Rudi Hayden
Urologist
Blood-tests Explained
Disclaimer:
This website is intended to assist with patient education and should not be used as a diagnostic, treatment or prescription service, forum or platform. Always consult your own healthcare practitioner for a more personalised and detailed opinion
Urine Culture
At a Glance
Why Get Tested?
To diagnose a urinary tract infection (UTI)
When to Get Tested?
When you experience symptoms of a UTI, such as frequent and painful urination, and/or when a urinalysis indicates you may have a UTI
Sample Required?
A mid-stream clean catch urine sample (women should spread the labia of the vagina and clean from front to back; men should wipe the tip of the penis; start to urinate, let some urine fall into the toilet, then collect one to two ounces of urine in the container provided, then void the rest into the toilet); sometimes a urine sample obtained via catheter
Test Preparation Needed?
Generally none, but you may be instructed 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.
The Test Sample
What is being tested?
Urine is the fluid that contains water and wastes and that is produced by the kidneys. It travels from the kidneys, through tubes called ureters to the bladder, and then is eliminated from the body through the urethra. 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).
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 as small circular colonies. 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. A laboratorian observes the colonies on the agar plate, counting the total number and determining how many types have grown. Ideally, if a good clean catch sample was collected for the test (see below), then the only bacteria present should be due to an infection. 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 to be due to contamination from the skin picked up during the urine collection.
The laboratorian will take a colony from each type and perform a gram stain. The laboratorian examines the microorganisms under the microscope. Different types of bacteria will exhibit characteristic colors and shapes. For instance, the bacterium Escherichia coli, which causes the majority of urinary tract infections, will appear as pink (gram-negative) rods under the microscope. Lactobacillus, which is a common vaginal contaminant in women's urine samples, will appear as thin purple (gram-positive) rods. Some of the bacteria, such as Lactobacillus, are easy for an experienced laboratorian to identify, are nonpathogenic, and do not require any further investigation. Others, such as gram-negative rods, represent groups of similar bacteria and will require additional testing to determine exactly which type of bacteria is present.
If there is no or little growth on the agar after 24 to 48 hours of incubation, the urine culture is considered negative for pathogens and the culture is complete. If there are one or more pathogens present, further testing is performed. Testing is done to identify which bacteria are present and susceptibility testing is done to identify antibiotics that are likely to cure the infection.
How is the sample collected for testing?
Because of the potential to contaminate urine with bacteria and cells from the surrounding skin during collection (particularly in women), it is important to first clean the genital area. Women should spread the labia of the vagina and clean from front to back; men should wipe the tip of the penis. Start to urinate, let some urine fall into the toilet, then collect one to two ounces of urine in the sterile container provided, then void the rest into the toilet. This type of collection is called a mid-stream clean catch urine.
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.
Is any test preparation needed to ensure the quality of the sample?
Generally none; however, you may be instructed not to urinate for at least an 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 sample. Follow the instructions provided for collecting a clean catch urine sample.
The Test
How is it used?
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.
When is it ordered?
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.
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.
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.
Is there anything else I should know?
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.