Over 35 million adults are currently living with HIV/AIDS, and 3 million of these people have started life-saving therapy in the last five years.  A CD4 count is a critical blood test used by doctors to monitor patients and to make decisions regarding treatment.  Flow cytometry is the most accurate way to obtain a CD4 count, but the cheapest flow cytometer costs around $35,000, not including substantial maintenance fees.  The technique also requires skilled technicians, and these people are rare in many countries.  In Malawi, for example, there are 300,000 patients with HIV and only 13 lab technicians capable of performing flow cytometry.  An affordable, accurate, and portable device for counting CD4-positive T cells would significantly improve healthcare in many resource-limited countries, especially in Africa. 

Researchers in the laboratory of William Rodriguez are working to create such a device.  Their simple machine filters whole blood through 3-um pores, and then passes the blood through a chamber coated with anti-CD4 antibodies.  Both CD4-positive T cells and CD4-positive monocytes stick to the antibodies, but the larger monocytes can be washed out of the chamber by adjusting the flow rate.  A lysis buffer is then sent through the chamber, causing the stuck CD4-positive T cells to release electrolytes and other charged molecules.  These molecules increase the conductance of the buffer, and this change can be measured electrically.  Thus, the number of CD4-positive T cells can be counted.  The measurement is not perfect, but it is accurate enough to provide useful diagnostic information.  If produced and distributed on a large scale, the devices being designed in the Rodriguez lab could fundamentally alter the way in which HIV is treated in countries lacking highly developed healthcare infrastructures. 

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