Saliva samples offer potential alternative to blood testing

Spitting into a cup or licking a diagnostic test strip could someday be an attractive alternative to having your blood drawn at the doctor’s office. Researchers have identified the largest number of proteins to date in human saliva, a preliminary finding that could pave the way for more diagnostic tests based on saliva samples. Such tests show promise as a faster, cheaper and potentially safer diagnostic method than blood sampling, they say.

“There is a growing interest in saliva as a diagnostic fluid, due to its relatively simple and minimally invasive collection,” says study leader Phillip A. Wilmarth, Ph.D., of Oregon Health & Science University School of Dentistry in Portland, Ore. “The same proteins present in blood are also present in saliva from fluid leakage at the gum line. It is considerably easier, safer and more economical to collect saliva than to draw blood, especially for children and elderly patients.”

The study of salivary proteins is described in the Oct. 11 print issue of the Journal of Proteome Research, a peer-reviewed publication of the American Chemical Society, the world’s largest scientific society.

While saliva tests won’t replace blood tests for all diagnostic applications, says Wilmarth, in the future they could prove to be a potentially life-saving alternative to detect diseases where early diagnosis is critical, such as certain cancers. Saliva collection also may be the only practical way to screen large numbers of patients in developing nations, the researcher adds.

Diagnostic assays using saliva are a relatively new but growing technology. This past spring, the FDA approved the first HIV test based on saliva rather than blood. Several other tests are in the pipeline for uses ranging from pregnancy testing to detection of chemicals such as alcohol and other drugs. One of the hurdles in developing new tests is a lack of understanding of the human proteome, or the study of large sets of proteins, particularly those that can serve as biomarkers for the presence of disease.

Most proteome studies have focused on specific tissues and human blood samples, but the current study represents one of only a few studies to date of the salivary proteome. “We’re just starting to map the saliva proteome,” Wilmarth says. “Not much is known yet, but more should be known in the near future.”

Using two-dimensional gel electrophoresis in combination with mass spectrometry, other researchers were able to identify up to 28 proteins in saliva, including 19 proteins only found in saliva and nine proteins also present in blood serum. The most important biomarkers for disease diagnosis are typically serum-derived proteins, the researcher adds.

In an effort to identify more serum proteins, which are a minor component of saliva, Wilmarth and his associates used a more sensitive analytical technique called two-dimensional liquid chromatography, combined with highly sensitive mass spectrometry. Using a single saliva sample from a healthy, nonsmoking male subject, the researchers were able to identify 102 proteins, including 35 salivary proteins and 67 common serum proteins. The study represents the first time the analytical technique has been applied to saliva, the researcher says.

“The number of serum proteins detected in this work is still far short of the number of proteins routinely seen in blood serum studies [800-1600 proteins], but it is a significant step toward identifying serum biomarkers in saliva,” Wilmarth says. Identifying all of the serum proteins present in saliva could take many more years, he estimates.

With advances in instrumentation, he predicts that the number of serum proteins identified in saliva will increase significantly, although it will probably never match the number found in blood, mainly because serum proteins are only a tiny part of saliva, described as a dilute, watery-solution containing electrolytes, minerals, buffers, as well as proteins.

Blood tests are a well-established, proven methodology, and it may take some time before saliva tests can become as reliable as serum tests, Wilmarth notes. “In the future, I think consumers can look forward to more saliva-based tests,” Wilmarth says. “It may make diagnostics as simple as licking the back of a test strip, mailing it in and getting your results. That’s a lot easier than getting stuck with needles and it’s potentially safer for health care workers.”