Urine culture and sensitivity

What is being tested?
The urine culture test detects and identifies bacteria and yeasts in the urine. Urine is produced by the kidneys, two fist-sized organs located on either side of the spine at the bottom of the rib cage. The kidneys filter waste from the blood and produce urine, a yellow fluid, to carry the waste out of the body. Urine travels from the kidneys through tubes called ureters to the bladder, where it is stored temporarily, and then out through the urethra when the bladder is emptied. Urine is normally sterile, but sometimes bacteria or, more rarely, yeasts can move from the skin outside the urethra and travel up the urinary tract, causing a urinary tract infection (UTI). In a urine culture, a small sample of urine is placed on one or more agar plates (a thin layer of nutrient culture medium) and incubated at body temperature. Any microorganisms present in the urine sample will grow over the next 24-48 hours into small circular colonies. The size, shape and colour of these colonies help identify which bacteria are present, and the number of colonies shows how many bacteria were originally present in the urine sample. A laboratory technician examines the colonies on the agar plate, counting the total number and determining how many types have grown. Ideally, if a good clean-catch sample has been collected, the only bacteria present should be due to an infection. Typically, this will be a single type of bacterium present in relatively high numbers. Sometimes more than one type of bacterium will be present. This may be due to an infection involving more than one pathogen; however, it is more likely to be due to contamination from the skin during urine collection. The laboratory technician will take a colony of each type of bacterium present in significant numbers or of significant type, smear it on a slide, dry it and stain it, carrying out a test called a Gram stain. The technician examines the organisms under the microscope. Different types of bacteria show characteristic colours and shapes. For example, the bacterium Escherichia coli, which causes most urinary tract infections, will appear as small pink gram-negative rods under the microscope. Lactobacillus, a common vaginal contaminant in women’s urine samples, will appear as thin purple gram-positive rods under the microscope. Some bacteria, such as Lactobacillus, are easy for an experienced laboratory technician to identify, are non-pathogenic and need no further investigation. Others, such as gram-negative rods, represent groups of similar bacteria and will need further tests to determine exactly which type of bacterium is present.

Examples of initial urine culture reports

Based on the information obtained, the laboratory technician gives the doctor an initial report on the amounts and types of bacteria or yeasts present in the urine. Examples would be:

If there is little or no growth on the agar after 24-48 hours of incubation, the urine culture is considered negative for pathogens and the culture is complete. If one or more pathogens are present, further tests are carried out. Biochemical tests are used to identify which bacteria are present, and susceptibility tests are carried out to identify antimicrobial agents that inhibit the growth of the bacteria. The results of these laboratory tests allow the doctor to choose the best antibiotic treatment to clear the infection

How is the sample collected for testing?
Urine for a culture can be collected at any time. Because the urine can become contaminated with bacteria and cells from the surrounding skin during collection (particularly in women), it is important to clean the genital area first. Women should spread the labia and clean from front to back; men should clean the tip of the penis. When you start to urinate, let some urine fall into the toilet, then collect 30-60 ml (one or two ounces) of urine in the sterile container provided, then pass the rest. This type of collection is called a clean-catch midstream urine sample.

How is it used?
To diagnose a urinary tract infection (UTI).

When is it requested?
A urine culture may be requested when symptoms suggest a possible urinary tract infection, such as pain and burning when urinating and a frequent need to urinate. Antibiotic treatment may be prescribed without a urine culture for young women with symptoms who have an uncomplicated lower urinary tract infection. If a complicated infection is suspected or symptoms do not respond to initial treatment, a urine culture is recommended. Pregnant women without symptoms may be tested for bacteria in the urine, which could affect the health and development of the fetus.

What do the test results mean?
The growth of a single type of bacterium in high colony numbers (more than 10,000 colony-forming units (CFU)/ml) is considered a positive urine culture. A culture with “no growth at 24-48 hours” or “fewer than 10,000 CFU/ml” usually indicates that there is no infection. If symptoms persist, however, the culture may be repeated to look for bacteria at lower colony counts (fewer than 10,000 CFU/ml) or other organisms that may cause these symptoms. The presence of white blood cells and low numbers of organisms in a patient with symptoms is a condition known as acute urethral syndrome.

If a culture shows the growth of several different types of bacteria, the growth is probably due to contamination. This is even more likely if the organisms present include Lactobacillus and/or other common non-pathogenic vaginal bacteria. If symptoms persist, the doctor may ask for the culture to be repeated on a more carefully collected sample.

A significant amount of a single type of bacterium usually indicates an infection. Susceptibility tests are carried out to guide antimicrobial treatment. Any bacterial infection can be serious and can spread to other parts of the body if not treated. Pain is often the first sign of an infection. Prompt treatment, usually with antibiotics, will help relieve the pain.

Is there anything else I should know?
Women have urinary tract infections (UTIs) more often than men. Girls of school age can also have frequent UTIs. For males with a UTI confirmed by urine culture, the doctor may request further tests to rule out a kidney stone or structural abnormalities that could be causing the infection.

If you have recurrent urinary tract infections, culture and susceptibility testing may be carried out for each episode. In patients with frequent UTIs, the bacteria can become resistant to antibiotics over time, so careful choice of antibiotics and completing the full course of treatment are extremely important. People with kidney disease and/or diseases affecting the kidneys, such as diabetes, and people with a weakened immune system may be more prone to recurrent UTIs.

1. The surgery called to say they need a new urine sample because the first one was contaminated. What happened?
If the skin and genital area were not cleaned properly before the sample was collected, the urine culture may grow three or more types of bacteria and is then considered contaminated. The culture will be discarded because it is impossible to tell whether the bacteria came from inside or outside the urinary tract. You can avoid giving a contaminated sample by carefully following the instructions on personal cleansing and on collecting a clean-catch midstream urine sample.

2. The doctor said I had symptoms of a urinary tract infection and prescribed antibiotics without doing a urine culture. Why?
The bacterium Escherichia coli (E. coli) causes most lower urinary tract infections. This organism is usually susceptible to a variety of antibiotics, such as trimethoprim-sulfamethoxazole, ciprofloxacin and nitrofurantoin. In most patients with uncomplicated disease, the UTI will clear up after treatment with one of these antibiotics. On the basis of this information, the doctor may prescribe one of these antibiotics without doing a urine culture.

3. What happens if my infection is not treated?
If the infection is not treated, it can move from the lower to the upper urinary tract and infect the kidneys themselves, and eventually enter the bloodstream, causing septicaemia. Symptoms of septicaemia include fever, chills, a high white blood cell count and tiredness. If the doctor suspects septicaemia, they will certainly request blood cultures and prescribe appropriate antibiotics.

4. What puts me at risk of recurrent urinary tract infections (UTIs)?
A wide variety of factors predispose a person to UTIs. After the newborn period, the incidence is higher in women than in men because of anatomical differences in the female urinary tract. In infants and young children, congenital abnormalities are linked to UTIs. In adults, sexual intercourse, use of a diaphragm, diabetes, pregnancy, reflux, neurological disorders, kidney stones and tumours are all factors that predispose to UTIs. In hospitals, care homes and home care settings, catheters and instruments used to examine the urinary tract are major contributors to the spread of UTIs.