Microchip could bring CD4 cell counting to poorest countries

A new CD4 cell counting machine that uses cheap technology developed for digital cameras is just as accurate as conventional laboratory techniques and could make CD4 cell counting feasible in many resource-limited settings within a few years, according to a study published in the July edition of Public Library of Science (PLoS) Medicine.

The test was developed by a collaborative group of researchers at Partners AIDS Research Centre in Boston, Harvard Medical School and the University of Texas, and field tested in Botswana and the United States in HIV-positive adults and children.

CD4 cell counting today

Measuring CD4+ lymphocyte levels can help determine when individuals might need antiretroviral treatment or prophylaxis against opportunistic infections. However CD4 cell counting requires a machine called a flow cytometer, which typically costs anywhere from $50,000 to $150,000, a reliable electricity supply, reagents to process the blood samples for flow cytometry and specially trained staff. Most developing countries can only afford to equip central teaching hospitals with flow cytometers, and the number of machines available would be totally inadequate to meet the need for CD4 cell monitoring when tens of thousands of people require treatment.

Whilst the cost of antiretrovirals has been forced down by competition between generic and branded manufacturers, the cost of CD4 cell counting remains high because the currently available technology is unsuitable for use in developing countries.

CD4 cell counting tomorrow?

Research groups in the United States and Europe have therefore been funded to investigate methods of CD4 cell counting that can be delivered cheaply and used at the point of care with the minimum of training, in primary health care settings and district hospitals.

The system uses a standard epifluorescence microscope and commercial digital image processing software, but the authors of the study say that a system using a simple light-emitting diode (LED) and battery power would be feasible.

To test the accuracy of their method, researchers recruited an HIV-negative control group in the United States, HIV-positive pregnant women in Botswana and six infants. Total lymphocyte counts, CD4 cell percentages and CD4 lymphocyte counts were measured using the experimental technology and compared with the results of standard flow cytometry. A total of 61 adults and six children were enrolled.

The study found that the microchip system underestimated CD4 cell counts by approximately 50 cells/mm³, and had a sensitivity of 0.86 at CD4 cell counts below 250 cells/mm³. The degree of underestimation was greater at higher CD4 cell counts.

A particular advantage of the microchip method, say the authors, is its utility in monitoring the immune system in children, where CD4 cell percentages rather than absolute CD4 cell counts must be used. The microchip method measured CD4 cell percentages in close agreement to flow cytometry.

Several other advantages of the method are also described:

• The test uses volumes of blood derived from finger prick rather than venous puncture, minimising hospital waste and risk of health care worker injury.
• Very low reagent quantities and costs, improving supply chain management and running costs.
• Results are available within 15 minutes.