Now scientists from two New York universities believe they have the solution: a hand-held sensor that checks the health of a patient's immune system in seconds. At the moment it can take a week to get the same results back from the lab, "and that's if they don't get lost", says Glenda Gray, a consultant physician and head of perinatal HIV research at the University of the Witwatersrand in Johannesburg, South Africa.
The sensor measures the quantity of key immune cells called CD4+ cells in the blood. A gradual depletion of CD4+ cells, which orchestrate the immune response to tumours and infections, is a sure sign that HIV is damaging a person's health: some clinical definitions of AIDS say that once a person's CD4+ count falls below 200 cells per microlitre of blood, he or she has developed the condition.
Doctors rely on CD4+ measurements to decide when to start drug treatments and to gauge whether a patient is responding to them.
To make the device, researchers from Cornell University in Ithaca and the University at Albany coated electrodes with antibodies specific to CD4+ cells. When a small sample of blood is put onto a chip bearing these electrodes, the antibodies grab hold of the CD4+ cells. The captured cells then impede the flow of current across the electrodes, allowing the density of CD4+ cells to be calculated.
To check that only CD4+ cells were sticking to the electrodes the team added a label to blood samples consisting of a fluorescent dye tagged to a further set of antibodies specific for CD4+ cells. They then used an electron microscope to check which cells had been captured. By counting the captured cells, the researchers devised a scale linking electrical resistance with the density of cells in the blood (Biosensors & Bioelectronics, vol 21, p 696).
"It is the first step towards a hand-held, simple, inexpensive device that will measure the number of CD4+ cells in human blood without the need for extensive infrastructure," says James Turner, a biophysicist at the University at Albany, who led the research.