A urine culture is a laboratory test designed to identify the presence and quantity of microorganisms, typically bacteria, in a urine sample. Its primary purpose is to diagnose urinary tract infections (UTIs). The process involves collecting a urine sample, allowing any bacteria present to grow in a controlled environment, and then counting the resulting bacterial colonies. This count, known as the “colony count,” is a significant part of the culture results and helps determine if an infection is present.
What Colony Count Means
Colony count refers to the estimated number of viable bacterial cells in a urine sample, expressed as Colony Forming Units per milliliter (CFU/mL). A Colony Forming Unit (CFU) represents a single bacterium or a cluster that forms a visible colony on a culture plate.
To determine the colony count, a precise volume of urine is spread onto a nutrient-rich agar plate. The plates are then incubated, allowing bacteria to multiply and form distinct colonies. Laboratory technicians then count these colonies to estimate the original number of bacteria in the sample.
Interpreting Normal Colony Counts
A “normal” or insignificant colony count in a urine culture generally indicates no active urinary tract infection. For a clean-catch midstream urine sample, less than 10,000 CFU/mL (<10^4 CFU/mL) is typically considered normal, often due to natural contamination from the skin or urethra during collection. Urine is usually sterile, but some contamination can occur during collection. These threshold values can vary between laboratories and clinical contexts. Samples collected via catheterization, for example, might have different interpretive criteria due to reduced contamination risk. A low count suggests that any bacteria present are likely not causing an infection.
Factors Affecting Colony Counts
Several factors can influence urine culture colony counts. One common issue is contamination during sample collection. If the “clean catch” technique is not followed, bacteria from the skin or genital area can enter the sample, resulting in higher counts that do not represent a true urinary tract infection. This can lead to a “mixed growth” report, especially if multiple types of bacteria are present at low counts.
A person’s hydration status can also affect the concentration of bacteria in urine. Being over-hydrated might dilute the urine, leading to a lower bacterial count even if an infection is present. Conversely, dehydration can concentrate bacteria, potentially contributing to bacterial growth and increasing the risk of UTIs.
Recent or current antibiotic use significantly impacts bacterial growth. Antibiotics suppress bacterial multiplication, which can result in a falsely low or negative colony count, masking an existing infection. Delays in processing or improper storage of the urine sample can also affect results, as bacteria can continue to multiply, leading to an artificially elevated count if the sample is not refrigerated or preserved within two hours of collection.
When Counts Indicate Infection
High colony counts typically signify a probable urinary tract infection. For a clean-catch urine sample, 100,000 CFU/mL (10^5 CFU/mL) or greater of a single type of bacteria is generally considered indicative of an active infection. Escherichia coli (E. coli) is the most frequent cause of UTIs. Other bacteria, such as Klebsiella species, Proteus species, and Enterococcus species, can also cause UTIs.
Counts between 10,000 and 100,000 CFU/mL may be considered indeterminate and could still suggest an infection, especially if the patient is experiencing symptoms. In such cases, or if symptoms persist, further clinical evaluation or repeat testing might be recommended. Beyond the count, identifying the specific type of bacteria and its susceptibility to various antibiotics guides appropriate treatment.