Fluorescent sensor provides low-cost diagnosis of cystic fibrosis

Cystic fibrosis is caused by two faulty copies of a gene that affects the flow of chloride in and out of cells, leading to damage to the lungs and digestive system. While there is no cure for cystic fibrosis, early detection is key to take steps to reduce fluid build up in the lungs and prevent malnutrition, allowing patients to live into their forties or fifties. It is often diagnosed by either a genetic test (which is expensive) or by a sweat test.

Currently, testing chloride levels in sweat is done by manual titration -- a labor-intensive technique that is subject to human error and can miss cases. But a new system created by researchers at Pennsylvania State University is based on a fluorescent dye that decreases in the presence of chloride, allowing the test to be automated. The research, described online in an August issue of the journal Chemical Science, is supported by the National Institute of Biomedical Imaging and Bioengineering (NIBIB), part of NIH.

"This is an important step towards faster, more reliable diagnosis," says Seila Selimovic, Ph.D., program director of the NIBIB program in Biological Sensors. "The new sensing technology is cheap and easy to apply and, if used as part of a point-of-care device may allow more clinics to provide early cystic fibrosis tests. That is a great thing for the developed world, but is a game changer for the economically developing world, since early intervention can save lives in dealing with this devastating and all too common disease."

To create the sensor, the researchers first developed a citrate-based dye that emits fluorescent light. In the presence of chloride, however, the amount of light given off by the molecule diminishes: the more chloride, the less fluorescence. After creating a detection system based on this principle, the team compared it to the chloride-detection method currently used in the clinic and found both tests gave similar results. But the new test can detect chloride over a wider range of concentrations and, because it's automated, it avoids the problem of human error.

Traditional tests such as ion-selective electrodes can also yield incorrect results because they can't distinguish between chloride and other similar ions, such as bromide and iodide, which can be elevated in the sweat as a result of certain medications or conditions. But, besides detecting chloride, the new fluorescence-based system can also tell the difference between three ions: chloride, bromide, and iodide. "Our methods are very sensitive and also very selective," says Jian Yang, Ph.D., professor of biomedical engineering at Pennsylvania State University and senior author of the paper. "So we are able to use one dye to distinguish three different ions in one biological fluid." The system could also be used in other diseases that can be diagnosed by altered chloride levels, such as metabolic alkalosis, Addison's disease, and amyotrophic lateral sclerosis (ALS), which present with abnormal amounts of chloride in urine, blood, and the fluid surrounding the brain, respectively.

Testing chloride levels can also help determine if a treatment is working, but getting a sweat test done currently requires a trip to a hospital or clinical lab. To get around this, the team created a prototype of their device that plugs into a cell phone to detect the light change. The portable version is easy enough to use that patients could monitor at home and skip the trip. "You just take the device, put the sweat in, and then you can use a cell phone to read the results," says Yang. Such a simple tool would enable small clinics and developing countries -- which can't afford the necessary personnel and expensive equipment for the current tests -- to reliably diagnosis the disease.

"We are stepping into the point-of-care device -- this kind of low cost, easy to use, home-use device," says Yang. "That can really make the detection of the sweat or even some other biological fluids like urine or serum much easier in the future because everyone can use it."