Laboratory tests for Urinary Tract Infection (UTI)

S Nilica Devi *

Urinary tract infections (UTIs) are among the most common bacterial infections, accounting for a considerable percentage of antibiotic prescriptions. The urinary tract consists of the kidneys, ureters (which connect the kidneys and the bladder), bladder and the urethra. These are the structures that the urine passes through before being eliminated from the body. Most UTIs are caused by bacteria. They can also be caused by fungi or viruses.

THE URINARY TRACT

The kidneys are located in the abdomen on either side of the spine. The kidneys filter the blood to remove excess water, salt, potassium, urea and other substances. The waste products are excreted out as urine.

The ureters are thin, spaghetti-shaped tubules. They carry urine from the kidney to the bladder.

The bladder is a small balloon-shaped organ located in the pelvis. In women, the bladder is located in front of the uterus. In men, it is located just above the prostate gland.

The urethra is the tube through which urine exits the bladder. The urethra in women is far shorter than it is in men. In men, it has to pass through the prostate and the penis.

URINARY TRACT INFECTION

The blanket term, UTI is frequently used, but a urinary tract infection may also be identified by the part of the urinary tract affected. Urethritis is an inflammation and/or infection of the urethra. Bladder involvement is called cystitis, and when one or both kidneys are inflamed or infected, it is called pyelonephritis.

Urine does not normally contain microorganisms, but if it is obstructed leaving the body or is retained in the bladder, it provides a good environment for bacteria to grow. Most UTIs are due to bacteria that are introduced in the opening of the urethra. They stick to the walls of the urethra, multiplying and moving up the urethra to the bladder.

Most UTIs remain in the lower urinary tract (urethra or bladder), where they cause symptoms such as urinary urgency and a burning sensation during urination. Most of these infections are considered uncomplicated but if they are not addressed in time, the infection may spread up through the ureters and into the kidneys. A kidney infection is more dangerous and can lead to permanent kidney damage.

CAUSES FOR UTIs

Conditions that cause an increased risk for developing UTI include:
o Anatomical problems, such as lowering of the urethra or ureters.
o Urine retention (bladder does not empty urine completely).
o Abnormal flow of urine from the bladder back to the ureters (vesicoureteral reflux).
o Kidney stones.
o Long bladder catheterisation.
o Spinal cord injuries.
o Diabetes.
o Kidney disease/transplant.
o Any condition that supresses the immune system.
o Enlarged prostate in men may inhibit the flow of urine.

SYMPTOMS

Although UTI symptoms vary, many people will experience:
o A strong, persistent desire to urinate.
o A burning sensation during urination.
o A cloudy, strong-smelling urine.
o Lower back pain.

Those with UTIs may also have pressure in the lower abdomen and small amounts of blood in the urine. If the UTI is more severe and/or has spread to the kidneys, it may cause fever, chills, nausea or vomiting. However, in some cases, it may not cause any symptoms at all.

COMMON LABORATORY TESTS

Most UTIs are detected performing a urinalysis and then confirmed with a urine culture. If there are pathogenic bacteria present, then susceptibility testing is done to make sure that the antibiotic that the doctor chooses will effectively treat the microorganism causing the infection.

o URINALYSIS: A clean catch (midstream) urine sample collection is important to minimise sample contamination. White blood cells (WBC, leucocytes), red blood cells (RBC), nitrites, and bacteria in the urine sample may indicate a UTI.

o URINE CULTURE: Urine is streaked on a thin layer of nutrient gel (agar plate), then incubated for 24- 48 hours. Any bacteria that grow on the agar are counted and identified. Usually, if a person has UTI, there will be a high colony count of one type of bacterium that will be present. If there are three or more types of bacteria present, then it is considered to be a contaminated sample and no further work is done on the culture. A repeat sample may need to be submitted if the person’s symptoms persist. If there is a predominant growth of one type of bacterium present in high number, then susceptibility testing is done. The ability of different antibiotics to inhibit the growth of the bacteria in a test tube or on an agar plate helps predict which antibiotics are likely to be effective in the treatment.

OTHER TESTS

If a doctor suspects that the UTI may have spread to the bloodstream, the doctor may ask for a blood culture. In case of recurrent or chronic UTI, other laboratory tests such as blood sugar (to check for diabetes) or BUN and creatinine (to evaluate kidney function) may also be done.

Imaging scans are also done to look for anatomical problems and/or signs of an underlying disease or a condition.

Kidneys and bladder ultrasound are done to study the structure of the kidneys and the bladder and to look for possible blockages.

Intravenous pyelogram (IVP) is done to look at the whole urinary tract. A series of x-ray scans are taken to look for obstructions or structural abnormalities.

Other procedures like voiding cystourethrogram (VCUG), nuclear scan and cystoscopy are also done to identify blockages and other abnormalities.