Urine tests often provide insights into potential infections. When a urine culture shows “mixed growth,” it can be confusing for individuals. This article clarifies what “mixed growth” signifies, distinguishing it from a definite infection and guiding interpretation.
Understanding Mixed Growth in Urine Culture
“Mixed growth” in a urine culture indicates multiple types of bacteria in the sample, typically two to three different kinds. These bacteria are usually found in low to moderate quantities. While urine within the bladder is generally sterile, bacteria can be introduced into the sample during the collection process itself. This finding suggests that the bacterial growth is not dominated by a single organism, which is a common characteristic of a true urinary tract infection. The diverse array of bacteria, without one clearly outnumbering the others, is what prompts the “mixed growth” designation from the laboratory, making it challenging to identify a single causative agent for infection.
Mixed Growth Versus True Infection
Mixed growth in a urine sample most often points to contamination, usually from the skin, genital area, or external environment during the collection process. The area around the urethra naturally harbors various bacteria, and these can easily enter the urine specimen if proper collection techniques are not followed. To minimize contamination, a “clean-catch” urine collection method is recommended. This involves:
Washing hands thoroughly.
Opening the specimen cup without touching the inside.
Cleaning the genital area with provided wipes (females: separate labia, wipe front to back; males: clean penis tip, retract foreskin if uncircumcised).
Passing the initial urine stream into the toilet.
Collecting the midstream portion into the sterile cup, filling it about halfway.
While contamination is the most common reason for mixed growth, there are instances where it might indicate a true infection. This can occur if specific pathogenic bacteria are identified among the mixed organisms, or if the overall bacterial counts are high despite being mixed. Most importantly, a doctor will consider the patient’s symptoms alongside the lab results. Common symptoms of a urinary tract infection (UTI) include a persistent urge to urinate, a burning sensation during urination, frequent urination with small amounts of urine, cloudy or foul-smelling urine, and lower abdominal or pelvic pain. If these symptoms are present, even with mixed growth, a true infection may be suspected.
What to Do After a Mixed Growth Result
Receiving a “mixed growth” result necessitates a discussion with a healthcare provider. They will interpret the findings in the context of your symptoms, medical history, and any underlying health conditions. Pregnant women or immunocompromised individuals might require a more cautious approach. If symptoms are absent or mild, retesting the urine sample with careful adherence to the clean-catch method is a common next step to rule out potential contamination. If symptoms of a UTI are strong, a doctor may decide to initiate treatment even with an initial mixed growth result, relying on their clinical judgment. This approach prioritizes addressing potential infection and alleviating discomfort while awaiting further clarity from a repeat sample.