What are Culture and Sensitivity Tests?
Culture and sensitivity tests are diagnostic tests that are done in medical settings to determine the cause of a patient’s infection or abnormality. These tests are done by collecting a sample of tissue, urine, blood or other body fluids from the patient and then growing it in sterile lab conditions to identify the bacteria or other pathogen causing the infection.
The sensitivity test is then done to find out which antibiotics or treatments will be most effective for that particular strain of bacteria or pathogen. This article will explore how to interpret culture and sensitivity results.
Why are Culture and Sensitivity Tests Important?
Culture and sensitivity tests play a crucial role in medicine as they help doctors to identify the appropriate drug therapy for patients who are suffering from various infections. These tests help doctors to determine the exact bacteria that is causing an infection and which antibiotic will help treat it. This way, the patient receives the correct treatment and ensures that there are no complications that may arise due to an incorrect diagnosis or treatment.
Reading Culture Results
When interpreting culture results, there are several factors to consider such as;
Specimen Type and Quality
The first step in interpreting culture results is to confirm that the specimen collected is of good quality, and no contamination occurred during the collection process.
Normal Flora
The next thing is to determine the presence of normal flora that is often present in most body fluids. These bacteria are harmless and are naturally occurring in the body. They do not cause any disease or infection.
Pathogenic Organisms
Finally, the lab technician needs to identify the presence of any pathogenic organisms. These organisms are not naturally present in the body and can cause disease or infection.
Reading Sensitivity Test Results
Sensitivity test results are often presented in a table format with the names of the antibiotics listed in the row headings, and the different bacterial strains in the column headings.
Zone of Inhibition
The zone of Inhibition is a clear area that appears around the antibiotic disc on the agar plate. A larger zone of inhibition indicates a more effective antibiotic.
Interpreting Results
When interpreting the sensitivity test results, there are three possible outcomes:
- Susceptible (S): The bacteria is sensitive to the antibiotic and is likely to respond to that drug
- Intermediate (I): The bacteria is moderately susceptible to the antibiotic. A higher dose may be needed for it to be effective.
- Resistance (R): The bacteria are not susceptible to the antibiotic and will not respond to this drug. Another antibiotic may be needed.
Tips on Interpreting Culture and Sensitivity Test Results
Here are some tips for interpreting culture and sensitivity results:
Gram-Positive and Gram Negative Bacteria
Determine whether the bacteria is Gram-negative or Gram-positive. Antibiotics are usually classified based on their effectiveness against specific Gram-negative or Gram-positive bacteria strains.
Interpret Susceptibility Results Carefully
Check for any errors in the results, such as incorrect interpretations or mislabeling of the samples. It is essential to interpret susceptibility results carefully, especially if the bacteria is resistant to multiple antibiotics.
Evaluate the Patient’s Clinical Response
Evaluate the patient’s clinical response to determine if the selected drug therapy has been effective or not. If the symptoms persist, re-test and consider different treatment options.
Understand the Clinical Context
Interpret the results within the context of the patient’s clinical presentation, such as the type of infection, the severity of symptoms, and the patient’s overall health status.
Common Questions and Answers
1. What does “pathogenic organisms” mean?
Pathogenic organisms are disease-causing organisms that are not normally present in the body.
2. What is a sensitivity test?
A sensitivity test is a laboratory test that evaluates the effectiveness of different antibiotics against a particular strain of bacteria.
3. How are culture and sensitivity tests done?
Culture and sensitivity tests are done by collecting a sample of tissue, urine, blood, or other body fluids from the patient, growing it in sterile lab conditions to identify the bacteria or other pathogen causing the infection, and then testing the bacteria against different antibiotics to determine which antibiotic will be most effective.
4. What is the significance of culture and sensitivity tests?
Culture and sensitivity tests are significant in identifying the exact bacteria that is causing an infection and which antibiotic will help treat it. This ensures that the patient receives the appropriate treatment, and there are no complications that may arise due to an incorrect diagnosis or treatment.
5. Can culture and sensitivity tests be wrong?
Yes, culture and sensitivity tests can be wrong due to various reasons such as specimen contamination, lab error, or incorrect interpretation of the results.
References
- Clinical microbiology procedures handbook, 4th ed. Washington, DC: ASM Press; 2016.
- Forbes BA, Sahm DF, Weissfeld AS, editors. Bailey & Scott’s diagnostic microbiology, 12th ed. St. Louis: Mosby; 2007.
- Patel R, ed. Clinical microbiology procedures handbook, 3rd ed. Washington, DC: ASM Press; 2010.