Antimicrobial susceptibility testing (AST) methods and importance in treating infections

In modern medicine, it has become essential to determine the most appropriate treatment for bacterial and fungal infections to ensure complete recovery and reduce the risk of drug resistance. The Antimicrobial Susceptibility Test (AST) is a key tool that helps doctors assess the effectiveness of antibiotics against disease-causing microorganisms. This test aids in selecting the optimal treatment, enhancing the chances of recovery, and minimizing the use of ineffective medications.If you are experiencing persistent or recurrent infections, or if you are wondering how to determine the most suitable treatment for you, an AST test might be the ideal solution. In this article, we will explore how this test is performed, how to interpret its results, and its critical role in improving treatment outcomes and preventing antibiotic resistance. Follow along with Dalily Medical to learn everything you need to know about AST and how to optimize infection treatment using this innovative technique.

What is the Antimicrobial Susceptibility Test (AST)?

The Antimicrobial Susceptibility Test (AST) is a laboratory test used to determine how susceptible microorganisms like bacteria and fungi are to various antimicrobial drugs. The goal of this test is to identify the most effective drugs for eliminating or inhibiting the growth of pathogens. This test helps doctors select the best treatment for a patient based on the results.

Do I need to fast before taking this test?

Typically, AST does not require fasting. However, if the test is part of a comprehensive exam that includes other tests which may require fasting, you may be asked to fast for a short period. It’s always best to follow your doctor’s or laboratory's instructions regarding any specific preparation needed.

Can I stop taking my medications before the AST?

It is not recommended to stop taking your medications before the test unless instructed by your doctor. If you have started taking antibiotics or other medications that might affect the test results, it is preferable to collect the sample before starting the treatment. Discontinuing medication may affect the accuracy of the results and make it harder to identify the microorganism properly.

Can the test results be inaccurate?

Yes, several factors can affect the accuracy of the antimicrobial susceptibility test, such as:

Improper sample collection: If the sample is not collected correctly or is contaminated, the results may be inaccurate.
Interaction between antibiotics and microorganisms: Some microorganisms may be more resistant in certain environments, leading to inaccurate results.
Environmental influences or medications: Some drugs may affect the growth of microorganisms before the sample is collected, potentially altering the results.

Can I take antibiotics while undergoing the AST?

It is generally advised not to take antibiotics before the AST, especially if the sample will be collected for the susceptibility test. Taking antibiotics may affect the effectiveness of the test, reducing its accuracy and impacting whether the microorganisms are sensitive or resistant to the drugs. If you have started treatment before the test, inform your doctor to determine the appropriate course of action.

What happens if bacteria are resistant to antibiotics?

If the test results show that the bacteria or microorganism are resistant to common antibiotics, your doctor may prescribe a different antibiotic or use alternative medications, such as broad-spectrum antibiotics. In cases of severe resistance, you may require special treatment with new antibiotics or targeted medications, with close monitoring to ensure successful treatment.

Can I request the AST on my own?

Typically, the AST is ordered by a doctor after evaluating symptoms and diagnosing the condition. If you are experiencing recurrent or persistent infections, your doctor may recommend this test. In rare cases, a doctor may request an antibiotic susceptibility test based on your health condition and treatment needs.

How long does it take to get the AST results?

The AST results usually take 24 to 48 hours to process in traditional laboratories. However, some complicated infections may require a longer time to obtain results. In some laboratories using advanced technologies, results may be available sooner.

Does the type of infection affect the choice of the right antibiotic?

Yes, the type of infection and the specific microorganism causing it (bacteria, fungi, etc.) greatly influence the choice of the appropriate antibiotic. For example:

Respiratory infections may require specific antibiotics.
Urinary tract infections may need different medications based on the bacteria involved.

Based on the AST results, the most effective antibiotic is chosen to treat the causative pathogen.

Types of Tests Used in Antimicrobial Susceptibility Testing

Disk Diffusion Test
In this test, a sample is cultured on a suitable nutrient medium in a petri dish, and discs containing various antibiotics are placed on the surface. The area of inhibition around each disc is observed to determine how effective each antibiotic is against the microorganisms. The larger the inhibition zone, the more effective the antibiotic.
Broth Dilution Test
This test involves adding different concentrations of an antibiotic to a liquid medium containing the microorganism. The minimum inhibitory concentration (MIC) is determined, which is the lowest concentration of antibiotic that prevents bacterial growth. This test helps identify the most effective antibiotics for killing microbes.
Solid Dilution Test
Similar to the broth dilution test, but solid agar is used instead of liquid medium. The microorganism’s reaction to the antibiotic is observed based on changes in microbial growth on the agar.

What Does This Test Reveal?

Microbial Sensitivity to Antibiotics
The test determines whether bacteria or fungi are sensitive or resistant to a range of antibiotics.
- Sensitive: The antibiotic will be effective in eliminating the microorganism.
- Resistant: The antibiotic will not be effective, and alternative medications are needed.
Minimum Inhibitory Concentration (MIC)
The test helps to determine the lowest concentration of antibiotic that effectively inhibits the growth of the microorganism.

MIC is the minimum concentration of an antibiotic required to prevent bacterial growth. This measure is crucial for selecting the appropriate treatment.
Identification of the Best Antimicrobials
The test reveals which antibiotics, antifungal agents, or antiviral drugs are effective against the infection.

This aids doctors in selecting the most appropriate treatment based on the type of microorganism and the test results.
Antimicrobial Resistance
The test shows if the microorganism is resistant to any antibiotics or other drugs.

This is particularly important for infections caused by drug-resistant bacteria like MRSA or ESBL.
Preventing Drug Resistance
AST helps reduce the overuse of ineffective antibiotics, which lowers the risk of resistance development.

The test assists in choosing the right treatment and limits the use of antibiotics that might lead to resistant microorganisms.
Monitoring Complex Infections
In cases of chronic or complicated infections (e.g., infective endocarditis or deep wounds), the test is used to select appropriate long-term treatment.
Response to Treatment
If an infection does not improve after treatment, AST can be repeated to determine if the dose is inadequate or if there is resistance to the drugs.

The test can also assist in adjusting the dosage for optimal results.

Examples of What the AST Reveals:

Sensitivity of Staphylococcus aureus to an antibiotic like methicillin.
Resistance of Escherichia coli (E. coli) to an antibiotic like ceftriaxone.
The best antifungal treatment, such as fluconazole for a Candida infection.
Resistance of enterococci to traditional antibiotics.

How is the Antimicrobial Susceptibility Test (AST) Performed?

Sample Collection
The first step in the AST is to collect a sample from the affected body fluids or tissues, such as:
- Blood
- Urine
- Sputum
- Wound or eye discharge
The sample must be collected under sterile conditions to prevent contamination that could affect the results.
Isolation and Growth of Microorganisms
After the sample is collected, bacteria or the microorganism are cultured in a special medium, such as agar or broth, in a microbiology lab. The microorganisms grow, and their type is identified using specialized techniques like colorimetric methods or biological assays.
Preparation of Antibiotics
A set of discs or solutions containing varying concentrations of antibiotics or other drugs to test their effectiveness against the microorganism is prepared.
Performing the Test using "Disk Diffusion" Method
One of the most common methods is the Disk Diffusion:

Discs with antibiotics are placed on the surface of an agar plate where bacteria have grown. After incubation, the size of the inhibition zone around the discs is measured to determine how sensitive the bacteria are to each antibiotic.
Testing with "Broth Dilution" Method
In this test:

Different concentrations of antibiotics are added to nutrient solutions containing the microorganism.

After incubation, the lowest concentration of antibiotic that inhibits bacterial growth (MIC) is identified.
Result Analysis
Based on the size of the inhibition zone or the MIC, the bacteria are classified as follows:
- Sensitive: The microorganism is affected by the antibiotic.
- Resistant: The microorganism is unaffected by the antibiotic.
- Intermediate Resistance: The microorganism is weakly affected, and the efficacy of the treatment may be limited.

Types of Antimicrobial Susceptibility Testing

Disk Diffusion Test
Discs are placed on agar, and the inhibition zone around the disc is measured to determine antibiotic effectiveness.
Broth Dilution Test
Different concentrations of antibiotics are added to tubes containing the microorganism to identify the minimum inhibitory concentration.
E-test (Epsilometer Test)
Uses strips with gradient antibiotic concentrations to determine how sensitive a microorganism is to specific antibiotics.

Importance of the Antimicrobial Susceptibility Test (AST)

Selecting the Best Treatment: Helps the doctor choose the most effective antibiotic for the infection, speeding up recovery.
Preventing Antibiotic Resistance: Helps avoid ineffective antibiotics, reducing the risk of bacterial resistance.
Monitoring Chronic Infections: For recurrent or complex infections, it assists in understanding if the bacteria have developed resistance to previous treatments.

5. Providing Patient Information

Medical History: The doctor or laboratory should be aware of the patient’s medical history, including any medications being taken or the presence of chronic diseases like diabetes or immune system disorders.

Treatment History: It is important to know whether the patient has recently taken antibiotics, as this could influence the choice of the most appropriate antibiotic for testing.

6. Storage and Transport

Samples should be stored appropriately during transport to the laboratory. Some samples may require cold or frozen transportation to maintain their viability and accuracy.

Critical Time: Some samples must be transported quickly to the laboratory to prevent the microorganisms from being affected or dying during transport.

7. Adherence to Global Standards

The laboratory must adhere to global standards for antimicrobial susceptibility testing, such as those provided by the Clinical and Laboratory Standards Institute (CLSI) or the European Committee on Antimicrobial Susceptibility Testing (EUCAST).

8. Interpreting Results

Once the test is performed, the results should be analyzed carefully. In some cases, additional review by specialized doctors may be needed to determine the most appropriate treatment based on the results of the susceptibility test.

9. Maintaining Confidentiality

Patient data must be kept confidential during sample collection, testing, and diagnosis to ensure privacy and comply with ethical standards.

10. Follow-up Medical Consultation

After receiving the test results, the patient should follow up with their doctor to discuss the appropriate treatment plan based on the antimicrobial susceptibility test results and choose the most effective antibiotic.

Interpreting Antimicrobial Susceptibility Test (AST) Results

The Antimicrobial Susceptibility Test (AST) is used to determine how effective antibiotics are against the microorganisms causing the infection, such as bacteria and fungi. Interpreting the results helps doctors choose the most appropriate treatment based on the microorganism’s sensitivity or resistance to antibiotics. Here’s how to interpret these results:

Sensitive Results:
Interpretation: If microorganisms show a positive response to the antibiotic, where their growth is inhibited or suppressed clearly in the area around the disc or in the dilution medium, this means the antibiotic is effective against the microorganism.

What it means for the patient: If bacteria or fungi are sensitive to a particular antibiotic, it means this antibiotic is the best choice for treating the infection. It will be prescribed as the primary treatment.

Clinical Indicators: The treatment is expected to be effective in clearing the infection or reducing symptoms quickly.
Resistant Results:
Interpretation: If the microorganisms continue to grow or show resistance to the tested antibiotic, this means the antibiotic is ineffective against them.

What it means for the patient: This indicates that this antibiotic will not be effective in treating the infection, and it should be replaced with another antibiotic that shows effectiveness against the pathogen.

Clinical Indicators: The treatment will need to be changed to another antibiotic, or broad-spectrum antibiotics might be used.
Intermediate Sensitivity Results:
Interpretation: If the results are intermediate, it means the microorganisms may respond to the antibiotic but at higher concentrations or with a longer duration of treatment. The treatment may require higher doses or a longer course of therapy to be effective.

What it means for the patient: These results mean the antibiotic may work against the infection, but its effectiveness is not optimal. It might be used if other options are limited.

Clinical Indicators: The treatment may be successful in some cases, but the doctor may recommend adjusting the treatment or exploring other options.

How to Determine Results:

Disk Diffusion Method:
The sample is cultured on a petri dish containing a nutrient medium, and discs containing various antibiotics are placed.

The inhibition zone (the area free of microorganism growth) is measured to determine the effectiveness of the antibiotic.
- Large inhibition zone: Indicates that the microorganisms are sensitive to the antibiotic.
- Small or no inhibition zone: Indicates that the microorganisms are resistant to the antibiotic.
Broth Dilution Method:
In this test, increasing concentrations of the antibiotic are added to a liquid medium containing the microorganism.

The minimum inhibitory concentration (MIC) is determined, which is the lowest concentration of the antibiotic that stops microorganism growth.
- Low MIC: Indicates sensitivity of the microorganisms.
- High MIC: Indicates resistance of the microorganisms.
Solid Dilution Test:
Similar to the broth dilution method but performed in a solid medium (like agar).

The antibiotics are mixed with the agar in varying concentrations, and the microorganisms are grown to observe which concentration of the antibiotic inhibits growth.

How the Doctor Decides on Treatment Based on Results:

Based on the antimicrobial susceptibility test results, the doctor will make an appropriate treatment decision, often relying on the following points:

Choosing the Optimal Antibiotic:
If the bacteria are sensitive to a particular antibiotic, it will be prescribed as the primary treatment.

If the bacteria are resistant to a particular antibiotic, another antibiotic showing effectiveness against the microorganism will be selected.
Monitoring Infection Progress:
In some cases, the doctor may decide to monitor the patient’s condition regularly to check for the response to treatment, especially if the results are intermediate or if the patient has other underlying health conditions.
Modifying the Treatment:
If intermediate or resistant results are found, the doctor may need to adjust the treatment or add adjunct medications to improve treatment effectiveness.
Combating Drug Resistance:
If resistance is detected, the doctor will decide whether to change the treatment immediately or in the future. This helps prevent recurrent infections and reduces the development of resistance.

Causes of Antibiotic Resistance

Excessive Use of Antibiotics: Improper use of antibiotics or not completing the prescribed course can lead to the development of resistance in microorganisms.
Overuse in Veterinary Medicine: The use of antibiotics in animal farming can lead to resistant bacteria being transferred to humans.
Infection Spread Without Proper Treatment: If infections are not treated with the appropriate antibiotics, bacteria may develop resistance to available drugs.

Tips for Preventing Antibiotic Resistance

Adhere to Prescribed Doses: Antibiotics should be taken exactly as prescribed by the doctor, and the treatment should not be stopped prematurely.
Don’t Use Antibiotics for Viral Infections: Antibiotics should not be used to treat viral infections like the common cold or flu.
Consult a Doctor Before Using Antibiotics: Do not use antibiotics without a prescription from a doctor.
Keep Medical Records: This can help the doctor identify the most appropriate treatments if you've had susceptibility tests in the past.

Tips for the Patient After an Antimicrobial Susceptibility Test (AST)

Complete the Prescribed Treatment: The patient should adhere to the prescribed treatment completely, even if they feel better, to avoid the recurrence of the infection or bacterial resistance.
Report Any Side Effects: If any unexpected side effects occur, the patient should contact the doctor immediately to adjust the treatment if necessary.
Do Not Use Antibiotics Without a Prescription: The patient should avoid using antibiotics indiscriminately or using someone else's medication.
Follow the Doctor's Instructions on Nutrition and Daily Habits: The patient should follow the doctor’s advice on how to take the medication (with or without food) and drink sufficient fluids.
Monitor the Progress and Follow Up with the Doctor: The patient may need to follow up regularly with the doctor to ensure symptoms improve or confirm the bacteria have been eradicated.
Prevent Future Infections and Antibiotic Resistance: Regular handwashing with soap and water is one of the best ways to prevent bacterial infections.
Understand the Prescribed Antibiotics: The patient should know the type of antibiotic prescribed, its properties, and the bacteria it is intended to treat.
Don’t Ignore Mild or Ongoing Side Effects: If side effects persist and are bothersome, the patient should inform the doctor to adjust the treatment.
Prevent Multiple or Chronic Infections: The patient should avoid sharing medications with others and ensure they keep up with preventive vaccinations.
Mental and Physical Rest During Treatment: Rest is essential for the healing process and to improve the body’s immunity during treatment.