A urine culture and sensitivity report is a laboratory document that serves as a detailed roadmap for treating a urinary tract infection (UTI). This two-part test first isolates and identifies the specific microorganism causing the infection in the urinary tract. The second part determines which antibiotic medications are most likely to kill the identified bacteria. The information contained within the report is the primary tool used by healthcare providers to select an effective treatment plan for the patient.
What the Culture Section Reveals
The first part of the report focuses on the growth and identification of bacteria from the urine sample. This section will typically report either “No Growth” or “Significant Growth,” which indicates whether an infection is likely present. A “No Growth” result suggests that a bacterial UTI is not the cause of the patient’s symptoms.
A finding of “Significant Growth” is followed by a number indicating the concentration of bacteria found, measured in Colony-Forming Units per milliliter (CFU/mL). For a clean-catch midstream urine sample, a count of \(10^5\) CFU/mL or greater of a single organism is the classic threshold. However, in symptomatic patients, lower counts, such as \(10^3\) to \(10^4\) CFU/mL, can still be considered meaningful, especially if the sample was collected via a catheter.
The report also identifies the specific organism, such as Escherichia coli (the most common cause of UTIs), Klebsiella pneumoniae, or Enterococcus species. If the lab finds “Mixed Flora” or multiple types of bacteria growing at low levels, this usually suggests the sample was contaminated by external skin bacteria during collection. In such cases, the results are inconclusive, and a repeat test may be necessary to identify a single, causative pathogen.
Interpreting Susceptibility Metrics
Once the organism is identified, the sensitivity portion of the report details how the bacteria reacts to a panel of antibiotics. This information is presented using a letter classification: Susceptible (S), Intermediate (I), or Resistant (R). The classification is based on a quantitative measure called the Minimum Inhibitory Concentration (MIC).
The MIC value is the lowest concentration of a specific antibiotic, measured in micrograms per milliliter (\(mu text{g/mL}\)), that visibly prevents the growth of the bacteria. Every antibiotic has an established cutoff, known as a breakpoint, against a particular bacterial species. The lab compares the MIC value to this breakpoint to assign the S, I, or R status.
A result of Susceptible (S) indicates that the antibiotic should be effective when used at the standard recommended dose. A lower MIC value within the susceptible range implies that less drug is needed to inhibit the bacteria, suggesting a better response.
A result of Resistant (R) means that the bacteria is not inhibited by concentrations of the antibiotic achievable in the body. The Intermediate (I) classification indicates that the drug may be effective only if a higher dose can be safely administered or if the drug concentrates particularly well at the site of infection. This category is now often described as “Susceptible, Increased Exposure,” acknowledging that clinical success is possible by adjusting the dosing regimen.
Factors Influencing Treatment Selection
Translating the sensitivity report into a successful treatment plan involves integrating the lab data with several factors specific to the patient. While an “S” result is a strong recommendation, a provider must still consider the patient’s existing drug allergies and the potential for drug-drug interactions. The local patterns of resistance for common uropathogens, often published in an antibiogram, are also considered before starting treatment.
The location of the infection is a major determinant in drug choice, as different antibiotics concentrate differently in the body. For an uncomplicated bladder infection (cystitis), drugs that achieve high concentrations in the urine are preferred. For example, nitrofurantoin is highly concentrated in the urine, reaching levels over \(200 mu text{g/mL}\), while maintaining very low, often undetectable, levels in the bloodstream and tissues. This makes it effective for lower UTIs but unsuitable for a kidney infection (pyelonephritis), which requires an antibiotic that penetrates tissue effectively.
Conversely, an antibiotic like fosfomycin achieves very high, prolonged concentrations in the urine, making it suitable for cystitis. Fosfomycin also demonstrates good distribution into tissues, including the prostate and kidneys, which expands its utility to some complicated infections. The Intermediate (“I”) result becomes particularly relevant for treating UTIs because the high concentrations of antibiotics reached in the urine may overcome the bacteria’s mild resistance, even if the drug’s tissue concentration would be too low for other types of infections.