A urine culture may be ordered when symptoms indicate the possibility of a urinary tract infection, such as pain and burning when urinating and frequent urge to urinate. Antibiotic therapy may be prescribed without requiring a urine culture for symptomatic young women 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 may be screened for bacteria in their urine, which could affect the health and development of the fetus.
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 normal flora from the skin, 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.
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 doctor 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 is typically performed to guide antimicrobial 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.
Females get UTIs more often than males. Even school-age females may have frequent UTIs. For males with a culture-proven UTI, the doctor may order further tests to rule out the presence of a kidney stone or structural abnormality that could cause the infection.
If someone has recurrent urinary tract infections, culture and susceptibility testing may be performed with each episode. 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. People with kidney disease and/or with diseases that affect the kidneys, such as diabetes and those with compromised immune systems, may be more prone to recurring UTIs.
This article was last reviewed on December 6, 2012. | This article was last modified on February 24, 2015.
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