When a healthcare provider suspects a urinary tract infection (UTI), a urine sample is sent for a culture. This test identifies and quantifies any bacteria present, confirming if an infection exists and guiding appropriate antibiotic treatment. Results are not always straightforward and require careful interpretation. Understanding the terminology on a lab report, such as the phrase “mixed growth,” is the first step in clarifying whether an infection is truly present.
Understanding the Term “Mixed Growth”
The phrase “mixed growth” on a urine culture report describes the presence of multiple different types of microorganisms in the cultured sample. This result is frequently reported when the laboratory identifies three or more distinct bacterial species growing on the culture plate. A typical positive UTI culture shows a high colony count, often exceeding 100,000 Colony-Forming Units (CFU) per milliliter, of one predominant organism, such as Escherichia coli. In contrast, mixed growth indicates a more varied and less concentrated population of bacteria, with no single species clearly dominating the culture.
The laboratory’s interpretation is based on both the number of species and their concentration, which is why mixed growth is often categorized as having “doubtful significance.” The presence of numerous organisms, each at a moderate or low concentration, suggests a less organized, non-infectious bacterial population. This finding is microbiologically distinct from a true infection, which is characterized by the rapid proliferation of a single, disease-causing microbe within the urinary tract.
Why Mixed Growth Usually Indicates Sample Contamination
In the majority of cases, a “mixed growth” result is not indicative of an actual infection within the bladder or kidneys, but rather points to contamination of the sample during collection. This happens when the urine passes over the normal, non-pathogenic bacteria, known as commensal flora, that live on the skin around the urethral opening and genital area. These external microbes are accidentally introduced into the collection cup, leading to the varied bacterial landscape seen in the laboratory.
The standard procedure for minimizing this contamination is called a “midstream clean catch” collection. This technique requires a patient to thoroughly clean the genital area before beginning to urinate. The patient must allow the first portion of urine to pass into the toilet, and then collect the middle portion of the stream in the sterile cup. The initial stream flushes out bacteria near the urethral opening, and the midstream portion is meant to be the most representative of the urine inside the bladder. Failure to follow these steps precisely allows the external flora to enter the sample.
These external bacteria, such as certain species of Staphylococcus or Lactobacillus from the skin, are not causing a urinary tract infection but are simply multiplying in the culture medium. The resulting report of multiple organisms, each without a high enough colony count to be considered a true pathogen, flags the sample as likely contaminated. If the patient has no symptoms, contamination is the most probable explanation for the result.
Clinical Scenarios Where Mixed Growth Requires Treatment
While contamination is the most common cause, a mixed growth culture may represent a true, complex polymicrobial infection in specific clinical circumstances. This is particularly relevant in patients whose urinary systems or immune defenses are already compromised. Individuals with indwelling urinary catheters are a prime example, as the device creates a surface for multiple types of bacteria to colonize and form a biofilm, making polymicrobial bacteriuria frequent.
Patients with complicated urinary tract anatomy, such as those with kidney stones, structural abnormalities, or recent urological procedures, may genuinely harbor infections caused by multiple organisms. In these cases, the multiple species are different pathogens working together within the urinary system, not just external skin flora. Immunocompromised patients, whose bodies struggle to contain even less virulent bacteria, may also experience a true infection from a mixed population of microbes. The interpretation also shifts if a high colony count is present despite the mixture, as a high density of two or three organisms is taken more seriously than a low density of many.
Interpreting the Result and Next Steps
The ultimate interpretation of a mixed growth result relies heavily on clinical correlation; the laboratory finding must be evaluated alongside the patient’s symptoms. If a patient reports no symptoms—such as painful urination, frequent urges, or fever—the result is almost always dismissed as contamination, and no treatment is necessary. Treating asymptomatic mixed growth with antibiotics is avoided to prevent unnecessary exposure and the development of antibiotic resistance.
If a patient is experiencing strong, classic UTI symptoms, the mixed growth result creates a diagnostic dilemma because contamination makes identifying the true causative pathogen impossible. In this situation, the healthcare provider will request a repeat urine culture, emphasizing the importance of a strict midstream clean catch technique. To obtain a truly sterile sample and bypass the risk of external contamination, a provider may opt for a catheterized specimen, collected directly from the bladder. Once a clean sample is secured, the culture can accurately identify a single pathogen, allowing the physician to prescribe targeted and effective antibiotic treatment.