How do health care professionals diagnose a bladder infection?
Bladder infection is generally diagnosed by a urinalysis (UA). In most cases, a voided urine specimen is use, however, there is a risk of contamination by skin bacteria. A catheterized urine sample is more accurate, but has the risk of introducing bacteria into the bladder.
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, in uncircumcised men, the foreskin should be retracted. 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 or not bacteria are in the bladder. If there is a concern for bacteria in the prostate, a rectal examination is performed and the prostate massaged 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 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, it is used to detect the presence of nitrite in the urine and leukocyte esterase. Nitrite is a chemical that is formed when bacteria in the urine break down a chemical called nitrate, which is normally present in the urine. In addition, 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. The absence of nitrite does not mean that there is not an infection, because not all bacteria can break down nitrate to nitrite.
A formal urinalysis with examination of the urine under the microscope is able to 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 are 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 of and type of bacteria in the urine as well as determine the sensitivity of the bacteria to a number of different antibiotics. The usual cutoff for a urinary tract infection is the presence of greater than 10,000 bacteria, however, in the presence of symptoms, even fewer bacteria in the urine is supportive of a urinary tract infection.
If symptoms of an upper urinary tract infection, fever, flank pain, nausea or vomiting are present, a blood test, a complete blood cell count (CBC) is often obtained. If there is a concern for a severe infection, a sample of blood will be cultured 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 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.