphy uses injections of tiny amounts of radioactive tracers. A scanning machine (scintillation or gamma camera) is then used to detect pictures of the tracer in the kidney. This information is recorded on a computer screen or on film.

Magnetic Resonance Imaging (MRI) or Computed Tomography (CT). Magnetic resonance imaging (MRI) and computed tomography (CT) scans are noninvasive advanced imaging techniques that are sometimes used when nuclear scans are inconclusive. A CT scan is useful for ruling out kidney stones or obstructions in women with recurrent UTIs.

X-Rays. Special x-rays can be used to screen for structural abnormalities, urethral narrowing, or incomplete emptying of the bladder, which can cause stagnation of urine and predispose to infection.

• Voiding cystourethrogram is an x-ray of the bladder and urethra. To obtain a cystourethrogram, a dye, called contrast material, is injected through a catheter inserted into the urethra and passed through the bladder.
• An intravenous pyelogram (IVP) is an x-ray of the kidney. For a pyelogram, the contrast matter is injected into a vein and eliminated by the kidneys. In both cases, the dye passes through the urinary tract and reveals any obstructions or abnormalities on x-ray images. Due to the possible risks to the fetus, x-rays are not performed on pregnant women.

Click the icon to see an image of a voiding cystourethrogram.

Click the icon to see an image of an intravenous pyelogram.

Cystoscopy. Cystoscopy is used to detect structural abnormalities, interstitial cystitis, or masses that might not show up on x-rays during an IVP. The patient is given a light anesthetic and the bladder is filled with water. The procedure uses a cystoscope, a flexible, tube-like instrument that the urologist inserts through the urethra into the bladder.

Click the icon to see an image of cystoscopy.

Other Diagnostic Tests for Kidney Infections and Severe UTIs

No noninvasive test will differentiate between upper and lower urinary tract infections. This is a particular problem because of the high percentage of women whose cystitis symptoms mask infections that also exist in the upper tract.

Antibiotic Trial. The best current test for pyelonephritis is the short-term antibiotic therapy given for cystitis. If the infection returns within 2 weeks after treatment, upper urinary tract infection is usually present.

Blood Cultures. If symptoms are severe, blood cultures will be taken to determine if the infection is in the bloodstream and threatening other parts of the body.

Review Date: 06/16/2006
Reviewed By: Harvey Simon, M.D., Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital