How do health care professionals diagnose a bladder infection?
A urinalysis (UA) is the initial evaluation for a bladder infection. In most cases, health care professionals use a voided urine specimen, however, there is a risk of contamination by skin bacteria. A "clean-catch" voided urine specimen involves voiding and collecting a urine sample "mid-stream" as opposed to at the very start or end of voiding. A catheterized urine sample is more accurate but has the risk of introducing bacteria into the bladder and may be uncomfortable in children. In infants, medical professionals can perform a suprapubic aspiration.
The method of collecting a voided urine sample differs between men and women, as well as between circumcised men and uncircumcised men. In circumcised men, there is no special preparation. However, uncircumcised men should retract the foreskin. If the source of the infection is unclear, three separate urine samples may be collected: the first void (the first 10 ml of urination) reflects whether or not bacteria are in the urethra, and the second sample is a midstream void (that which occurs after the first 10 ml) and reflects whether bacteria are in the bladder. If there is a concern for bacteria in the prostate, a medical professional performs a rectal examination and massages the prostate to express fluid from the prostate into the urethra, and the third urine sample is obtained after the prostate massage. In both males and females, the voided urine should be collected midstream. It is unclear if washing the penis or perineum with gauze or an antibiotic wipe is more effective in preventing contamination from the skin. In children who are not toilet-trained, a catheterized specimen is more accurate than placing a collection bag over the urethra. In infants, a health care professional can perform a suprapubic aspiration (placing a small needle through the lower abdomen into the bladder and withdrawing a urine sample). In toilet-trained children, a health care professional may obtain a voided urine sample.
A quick office-based urinalysis, called a urine dipstick, is unable to detect if bacteria are present. However, health care professionals use it to detect the presence of nitrite in the urine and leukocyte esterase. Nitrite is a chemical that forms when bacteria in the urine break down a chemical called nitrate, which is normally present in the urine. The nitrite test is not positive for all bacterial infections of the bladder, as not all bacteria can break down nitrate to nitrite. Besides, the urine dipstick is unable to determine the number of white blood cells (infection cells) present in the urine but assesses whether or not white blood cells are present by measuring leukocyte esterase activity. Leukocyte esterase is a chemical produced by white blood cells. The presence of both nitrites and an elevated leukocyte esterase are very suggestive of a urinary tract infection. A formal urinalysis with an examination of the urine under the microscope can identify whether or not bacteria are present in the urine as well as determine the number of white blood cells present in the urine. Examination of the urine under the microscope can also determine if yeast is present in the urine. Viruses cannot be seen under the routine microscope and require special tests to identify.
The definitive test to determine if there is a bladder infection is the urine culture. The urine culture identifies the number and type of bacteria in the urine as well as determines the sensitivity of the bacteria to several different antibiotics. The usual cutoff for a urinary tract infection is the presence of greater than 100,000 bacteria, however, in the presence of symptoms, a positive leukocyte esterase or > 10 white blood cells on urinalysis, even fewer bacteria in the urine is supportive of a urinary tract infection.
A recent publication in the Infectious Disease Clinics of North America highlights the importance of differentiating between asymptomatic bacteriuria, possible, probable, and definite urinary tract infection. Localizing symptoms and signs (pain with urination, urinary frequency, suprapubic/bladder pain, bloody urine, pain in the flank, new or worsening urgency or urinary incontinence), pyuria (white blood cells in the urine), or positive leukocyte esterase on urinalysis are the most important predictors of a urinary tract infection. They note that the diagnosis of a urinary tract infection requires three components: (1) clinical symptoms of infection localizing to the urinary tract or nonspecific symptoms of infection (fever, chills, change in mental status) in the absence of symptoms suggesting infection in another part of the body, (2) the presence of white blood cells in the urine and bacteria in the urine, and (3) absence of another infection or noninfectious process accounting for the symptoms and signs.
If symptoms of an upper urinary tract infection (pyelonephritis), fever, flank pain, nausea, or vomiting are present, health care professionals will obtain a blood test (a complete blood cell count [CBC]). If there is a concern for a severe infection, medical professionals will culture a sample of blood to see if there are bacteria in the bloodstream. Radiologic studies are not routinely obtained in the case of cystitis, however, if there are signs of a kidney infection (pyelonephritis) such as flank pain, fevers/chills, or there is a failure to respond to antibiotics (with persistent or worsening symptoms), then radiologic testing (renal ultrasound, CT scan, or MRI) can be performed to rule out an abscess or other abnormalities. If a physician suspects constipation, a plain X-ray of the abdomen may be obtained to assess the severity of constipation.