A Culture and Sensitivity (C&S) report is a specialized laboratory document that provides detailed information about a bacterial infection. Its primary purpose is to identify the specific microorganism responsible for the illness, a process known as the culture. The report then determines which antibiotic medications are effective against that particular pathogen, which is the sensitivity or susceptibility component. This diagnostic tool moves treatment away from generalized or “empiric” antibiotic use toward targeted therapy, which is a method that improves patient outcomes and helps combat the development of antibiotic resistance.
Understanding the Culture Results
The initial section of the report details the culture results, confirming whether a pathogen was isolated from the collected sample. The report first notes the specimen source (e.g., blood, urine, or a wound swab), as the site of infection guides the interpretation of the results and the subsequent choice of therapy.
The most prominent result is the Organism Identification, which lists the specific bacteria found, typically using its genus and species name, such as Escherichia coli or Staphylococcus aureus. If no microbial growth occurs after incubation, the report indicates “No Growth,” suggesting either no infection or a non-bacterial pathogen. The presence of an organism leads to the sensitivity test.
The Significance of Growth is crucial when interpreting the identification. Not all bacteria found in a sample cause infection, as the body contains many harmless organisms known as normal flora. A report may note “Mixed Flora” or identify a common skin bacterium, like Staphylococcus epidermidis, which can be a contaminant introduced during collection.
In specific culture types, such as urine culture, the lab relies on quantitative data like the colony count. This count measures the number of colony-forming units (CFUs) per milliliter to differentiate contamination from a true infection. A count greater than \(10^3\) CFUs per milliliter in urine is often considered significant in a symptomatic patient. The lab may also describe the growth as “heavy,” “moderate,” or “scanty,” which helps the clinician evaluate the bacterial load.
Deciphering the Sensitivity Panel
The sensitivity panel, also called the antibiotic susceptibility test, lists various antibiotics tested against the identified organism. This Antibiotic List details the drugs introduced to the cultured bacteria to predict whether the medication will be effective in the patient.
The core results are summarized using The S, I, R Key, which stands for Susceptible, Intermediate, and Resistant. An S (Susceptible) result indicates a high likelihood that the standard dose of the antibiotic will successfully treat the infection.
An R (Resistant) result means there is a high probability of therapeutic failure, signaling that the antibiotic should not be used because the organism has mechanisms to overcome the drug. The final category, I (Intermediate or Susceptible, Increased Exposure), indicates that a positive outcome is likely only if the drug concentration is increased, such as through a higher dose or if the drug naturally concentrates at the site of infection.
The S/I/R categorization is derived from a measurement known as the Minimum Inhibitory Concentration (MIC). The MIC is the lowest concentration of an antibiotic, measured in micrograms per milliliter, that prevents the visible growth of the bacteria in the laboratory test. This value is a quantitative measure of the organism’s tolerance to the drug.
The laboratory compares the organism’s MIC value to a pre-established “clinical breakpoint,” which is a threshold determined by regulatory bodies. If the MIC is lower than the breakpoint, the drug is typically labeled as Susceptible. Generally, a lower MIC value for an antibiotic suggests better activity against the organism; however, the MIC for one antibiotic cannot be directly compared to the MIC of a different antibiotic because their properties and breakpoints are different.
Clinical Application and Treatment Decisions
The physician uses the C&S report to transition the patient from initial empiric therapy—a broad-spectrum antibiotic started before results are ready—to a targeted definitive treatment. This involves synthesizing data from the organism identification and the sensitivity panel, focusing on an antibiotic that showed an ‘S’ result against the identified pathogen.
Selecting the optimal therapy requires looking Beyond ‘S’ and integrating several patient-specific factors. For example, a susceptible drug may not be appropriate if the Site of Infection is the central nervous system, as many antibiotics cannot effectively penetrate the blood-brain barrier. The physician must choose a drug that can reach a high enough concentration at the infection site to eradicate the bacteria.
Crucially, Patient Factors heavily influence the final choice, even among multiple ‘S’ options. These factors include:
- Known drug allergies.
- The patient’s age and the status of their kidney or liver function, as these organs process and eliminate the medication.
- Drug availability and route, determining whether an intravenous (IV) formulation is necessary for severe infections.
- Whether a convenient oral medication is sufficient.
A physician must also weigh the Implications of ‘I’ and ‘R’ results. A Resistant drug should be avoided entirely, as it will likely fail to clear the infection and prolong the patient’s illness. An Intermediate drug is generally avoided unless no other susceptible options exist, or the drug is known to reach high concentrations at the infection site (e.g., a urinary tract infection that is excreted primarily through the urine). The C&S report serves as a powerful guide, but the final treatment decision requires clinical judgment that incorporates the lab data with the patient’s overall health picture.