What is Yeast Susceptibility test? 

Yeast is a type of fungus that makes an important part of the natural flora of the body. However, an imbalance in the yeast population can lead to a condition called candidiasis (vaginal yeast infection).

A sensitivity or susceptibility test for microbes is used to determine the ability of a drug to kill a particular microorganism. Since more than one strain of a particular microbe may cause infection, this test helps select a suitable drug that can help treat an infection (bacterial, fungal or yeast) in an individual.

The yeast sensitivity test is a type of antimicrobial susceptibility panel test (also known as antifungal susceptibility test [ASFT]) that determines the effectiveness of antifungal medications at different concentrations on the growth of yeast.

It aids in determining the minimum inhibitory concentration (MIC), which is the lowest concentration of the antifungal drug at which there is complete inhibition of fungal growth.

Some of the commonly used drugs for the test are amphotericin, itraconazole, fluconazole and 5-flucytosine.

The AFST is usually performed on Candida species since candidiasis is the most common yeast infection in humans. Candida may cause superficial as well as systemic diseases. It is the fourth leading cause of bloodstream infections originating in hospitals. Among all the Candida species, Candida albicans is highly prevalent and shows resistance (growth even in the presence of drug) against the azole group of antifungal drugs. Other species tested for antifungal susceptibility are C. glabrata, C. guilliermondii, C. krusei, C. parapsilosis and C. tropicalis.

  1. Why is Yeast Susceptibility test performed?
  2. How do you prepare for Yeast Susceptibility test?
  3. How is Yeast Susceptibility test performed?
  4. What do Yeast susceptibility test results mean?

Your healthcare practitioner may order this test in the following situations:

  • Refractory (stubborn) oropharyngeal Candida infections, which do not respond to standard treatments. Some of the symptoms of oropharyngeal Candida infection are:

    • Redness or soreness in the throat or mouth 
    • Loss of taste 
    • Pain while eating and swallowing 
    • Cracking and redness at the corners of the mouth 
    • White patches on inner cheeks, tongue or roof of the mouth 
    • Difficulty in swallowing
  • Invasive infections caused by Candida species, when the benefit of azole antifungals is not certain. In an invasive infection, Candida enters the bloodstream and can spread to the brain, heart, bones and other body parts.
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This test does not require any special preparation.

Depending on the site of infection, the procedures for sample collection are as follows:

  • Blood sample: Your healthcare practitioner will draw blood from a vein in your arm. 

    • He/she will tie a tourniquet (band) around your upper arm and ask you to make a tight fist. This will help him locate a vein for sample collection.
    • Once the right vein is located, the doctor will insert a sterile needle attached to a small test tube into it, and will draw the required amount of blood. 
    • He/she will remove the tourniquet and the needle, label the sample and send it to the lab for testing.
  • Urine sample: For collecting a urine sample, you’ll be provided with a sterile container along with the following instructions: 
    • Write your name and date of sample collection on the container.
    • Before collecting the sample, wash your hands properly.
    • Now, start to pee into the urinal. 
    • Collect a mid-stream sample, i.e. avoid taking the first and the last part of the urine. 
    • After collecting the sample, close the lid tightly and submit the sample to the laboratory for testing.
  • Vaginal swab:
    • Your doctor will ask you to lie on the examination table with your legs in stirrups. 
    • He/she will insert a speculum into your vagina to open it up for inspection and will insert a cotton swab about 2 inches into the opening of your vagina.
    • The doctor will then gently rotate the swab for about 30 seconds so that the moisture from the walls of the vagina is absorbed by the swab. 
    • After collecting the sample, the doctor will withdraw the swab from your vagina and transfer it into a container.
    • The sample will be immediately sent to the laboratory for testing. 
  • Sputum sample:
    • Your healthcare practitioner will give you a container and will ask you to cough up sputum and submit it for testing.
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Based on the MIC, the results for some of the drugs for Candida species are given in the table below:

Drug MIC range in microgram per millilitre (mcg/mL)
Susceptible  Resistant
Fluconazole  < or = 8 >32
Itraconazole  < or = 0.125 >0.5
Flucytosine < or = 4 >16

The MIC range between susceptible and resistant is considered as intermediately susceptible. Based on the test results, your doctor will choose the appropriate antifungal drug for the treatment of the infection.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational point of view and is in no way a substitute for medical advice by a qualified doctor.

References

  1. Centers for Disease Control and Prevention [internet]. Atlanta (GA): US Department of Health and Human Services; Candidiasis
  2. Penn Medicine: University of Pennsylvania [internet]. Philadelphia, Pennsylvania, United States; University of Pennsylvania Medical Center Guidelines for Antibiotic Use: Antimicrobial Susceptibility Testing
  3. Lab Tests Online [Internet]. Washington D.C. American Association for Clinical Chemistry; Antibiotic Susceptibility Testing
  4. Merck Manual Professional Version [Internet]. Kenilworth (NJ): Merck & Co. Inc.; c2019. Susceptibility Testing
  5. Bayot ML, Bragg BN. Antimicrobial Susceptibility Testing. [Updated 2019 Mar 18]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan
  6. Pappas PG, Rex JH, Sobel JD, et al. Guidelines for treatment of candidiasis. Clin Infect Dis. 2004;38(2):161–189. PMID: 14699449.
  7. Sanguinetti M, Posteraro B. New approaches for antifungal susceptibility testing. Clin Microbiol Infect. 2017;23(12):931–934. PMID: 28377311.
  8. Marak Munmun B. and Dhanashree Biranthabail. Antifungal Susceptibility and Biofilm Production of Candida spp. Isolated from Clinical Samples. International Journal of Microbiology. 2018 Oct; (7495218).
  9. University of Rochester Medical Center [Internet]. Rochester (NY): University of Rochester Medical Center; Tinea Infection
  10. Hinkle J, Cheever K. Brunner & Suddarth's Handbook of Laboratory and Diagnostic Tests. 2nd Ed, Kindle. Philadelphia: Wolters Kluwer Health, Lippincott Williams & Wilkins; c2014. Fungal Serology; 312 p.
  11. Merck Manual Consumer Version [Internet]. Kenilworth (NJ): Merck & Co. Inc.; c2018. Candidiasis (Yeast Infection)
  12. Mount Sinai [Internet]. Icahan School of Medicine. New York City (NY), U.S.A. Skin or nail culture
  13. National Heart, Lung, and Blood Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; Blood Tests
  14. Mônica Bastos de Lima Barros, Rodrigo de Almeida Paes, and Armando Oliveira Schubach. Sporothrix schenckii and Sporotrichosis. Clin Microbiol Rev. 2011 Oct; 24(4): 633–654. PMID: 21976602.
  15. National Health Service [internet]. UK; How should I collect and store a pee (urine) sample?
  16. Harvard Health Publishing. Harvard Medical School [internet]: Harvard University; Testing for Vaginitis (Yeast Infections, Trichomonas, and Gardnerella)
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