Urine Culture

The urine culture is used to diagnose a urinary tract infection (UTI) and to identify the bacteria or yeast causing the infection. It may be done in conjunction with susceptibility testing to determine which antibiotics will inhibit the growth of the microbe causing the infection. The results will help a healthcare practitioner determine which drugs are likely to be most effective in treating a person's infection.

Urine is produced by the kidneys, a pair of bean-shaped organs that are located at the bottom of the ribcage in the right and left sides of the back. The kidneys filter waste out of the blood and produce urine, a yellow fluid, to carry wastes out of the body. Urine travels through tubes called ureters from the kidneys to the bladder, where it is stored temporarily, and then through the urethra as it is voided. Urine is generally sterile, but sometimes bacteria or, more rarely, yeast can move from the skin outside the urethra and migrate back up the urinary tract to cause a urinary tract infection.

Most UTIs are considered uncomplicated and are easily treated. However, if they are not addressed, 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. In some cases, a urinary tract infection may lead to an infection in the bloodstream (sepsis, septicemia) that can be life-threatening.

People with kidney disease or with other conditions that affect the kidneys, such as diabetes or kidney stones, and people with weakened immune systems may be more prone to frequent, repeated and/or complicated UTIs.

For more on how the urine culture is performed, see the "What is being tested?" section.

A urine culture may be ordered when a person has symptoms that indicate the possibility of a urinary tract infection (UTI), such as:

• A strong, persistent desire to urinate
• A burning sensation during urination
• A cloudy, strong-smelling urine
• Lower back pain

People 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 into the kidneys, it may cause flank pain, high fever, shaking, chills, nausea or vomiting.

Sometimes, antibiotics may be prescribed without requiring a urine culture for young women with signs and symptoms of a UTI and who have an uncomplicated lower urinary tract infection. If there is suspicion of a complicated infection or symptoms do not respond to initial therapy, then a culture of the urine is recommended.

Pregnant women without any symptoms are recommended to be screened during the first trimester or first prenatal visit for bacteria in their urine, which could affect the health of the developing baby.

A urine culture may be ordered with a urinalysis or as follow up to abnormal results on a urinalysis.

Results of a urine culture are often interpreted in conjunction with the results of a urinalysis and with regard to how the sample was collected and whether symptoms are present. Since some urine samples have the potential to be contaminated with bacteria normally found on the skin (normal flora), care must be taken with interpreting some culture results.

Typically, the presence of a single type of bacteria growing at high colony counts is considered a positive urine culture. For clean catch samples that have been properly collected, cultures with greater than 100,000 colony forming units (CFU)/mL of one type of bacteria usually indicate infection. In some cases, however, there may not be a significantly high number of bacteria even though an infection is present. Sometimes lower numbers (1,000 up to 100,000 CFU/mL) may indicate infection, especially if symptoms are present. Likewise, for samples collected using a technique that minimizes contamination, such as a sample collected with a catheter, results of 1,000 to 100,000 CFU/mL may be considered significant.

Although a variety of bacteria can cause UTIs, most are due to Escherichia coli, bacteria that are common in the digestive tract and routinely found in stool. Other bacteria that may cause UTIs include species of Proteus, Klebsiella, Enterococcus, and Staphylococcus. Occasionally, a UTI may be due to a yeast, such as Candida albicans; urethritis is often due to a sexually transmitted disease such as herpes, chlamydia, or gonorrhea.

A culture that is reported as "no growth in 24 or 48 hours" usually indicates that there is no infection. If the symptoms persist, however, a urine culture may be repeated on another sample to look for the presence of bacteria at lower colony counts or other microorganisms that may cause these symptoms. The presence of white blood cells and low numbers of microorganisms in the urine of a symptomatic person is a condition known as acute urethral syndrome.

If a culture shows growth of several different types of bacteria, then it is likely that the growth is due to contamination. This is especially true in voided urine samples if the organisms present include Lactobacillus and/or other common nonpathogenic vaginal bacteria in women. If the symptoms persist, the healthcare practitioner may request a repeat culture on a sample that is more carefully collected. However, if one type of bacteria is present in significantly higher colony counts than the others, for example, 100,000 CFUs/mL versus 1,000 CFUs/mL, then additional testing may be done to identify the predominant bacteria.

If a culture is positive, susceptibility testing may be performed to guide treatment. Any bacterial infection may be serious and can spread to other areas of the body if not treated. Pain is often the first indicator of an infection. Prompt treatment, usually with antibiotics, will help to alleviate the pain.

Women and girls get urinary tract infections (UTIs) more often than men and boys. Even preteen girls may have frequent UTIs. For men and boys with a culture-proven UTI, further tests may be done to rule out the presence of a kidney stone or structural abnormality that could cause the infection.

If someone has frequent and/or recurrent UTIs, culture and susceptibility testing may be performed with each infection. For those who have frequent UTIs, their bacteria may become resistant to antibiotics over time, making careful selection of antibiotic and the full course of treatment essential.

If the skin and genital area were not cleaned well prior to collecting the sample, the urine culture may grow three or more different types of bacteria and is assumed to be contaminated. The culture will be discarded because it cannot be determined if the bacteria originated inside or outside the urinary tract. A contaminated specimen can be avoided by following the directions to carefully clean yourself and by collecting a mid-stream clean catch urine sample.