Another breath test determines whether a transplanted organ is taking or being rejected by a patient. If a patient's body is rejecting a new heart, the breath test will detect alkanes in the breath; alkanes are the by-products of chemical reactions set off by free radicals (unstable, cell-damaging molecules) that are produced by the body when it rejects donated organs.
There are many reasons breath tests could be the next frontier in medical diagnosis, Risby says, noting that they are relatively inexpensive to administer, safe (the only requirement is that a person is breathing), and noninvasive (there's no need to stick patients with needles or take a slice of tissue to biopsy).
A research team led by physicist Jun Ye at the University of Colorado at Boulder (U.C.B.) is currently testing a breath analyzer that scans for abnormally high levels of about 10 disease-related molecules within two minutes. Among them: ammonia (a marker for liver and kidney disease); acetone (an indicator of diabetes); and ethane, a potential cancer marker.
The liver and the kidneys normally filter ammonia from the blood. If these organs are not doing their jobs properly, ammonia builds up in the body and is exhaled at unusually high levels, says Michael Thorpe, a physicist in Ye's lab group. He notes that the team's breath analyzer also screens for elevated levels of acetone, a marker for diabetes, a disease in which cells cannot efficiently absorb glucose, their primary fuel source. When the body cannot get energy from glucose, it breaks down fat for fuel; acetone is one of the by-products of fat breakdown.
The experimental breath analyzer also searches for ethane. Ethane tends to accumulate in the body when cells are damaged by free radicals, which may lead to cancer, Thorpe says. But he says more research is needed to identify a collection of breath chemicals associated with cancer, given that ethane may also be linked to other conditions such as Alzheimer's disease and arteriosclerosis (hardening of the heart arteries that can lead to heart attacks).
Menssana Research in Newark, N.J., is currently conducting a clinical trial on a breath test for lung cancer. Founder and CEO Michael Phillips (profiled by Scientific American in 2003) has developed a technology that screens a set of 10 to 15 molecules typically released in the breath of lung cancer sufferers. Scientists suspect toxins (such as components of tobacco) activate a system of enzymes (proteins) that work to expel the poisons, producing chemical by-products released in the breath. (People with types of cancer other than lung also emit telltale cancer molecules from their bodies—in fact, dogs may be able to identify people with various types of cancer by sniffing their skin and breath, studies suggest).
Phillips says Breathalyzers could reduce the need for computerized tomography (CT) scanning, which is currently used to screen high-risk patients (smokers and former smokers) for lung cancer: The x-rays not only expose patients to potentially dangerous amounts of radiation, but they cost around $1,500 to $2,000 a pop to administer compared with $150 to $200 for a breath test. He notes that Breathalyzers would not replace CT scans, but that doctors could use them to determine whether CT scans are necessary to confirm – or dispute – results.
"I believe very strongly," Ddweik says, that breath analyzers are "the future of medical tests."