A urine culture is a laboratory test performed to identify the presence and quantity of bacteria in a urine sample, helping determine if a urinary tract infection (UTI) is present. While the urinary tract is typically considered sterile, bacteria from the surrounding skin can contaminate the sample during collection. This test is designed to distinguish between true infection and simple contamination to ensure accurate diagnosis and appropriate treatment. Interpreting the results hinges on correctly understanding the numerical output, which quantifies the bacterial load.
Understanding Colony-Forming Units (CFU/mL)
The standard way to measure the concentration of bacteria in a urine culture is through Colony-Forming Units per milliliter (CFU/mL). This unit represents the estimated number of viable bacterial cells present in one milliliter of the urine sample. It is a calculated value, not a direct count of every single bacterium.
To determine this number, a small, measured amount of urine, often 1 or 10 microliters, is spread onto a nutrient-rich agar plate. The plate is then incubated for 24 to 48 hours, allowing viable bacteria to multiply and form visible clusters, or “colonies.” Each colony is assumed to have originated from a single bacterial cell or small cluster.
The lab technician counts the colonies and multiplies that count by a factor based on the volume of urine plated to determine the final CFU/mL result. For example, if \(10\mu L\) of urine was plated and 10 colonies grew, the result would be \(1,000\text{ CFU/mL}\). This quantitative approach provides a standardized measure of the bacterial load for clinical interpretation.
What Colony Counts Indicate a Negative Result?
A negative or non-significant result in a urine culture indicates that a clinically relevant infection is unlikely. For a midstream clean-catch urine sample, a count of less than \(10,000 \text{ CFU/mL}\) is generally considered negative. This low level of bacterial growth often suggests contamination from the urethra or external skin during collection.
Some laboratories may use a stricter definition, classifying results below \(1,000 \text{ CFU/mL}\) as definitively negative. When a result shows no bacterial growth, it is reported as “No Growth Observed,” suggesting the absence of bacteria in the urinary tract. Low-range results, especially those with multiple types of bacteria growing, are frequently attributed to contamination rather than true infection.
A low colony count, such as \(1,000 \text{ CFU/mL}\) to \(10,000 \text{ CFU/mL}\), indicates a lower likelihood of a urinary tract infection. However, even a low count must be interpreted alongside the patient’s symptoms and other lab tests, such as the urinalysis, to rule out infection.
Thresholds for Diagnosing a Urinary Tract Infection (UTI)
The standard threshold for diagnosing a definitive urinary tract infection, particularly in uncomplicated cases, has historically been \(100,000 \text{ CFU/mL}\) (\(10^5 \text{ CFU/mL}\)) of a single type of pathogen. This high concentration, often called “significant bacteriuria,” was established to identify true infection, even in people without symptoms.
However, this traditional cutoff is not universally applied, especially in symptomatic patients. Clinical guidelines recognize that lower counts can be significant when a person is experiencing symptoms like painful urination (dysuria) or frequent urge to urinate. For example, in women with acute cystitis, bacterial counts as low as \(100 \text{ CFU/mL}\) of a primary uropathogen may indicate infection.
A threshold of \(10,000 \text{ CFU/mL}\) to \(50,000 \text{ CFU/mL}\) may be considered positive in certain contexts, especially if the bacteria is a known uropathogen like Escherichia coli. This lower threshold is often applied to specimens collected via catheterization, which have a lower risk of contamination than midstream samples. For asymptomatic bacteriuria (bacteria without symptoms), diagnosis generally requires isolating \(\ge 100,000 \text{ CFU/mL}\) on two consecutive samples in women or a single sample in men.
Why Colony Count Alone Isn’t the Only Factor
Relying solely on the colony count can lead to misdiagnosis because several factors complicate the interpretation of the numerical result. The method used to collect the urine specimen drastically influences the significance of the count.
Collection Method
A midstream clean-catch sample, while common, carries the highest risk of contamination from external flora. In contrast, a specimen collected via catheterization has a much lower contamination risk, meaning growth of \(\ge 100 \text{ CFU/mL}\) or \(\ge 1,000 \text{ CFU/mL}\) may be considered significant. The most sterile collection method is suprapubic aspiration, which involves a needle inserted directly into the bladder. Any bacterial growth from this sample type is often considered indicative of infection.
Type of Bacteria and Clinical Context
The type of bacteria identified is also a major consideration. Growth of a known uropathogen, such as E. coli, is more likely to indicate a true infection than the growth of a common skin contaminant. A result showing three or more different types of bacteria, especially at low levels, is often reported as “mixed bacterial flora” and strongly suggests sample contamination.
The colony count must be correlated with the patient’s clinical presentation and other laboratory findings to reach a definitive diagnosis. The presence of white blood cells in the urine, known as pyuria, indicates the body’s inflammatory response to an infection. Pyuria is a strong supporting factor for a UTI diagnosis, even when colony counts are lower than the traditional \(100,000 \text{ CFU/mL}\) threshold. Ignoring symptoms or pyuria and only treating the number can result in delayed care or unnecessary use of antibiotics.