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What is a Cryptococcal Antigen test?
A cryptococcal antigen test detects the presence of the fungus Cryptococcus in blood for diagnosing cryptococcal infection.

Cryptococcus usually does not cause any infection in a healthy host. It is categorised as an opportunistic infection, ie, it affects an individual with compromised immunity.
The spores of Cryptococcus are generally found in soil and bird droppings. When these spores enter the body of an immunocompromised person, it causes cryptococcal meningitis.

In patients with human immunodeficiency virus (HIV) infection, with a lower CD4-positive cell count, this meningitis is generally fatal.

As a screening test for the early diagnosis of cryptococcal infection, a cryptococcus test helps to prevent mortality due to meningitis.

  1. Why is a Cryptococcal Antigen test performed?
  2. How do you prepare for a Cryptococcal Antigen test?
  3. How is a Cryptococcal Antigen test performed?
  4. What do a Cryptococcal Antigen test results indicate?

A cryptococcal antigen test is performed for diagnosing a cryptococcal infection or when an immunocompromised individual has the following symptoms of meningitis:

Cryptococcal meningitis or cryptococcal infection is usually suspected in individuals with a compromised immune system due to HIV infection, liver cirrhosis, uncontrolled diabetes, lymphoma, leukaemia or sarcoidosis or in case of organ transplant.
In addition, a cryptococcal antigen test is also performed as a screening test in immunocompromised individuals with the following criteria:

  • HIV/acquired immunodeficiency syndrome (AIDS) 
  • Initiation of antiretroviral therapy (ART) for HIV 
  • CD4-positive cell count of <200
  • Oral thrush (candidiasis)

No special preparations are needed for this test.

Blood or cerebrospinal fluid (CSF) is used for evaluating the presence of cryptococcal antigen.

Blood test: Blood collection is a simple procedure in which a blood sample is drawn from a vein in the arm using a needle.

There is a minimal risk of pain, light-headedness and bruising at the site of injection associated with the test. However, at most times, these symptoms disappear quickly. Rarely, an infection may occur at the site of blood withdrawal.

CSF collection: Collection of CSF may require a lumbar puncture (spinal tap), ie, removal of spinal fluid from the spine using a needle. A local anaesthetic agent is injected before the procedure; a spinal needle is inserted in the lower back area (lumbar), and about 5-10 millilitres (mL) of the sample is collected in four vials. After sample collection, the needle is removed, and the area is covered with a bandage. 

Lumbar puncture carries a risk of bleeding into the spinal canal or around the brain (subdural hematoma) and pain or discomfort during the test or infection at the site of puncture. CSF may leak from the punctures site (and can cause headache).

Normal results: Absence of cryptococcal antigen in patient samples indicates normal results, which means either they don't have a cryptococcal infection (meningitis) or some other pathogen is the cause of meningitis (in an immunocompromised patient).

Abnormal results: Presence of cryptococcal antigen indicates that the cause of meningitis is Cryptococcus.

Cryptococcal meningitis has a very high death rate as the disease progresses rapidly in a short span. However, according to the Center for Disease Control, this mortality can be prevented in patients who have already been diagnosed with HIV or have started ART. 

The World Health Organisation recommends the implementation of a cryptococcal screening program as a strategy to prevent death. This screening is mainly performed in individuals with low CD4-positive cells (<200 cells), advanced HIV infection or developed AIDS and those on ART or who have developed oral thrush (candidiasis) since these individuals are more prone to develop cryptococcal meningitis.

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.

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References

  1. Center for Disease Control and Prevention [internet], Atlanta (GA): US Department of Health and Human Services; Cryptococcal Screening
  2. Center for Disease Control and Prevention [internet], Atlanta (GA): US Department of Health and Human Services; Fungal Meningitis
  3. Pennstate Hershey. [Internet] Milton S Hershey Medical Center, U.S. Cryptococcosis
  4. UF Health. [Internet] University of Florida Health, Florida, U.S. Cryptococcosis
  5. Gerald L Mandell, John E Bennett, Raphael Dolin. Principles and Practice of infectious diseases 8th Edition U.S : Elsevier Publishing, 2015, Chapter 264
  6. Labtests Online Australasia. [Internet] Department of Health, Australian Government. What to expect when having a blood test?
  7. Pennstate Hershey. [Internet] Milton S Hershey Medical Center, U.S. CSF glucose test
  8. Children's Minnesota. [Internet] Children's Hospital and Clinics of Minnesota Cryptococcus Antigen