Most of the substances found in blood can also be detected and analyzed from saliva. But biomarkers in saliva are at much lower concentrations, challenging researchers to develop more sensitive biosensors for a variety of diagnostics.
Who wouldn't prefer to avoid syringes and blood draws given an easier alternative?
Now researchers working via a $12.5 million grant from the National Institute of Dental and Craniofacial Research have developed an ultra-sensitive optical protein sensor, a first for a salivary diagnostic test.
Chih-Ming Ho, PhD, a scientist at the University of California Los Angeles (UCLA), and his team report that the sensor can be integrated into a specially designed lab-on-a-chip, or microchip assay, and preprogrammed to bind a specific protein of interest, generating a sustained fluorescent signal as the molecules attach.
A microscope then reads the intensity of the fluorescent light – a measure of the protein's cumulative concentration in the saliva sample – and scientists gauge whether it corresponds with levels linked to developing disease.
"The biggest the challenge is that the biomarkers in saliva are at very low concentrations compared to blood, so we need a highly sensitive sensor," Ho told Medical Device Daily. "Because saliva contains all the same markers found in blood, in principal, you could detect many of the diseases that you can in the blood. Our first task is to try to detect oral cancer and we have proved that it is possible."
In their early experiments, the researchers primed the optical protein sensor to detect the IL-8 protein, which at higher than normal concentration in saliva is linked to oral cancer. Using saliva samples from 20 people – half healthy, the others diagnosed with oral cancer – the sensor correctly distinguished in all cases between health and disease.
What's unique about the sensor is that it achieved a limit of detection for IL-8 that is roughly 100 times more sensitive than the current blood-based Enzyme-Linked ImmunoSorbent Assay (ELISA) tests, the "gold standard" technique to measure protein in bodily fluid.
The limit of detection (LOD) refers to a sensor's ability to distinguish the lowest concentration of a protein or other target molecule apart from competing background signals.
Ho and his team wanted their sensor to have the kind of extremely high sensitivity that some future salivary diagnostic tests are sure to require. To do that, and extend the lower limit of the LOD, they had to turn down the noise, also known as the various ambient molecules in the saliva sample that typically stray to the sensor and bind which creates the visual equivalent of static and can lead to false positives.
"It's really a matter of figuring out how to reduce the background noise and make the signal stand above the noise," Ho said.
So the group filtered out other wavelengths of light that might pollute the signal. Then they needed a better optical tool to collect the light and decided to use a confocal microscope, which gathers all of the fluorescence and has the added advantage of locking onto a single slice of light and then viewing it from multiple points. They were able to identify a location that has maximum signal-to-noise ratio, ultimately extending the LOD.
Ho said his group plans to move into clinical tests with healthy people within a few months. That first test will likely include up to 200 people, but the study design has not been finalized.
The intellectual property is owned by UCLA; the technology was developed in Ho's lab. As yet, there is no talk of outlicensing the work to a company.
Whoever develops the saliva test, Ho said it will be a relatively inexpensive technology.
"It will be less expensive than blood tests," Ho said, adding that the test could be easily incorporated into dental visit check-ups. "Whenever a person goes for a teeth cleaning, the dentist could do this test."
The test is anticipated to take well under an hour to complete.
"If everything goes right, I can see this on the market in, maybe, five years," Ho said.
Saliva-based point-of-care diagnostics aren't new, but the refinements continue and applications broaden. Many are focused on genetic risk factors. For example, earlier this year MDD reported that Smart Genetics (Philadelphia), a direct-to-consumer genetic testing firm, launched an Alzheimer's test to be used at home.
The $399 test kit can be ordered online. Customers then send it with a sample of their salvia to be tested at a CLIA-certified lab. Within about three weeks they are given the results over the phone by a genetic counselor and they receive a lengthy report detailing what their genetic and non-genetic risk factors mean and what they might want to do about it (MDD; June 4, 2008).
There are other oral cancer tests in the works too. Researchers at Sandia National Laboratories (Livermore, California) have developed a desktop, phone-sized, point-of-care device, called the IMPOD, to test saliva, which they believe can one day be a simple device to test for oral cancer and perhaps be further reduced in size to a hand-carried modality (MDD, April 18, 2007).