Background

Bacteria play a role in the spread of diseases. It is important to be able to grow these microorganisms, in laboratories to understand which antibiotics work against them and which ones they are resistant to. Healthcare providers use susceptibility testing (AST) to decide on the treatment, for each patient. This test also assesses the effectiveness of healthcare services offered by hospitals, clinics and government programs in fighting and preventing illnesses. Various clinical labs use methods based on the tests they conduct. 

The Minimal Inhibitory Concentration (MIC) serves as a measure in evaluating how sensitive a microorganism is to an agent.  

Another method used to determine sensitivity is the disk diffusion technique. This process entails applying disks infused with antibiotics onto an agar plate infected with the patient’s organism. Over an incubation period the antibiotics diffuse outward from the disks and the diameter of the zone where bacterial growth is inhibited is measured. 

To determine how sensitive the organism is, to medication the size of this area is compared to measures. A new version of this examination involves a diffusion gradient called an E test. This technique uses a strip that absorbs with a gradient of levels along its length. When placed on an agar plate containing the microorganism being studied the antibiotic moves into the substance. 

A study by Webber et al. Suggests that after 6 hours of incubation results from resistance testing using disk diffusion are comparable to those obtained after 24 hours. Tests conducted for 24 hours. Those for 6 hours showed agreement at a rate of 96.7% while tests performed over 10 hours and those, over 24 hours had concordance at the percentage. 

Indications/Applications

Testing for susceptibility is crucial whenever a sample is collected from a suspected site of infection. When an infection is active doctors can utilize this data to select the antimicrobial therapy. Understanding the bactericidal concentration (MBC). Mic can also help in determining the right dosage. 

Clinical Significance 

In some instances, it may be necessary to provide a sample because a negative culture result from a lab does not always mean that the patient is free from infections. The results of culture and susceptibility testing play a role in guiding healthcare providers decisions regarding use and treatment duration. In cases where infections are caused by known or severe pathogens this type of testing may not be mandatory. Factors such, as treatment duration, disease complexity and laboratory availability can impact when test results become available. For instance, if a common urinary tract infection only lasts three days, susceptibility and culture test results may not significantly alter the treatment plan.  

Reference Range

Antimicrobial susceptibility testing helps identify which antibiotics a bacterium or fungus can be effectively treated with. This testing is often used in conjunction with culture and Gram stain tests providing results. By conducting susceptibility tests physicians can select the appropriate medication and dosage for challenging infections. 

Reports typically include an interpretation and a quantitative value in µg/mL.  

Results are usually classified into categories such as Susceptible (S), Intermediate (I), Resistant (R), Sensitive dose dependent (SDD) and no susceptible (NS).  

Interpretation

There is no value in these test results, it is generally assumed that the baseline is “susceptible (S) ” except in cases involving resistant antibiotics. 

Collection And Panels

Sample type: blood, cerebrospinal fluid, urine or other body fluids 

Sample container: swabs, sterile cups for liquids, vacutainers for fluid or bronchoalveolar traps. 

The collection process should be done in conditions that minimize contamination whether in a room or a clean office setting. Keep in mind that complete sterility may not be achievable during an infection. The goal is to isolate the specific bacterium causing the infection. Test results are typically available within two days from the lab; however not all tests include both culture and susceptibility results and Gram stains are not always included. 

Modifying factors: 

Issues like sample quality using colonies for testing inconsistent bacterial suspension, preparation standards and deep agar plates can lead to inaccurate test results. It’s crucial to monitor and address these issues to prevent wastage of resources and time.  

Providing alternative support therapy, for medications that’re not in line, with the hospitals approved list may not be advantageous when also offering laboratory services. 

References

https://www.ncbi.nlm.nih.gov/books/NBK539714/ 

https://www.mdpi.com/2079-6382/11/4/427