A urine culture is a laboratory test performed to identify the presence and type of microorganisms, primarily bacteria, in a urine sample, which helps diagnose a urinary tract infection (UTI). When a urine culture result returns with a finding of “mixed flora,” it signals the presence of multiple different species of bacteria growing in the culture dish. This result is often confusing because it deviates from the typical UTI diagnosis, which usually involves a large concentration of a single type of bacteria. Understanding mixed flora requires examining the difference between harmless contamination and a true infection.
What Mixed Flora Means on a Lab Report
The term “flora” refers to the diverse community of microorganisms, predominantly bacteria, that naturally inhabit various parts of the body, such as the skin and the genital area. A mixed flora finding on a urine culture specifically indicates that the laboratory has identified the growth of several different bacterial species, typically three or more, in the collected sample. This contrasts sharply with the classic result for an uncomplicated UTI, which is characterized by a high concentration of just one predominant type of bacteria, such as Escherichia coli.
When the lab reports mixed flora, it signifies that no single organism is growing in a clearly dominant quantity. The number of bacteria is quantified in Colony Forming Units per milliliter (CFU/mL). A true infection usually involves counts exceeding 100,000 CFU/mL of a single pathogen, so a mixed flora result with lower counts, such as less than 10,000 CFU/mL, usually indicates the result may not represent an infection.
The Most Common Scenario: Sample Contamination
The most frequent explanation for a mixed flora result is contamination of the urine sample during the collection process itself. The urethra and the surrounding genital skin naturally harbor many types of bacteria that are part of the normal urogenital flora. If these external bacteria are inadvertently transferred into the sterile collection cup, they will grow in the culture, leading to the mixed flora finding.
The standard procedure for minimizing this contamination is the “clean catch” midstream technique. This technique requires the patient to first clean the genital area with a provided wipe, begin urinating into the toilet, and then collect only the middle portion of the urine stream in the sterile container. Failure to follow these steps can cause bacteria from the skin, vagina, or rectum to wash into the sample cup. In the absence of symptoms, a mixed flora result is generally treated as contamination that does not require antibiotic treatment.
When Mixed Flora Indicates a True Infection
While contamination is the most common cause, a mixed flora result can occasionally signify a true polymicrobial infection involving multiple types of bacteria. Clinicians use two primary factors to distinguish a true infection from contamination: the bacterial quantity and the patient’s symptoms. Even if the culture shows multiple species, a high overall colony count—suggesting a large quantity of bacteria—is more concerning for an actual infection.
Patient symptoms provide important context for interpreting the lab result. The presence of symptoms such as urinary urgency, frequency, painful urination (dysuria), or fever strongly suggests a true infection, even if the culture is mixed. Polymicrobial UTIs are more common in specific patient populations, including those with indwelling urinary catheters, structural abnormalities of the urinary tract, or compromised immune systems. In these cases, the mixed flora result cannot be automatically dismissed as a collection error.
Interpreting the Results and Next Steps
After receiving a mixed flora result, a healthcare provider must perform a clinical correlation, matching the laboratory finding with the patient’s overall health and symptoms. If the patient is asymptomatic and the bacterial count is low, contamination is likely suspected, and the provider will typically recommend repeating the culture. This repeat test is done with strict adherence to the clean-catch technique to obtain a more reliable sample.
If the patient has strong symptoms of infection and the culture shows a high bacterial count, the mixed flora may be treated as a true polymicrobial infection. In this situation, the provider will work to identify the dominant bacteria and may prescribe an antibiotic that can effectively target a broad range of organisms. The goal is to ensure appropriate treatment while avoiding unnecessary antibiotic use for a simple contaminated sample.