In vitro diagnostics - CD4+T-Cell counting technology

7 September 2004 | Q&A

Promote and facilitate access to safe, reliable and appropriate in vitro diagnostic technologies and laboratory services in an equitable manner

What are CD4 T-cells?

CD4+ T-cells are also known as helper T-cells and act as an co-ordinator of the immune response, unfortunately, CD4+ T-cells are also the main targets of HIV. HIV destroys infected CD4+ T-cells and leading to an overall weakening of the immune system.

What is the CD4+ T-cell count?

Lower numbers of circulating CD4+ T-cells indicates a weakening of the immune system and advancement in the progression of HIV disease. The CD4+ T-cell count can also be indicative of the success or failure of anti-retroviral therapy (ARV).

How are CD4+ T-cells enumerated?

CD4+ T-cell counts are performed using manual and automated methods.

Manual

Microscopy

  • Optical or fluorescence microscopy
  • Requires a centrifuge and other laboratory consumables e.g. beads
  • Designed for low sample throughput laboratories in resource-limited settings
  • Currently marketed products include; Cyto-Spheres [Coulter Corporation, USA] and Dynabeads® [Dynal Biotech, Norway]
Automated Systems

Flow Cytometery

  • Gold Standard for accuracy, precision, reproducibility
  • Expensive and technically demanding, requiring extensive technician training
  • Capable of high sample throughput in well-resourced laboratory
  • Can be used for other laboratory applications
  • Requires regular equipment maintenance

Dedicated Cytometer

  • Cytometer used exclusively to perform CD4+ and CD8+ T-cell enumeration
  • Simpler and cheaper than flow cytometers but lacking in general laboratory versatility
  • Currently marketed products include; FACSCount™ [Becton Dickinson, USA]

What is HIV-1 viral load?

HIV-1 viral load refers to the number of viral particles found in each millilitre. The more HIV-1 viral particles in the blood, the faster the CD4+ T-cells are likely destroyed and the faster the progress toward AIDS.

How are CD4+ T-cell counts and HIV viral load used?

CD4+ T-cell counts are used, together with the Viral Load test, to get a complete picture about how the immune system is fighting the virus. As HIV reproduces within the body, the viral load increases and HIV destroys the CD4+ T-cells and thus lowers the amount of cells present. Generally, the higher the HIV viral load, the more CD4+ T-cells are being destroyed. The goals are to keep CD4+ T-cell count high and viral load low.

  • The number of CD4+ T-cells and virus levels will guide a patient and their doctor in deciding when to start anti-viral treatment.
  • Monitoring CD4+ T-cell counts and viral loads during treatment helps the doctor assess how well the patient responds to their prescribed treatment.

How is Viral Load measured?

The viral load test should consistently detect and measure virus levels down to 50 copies/mL, have a high specificity and provide reproducible results. The technologies used are advanced and very sensitive for measuring the amount of HIV genetic material present in the blood.

What is an ELISA test?

Enzyme-Linked Immunosorbent Assays (ELISAs) are the most widely used type of assay. They have evolved from viral lysate tests to tests containing recombinant protein and synthetic peptide antigens:

  • They have high sensitivity and specificity.
  • ELISAs are designed specifically for screening large numbers of specimens at a time, making them suitable for use in surveillance and centralized blood transfusion services.
  • As ELISAs require sophisticated equipment and skilled technicians to perform the tests, their use is limited to certain circumstances.

What is a Simple/Rapid test?

Simple/Rapid tests are designed for use where a preliminary screening test result is required and are especially useful in resource-limited countries

  • High quality, easy-to-use tests for use in resource poor settings.
  • Tests based on agglutination, immuno-dot, immuno-chromatographic and/or immuno-filtration techniques.
  • Quick and easy to perform – 10 minutes to 2 hours – and require little or no additional equipment.
  • Are designed for use with individual or a limited number of samples, which make them more economical then ELISAs in low throughput laboratories.
  • Possibility to store at room temperature for extended period of time.
  • Same-day results provide timely treatment interventions.

What is the difference between a ELISA and a Simple/Rapid test

ELISAs are highly sensitive and specific, and are able to detect HIV-1/ HIV-2 and variants. They require sophisticated equipment that must be regularly maintained, a constant electricity supply and skilled technicians. They are really not suitable for small laboratories, but for testing large numbers of samples per day, as well as in blood banks or for surveillance studies. The Simple/Rapid tests are better for emergency testing, and in smaller laboratories with low numbers of tests per day.

What is a confirmatory assay?

Confirmatory Assays are used to confirm whether specimens found reactive with a particular screening test contain antibodies specific to the virus they have been tested for.

  • Expensive tests, first generation tests have proven to be prone to producing large numbers of indeterminate results.
  • LIAs ( Line immune-assays ) have been developed which in general produce fewer indeterminate results.
  • Studies have shown that combinations of ELISAs or Simple/Rapid assays can provide results as reliable as the confirmatory assays at a much lower cost.

What is the "window-period" and how long is it?

This is the time between original infection with HIV and the appearance of Detectable antibodies to the virus, normally a period of about 14-21 days.

Is urine or saliva feasible to use as a source of HIV detection?

Tests are currently available for anti-HIV testing using urine and saliva specimens. However, the level of antibodies in these specimen types is lower than serum or plasma. Therefore, rigourous sample collection conditions, testing procedures, result validation and interpreation must be followed.