A few ounces of urine; the mid-stream clean catch urine sample is the most common type of sample collected. (The genital area is cleaned before collecting your urine.) Urine may also be collected using a catheter and, rarely, a needle is used to aspirate urine directly from the bladder. For infants, a collection bag may be attached to the genital area to catch any urine produced. For additional details, see "What is being tested?" below.
Generally none, but depending on the type of culture, you may be given special instructions. For example, you may be asked not to urinate for at least one hour before the test and/or to drink a glass of water 15-20 minutes before sample collection. This will help to ensure that you can produce enough urine for the test. Antibiotics taken prior to the test may affect your results. Tell your healthcare practitioner if you have taken antibiotics recently.
Urine is the fluid produced by the kidneys that carries water and wastes through the urinary tract and then is eliminated from the body. The urine culture is a test that detects and identifies bacteria and yeast in the urine, which may be causing a urinary tract infection (UTI).
The kidneys, a pair of bean-shaped organs located at the bottom of the ribcage in the right and left sides of the back, filter wastes out of the blood and produce urine, the yellow fluid that carries 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 contains low levels of microbes, such as bacteria or, yeast which move from the skin into the urinary tract and grow and multiply, causing a urinary tract infection.
Most UTIs are considered uncomplicated and are easily treated. However, if they are not addressed, the infection may spread from the bladder and ureters into the kidneys. A kidney infection is more dangerous and can lead to permanent kidney damage. In some cases, an untreated urinary tract infection may spread to the bloodstream (septicemia) and cause sepsis, which can be life-threatening.
Women and girls get urinary tract infections more often than men and boys. Even preteen girls may have frequent UTIs. For men and boys with a UTI confirmed by a urine culture, further tests may be done to rule out the presence of a kidney stone or structural abnormality that could cause the infection.
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 a urine culture, a small sample of urine is placed on one or more agar plates (a thin layer of a nutrient media) and incubated at body temperature. Any bacteria or yeast that are present in the urine sample grow over the next 24 to 48 hours.
- A laboratory professional studies the colonies on the agar plate, counting the total number and determining how many types have grown. The size, shape, and color of these colonies help to identify which bacteria are present, and the number of colonies indicates the quantity of bacteria originally present in the urine sample. The quantity can differentiate between normal levels of bacteria versus infection.
- Ideally, if a good clean catch sample was collected for the test, only bacteria causing a UTI are present. Typically, this will be a single type of bacteria that will be present in relatively large numbers.
- Sometimes, more than one type of bacteria will be present. This may be due to an infection that involves more than one pathogen; however, it is more likely due to skin, vaginal, or fecal contamination picked up during the urine collection.
- The laboratorian will take a colony from each type and perform other tests, such as a gram stain, to identify the type (species) of bacteria or other microbe (i.e., yeast). Susceptibility testing may be done to determine which antibiotics will likely cure the infection.
If there is no or little growth on the agar after 24 to 48 hours of incubation, the urine culture is considered negative and the culture is complete, suggesting an infection is not present.
How is the sample collected for testing?
Although there are several types of urine samples, the mid-stream clean catch is the type most commonly submitted for culture.
- It is important to first clean the genital area before collecting your urine because of the potential to contaminate the urine with bacteria and cells from the surrounding skin during collection (particularly in women).
- Start first by washing your hands.
- Women should then spread the labia of the vagina and clean from front to back using a wipe provided by your healthcare practitioner or the laboratory. It is recommended to repeat with a second towel or wipe.
- Men should wipe the tip of the penis.
- Start to urinate, let some urine fall into the toilet, and then collect one to two ounces of urine directly into the sterile container provided, then void the rest into the toilet. Do not allow the inside of the container to come into contact with skin and do not scoop the urine from the toilet (or any other container).
For catheterized specimens, a urine sample is taken by inserting a thin flexible tube or catheter through the urethra into the bladder. This is performed by a trained healthcare practitioner. The urine is collected in a sterile container at the other end of the tube. Rarely, a needle and syringe may be used to collect by aspirating urine directly from the bladder. For infants, a collection bag may be placed on the genital area to collect any urine produced.
Is any test preparation needed to ensure the quality of the sample?
Generally, no preparation is needed, but depending on the type of culture, you may be given special instructions. For example, you may be asked not to urinate for at least one hour before the test and/or to drink a glass of water 15-20 minutes before sample collection. This will help to ensure that you can produce enough urine for the test. Sometimes you may be instructed to collect the first urine you void in the morning. Antibiotics taken prior to the test may affect your results. Tell your healthcare practitioner if you have taken antibiotics recently.
How is the test used?
The urine culture is used, along with results from a urinalysis, to diagnose a urinary tract infection (UTI) and to identify the bacteria or yeast causing the infection. If a urine culture is positive, susceptibility testing may be done 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 your infection.
A urine culture is used, as recommended by several health organizations, to screen pregnant women for asymptomatic bacteriuria, a condition in which significant amounts of bacteria are in the urine but do not cause symptoms. About 2%-10% of pregnant women in the U.S. have this condition that can lead to more serious kidney infection as well as increased risk of preterm delivery and low birth weight.
When is it ordered?
Some signs and symptoms of a UTI include:
- A strong, frequent urge to urinate, even when you have just gone and there is little urine voided
- Pain and/or a burning sensation during urination
- Cloudy, strong-smelling urine
- Lower back pain
You 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 with a urine culture early in their pregnancy (e.g., during the second trimester) or during the first prenatal visit for bacteria in their urine.
What does the test result mean?
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.
Positive urine culture: 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)/milliliter 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.
Results from a urinalysis can be used to help interpret results of a urine culture. For example, a positive leukocyte esterase (a marker of white blood cells) and nitrite (a marker for bacteria) help confirm a UTI.
If a culture is positive, susceptibility testing may be performed to guide treatment. (See the article on Antibiotic Susceptibility Testing for more details on results.)
Although a variety of bacteria can cause UTIs, most are due to Escherichia coli (E. coli), bacteria that are common in the digestive tract and routinely found in stool.
Other bacteria that commonly cause UTIs include:
Occasionally, a UTI may be due to a yeast, such as Candida albicans.
Negative urine culture: 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.
Contamination: 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.
Can a urine culture be used to test for infections other than UTIs, such as sexually transmitted diseases (STDs)?
Yes. Urine cultures can detect some sexually transmitted diseases. However, a urine culture is not the test of choice for sexually transmitted diseases in adults. Some STDs such as chlamydia may be tested using a urine sample, but the testing method used detects chlamydia genetic material in the urine and is not a culture. Tell your healthcare practitioner if you think you have a sexually transmitted disease, so the practitioner can order the appropriate test. (For examples, read the articles on Chlamydia Testing and Gonorrhea Testing.) Urine cultures may be used to test for STDs in children.
For another example, a urine culture may be used to help diagnose infections of the urinary tract and genital tract caused by mycobacteria. Typically, this test requires that the first urine voided in the morning be collected.
My healthcare practitioner's office called to say they need a new urine sample, the first was contaminated. What happened?
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.
My healthcare practitioner said I had symptoms of a urinary tract infection and prescribed antibiotics without performing a urine culture. Why?
Bacteria known as Escherichia coli (E. coli) cause the majority of lower urinary tract infections. This microbe is usually susceptible to a variety of antibiotics, such as trimethoprim-sulfamethoxazole, ciprofloxacin, and nitrofurantoin. In most people with an uncomplicated UTI, the infection will be cured after treatment with one of these antibiotics. Based on this information, your healthcare practitioner may prescribe one of them without performing a culture.
What happens if my infection goes untreated?
If your infection is not treated, it can move from the lower urinary tract to the upper urinary tract and infect the kidneys and possibly spread to the bloodstream, causing septicemia and sepsis, a serious and potentially life-threatening condition. Signs and symptoms of septicemia include fever, chills, elevated white blood cell count, and fatigue. If a healthcare practitioner suspects septicemia, the practitioner will typically order a blood culture as well as other tests and will prescribe antibiotics accordingly.
What puts me at risk for recurrent urinary tract infections?
There are a wide variety of factors that predispose a person to get a UTI. UTIs are more common in girls and women than in boys and men because of the differences in their genitals and urinary tracts. Some infants and young children have abnormalities of the urinary tract that they are born with (congenital) that increase their risk of UTIs. In adults, sexual intercourse, diabetes, pregnancy, poor bladder control, kidney stones, and tumors are examples of factors that increase risk of UTIs. In a hospital, nursing home, or home care setting, urinary catheters are major risk factors for UTIs.
Is there anything else I should know?
If you have frequent and/or recurrent UTIs, culture and susceptibility testing may be performed with each infection. If you have frequent UTIs, careful selection of antibiotics and completing the full course of treatment can be important.