Dear Doctor: How accurate is the blood test for irritable bowel syndrome? My gastroenterologist says that one exists but that he doesn't feel it has been proved effective.

Dear Reader: Irritable bowel syndrome (IBS) is a disorder characterized by abdominal pain in association with altered bowel movements. Sufferers can have diarrhea or constipation, or they can alternate between the two. IBS affects about 11 percent of the population and accounts for anywhere from 25 to 50 percent of all visits to gastroenterologists. It affects women more than men, with 14 percent of women affected and 9 percent of men. IBS is what's called a "functional bowel disorder," meaning that it's not related to inflammation but to how the bowels contract and relax.

To diagnose IBS, a doctor must rule out other causes for a patient's symptoms. But newer blood tests can help. One assesses levels of an antibody to the cell membrane protein vinculin; and the other assesses levels of an antibody to a toxin produced by the gut (anti-CdtB). A 2015 study looked at 2,375 people with diarrhea-associated IBS and compared their blood levels of these two antibodies to the levels in people with inflammatory bowel disease (IBD), celiac disease or no bowel issues. In those with IBS, the authors found elevated anti-CdtB and anti-vinculin levels in 43.7 and 32.6 percent of patients respectively. In other words, these are not very sensitive tests for detecting IBS.

Still, a positive test can point doctors in the right direction. In this study, 91.6 percent of those who tested positive for anti-CdtB had diarrhea-associated IBS, while 83.8 percent of those who tested positive for anti-vinculin had the condition. People with inflammatory bowel disease had similar antibody levels as healthy adults. This means that, if someone does test positive for the high antibody levels, they likely have diarrhea-associated IBS.

A 2017 study additionally evaluated the antibody levels in people with constipation-associated IBS and people with IBS that fluctuated between constipation and diarrhea. In this study, a positive result for high levels of either anti-CdtB or anti-vinculin was found in 58.1 percent of people with diarrhea-associated IBS; 44 percent of those with fluctuating IBS; 26.7 percent of those with constipation-associated IBS; and 16.3 percent of healthy controls. The authors concluded that the results of antibody-level testing in people with constipation-associated IBS was not much different than it was for healthy people.

It's still important to rule out other potential causes for IBS symptoms of abdominal bloating and altered bowel movements. This can be done with blood tests for C-reactive protein and celiac-disease antibodies; a stool test assessing potential bacterial causes; or a stool calprotectin test, which is positive in inflammatory bowel disease.

Nonetheless, the antibody tests for CdtB and vinculin can help solidify the diagnosis of diarrhea-associated IBS or of IBS with mixed diarrhea and constipation.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: Could one's sense of smell really be used to predict Alzheimer's? I've never had a very good sense of smell, so does that mean I'm doomed?

Dear Reader: It's true that recent research has suggested a link between an impaired sense of smell and an increased likelihood of developing dementia. But before we get into the details, it's important to note the researchers agree that this dysfunction is not a risk factor for, and not a cause of, dementia. Instead, the results of the study suggest that an impaired sense of smell may be an early warning sign that someone is in the process of developing the condition.

In the study, which was published last year in the Journal of the American Geriatrics Society, about 3,000 participants between the ages of 57 and 85 had their sense of smell evaluated. They were asked to sniff a series of "odor pens," which look somewhat like a felt-tip marker. They were then asked to choose the scent they detected from four possible options. The five scents they were asked to identify were orange, rose, peppermint, fish and leather. A solid majority -- 78 percent -- correctly identified four out of the five scents. (Of the five scents, leather proved to be the most challenging.) Three scents were correctly identified by 13.8 percent of participants, 4.9 percent got two scents right, and 2.2 percent correctly named one scent. One percent of study participants were unable to identify any of the five scents.

Five years later, researchers conducted home visits with either the study participants themselves, or a proxy, such as a friend or relative, if the individual was too ill or had passed away. They discovered that those individuals who could not correctly identify at least four of the five scents were twice as likely to have developed dementia. There was also a correlation between the fewer scents identified and an increase in the likelihood of a dementia diagnosis.

What's going on? The researchers suspect that our sense of smell is so closely connected to brain function that when it declines, it can be an indicator of more serious and complex problems. Our sense of smell takes place via the olfactory nerve, which leads from the mechanisms in the nose that identify odors and connects directly to the brain. In addition, the scent sensors in the nose are constantly being replaced, with olfactory stem cells producing new cells when the old ones die off. This has led scientists to wonder whether the decrease in the ability to smell indicates that the brain's ability to rebuild certain vital components may be in decline and could be related to changes connected to dementia.

As for your own poor sense of smell, that appears to be a condition you've lived with for some time. As we mentioned earlier, flunking the smell test doesn't amount to either a risk factor or a cause of dementia. Instead, it means that in your case, the smell test would not be an appropriate diagnostic predictor.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: I smoked for more than 20 years. And although I stopped about 20 years ago -- I'm now in my early 60s -- I worry about what changes it might have wrought in my cells. Should I get a CT scan just to be sure?

Dear Reader: Your concern is understandable. Tobacco smoke is a potent carcinogen, and lung cancer is the leading cause of cancer death among both men and women in the United States. On the plus side, detection of budding lung cancers saves lives. Among people diagnosed with Stage 1 lung cancer, the survival rate is 92 percent at five years; the survival rate for those with Stage 4 lung cancer is 6 to 8 percent.

As for whether CT scan screening specifically can reduce deaths from lung cancer, let's look at the numbers. A 2011 study analyzed data of 53,454 men and women who had a 30 pack-year or greater smoking history; a 30 pack-year is the equivalent of smoking one pack of cigarettes daily for 30 years. The participants were either current smokers or had stopped within the last 15 years. The study compared the results of having either yearly CT scans or yearly X-rays for three years, and the participants were followed on average for just over six years.

Screening X-rays had no real benefit in reducing the lung cancer death rate, but CT screening did -- a 20 percent reduction, in fact. That's because CT scans were much more likely to find cancers. Previous trials of CT screening had not shown the same survival benefits because those trials were much smaller.

CT screening for lung cancer is not without risk. For starters, CT scans detect many nodules that are not cancer. This can lead to additional scans to evaluate the nodule or to a biopsy of the lesion, potentially requiring major surgery. In the 2011 study, 96 percent of nodules 4 millimeters or greater -- considered a positive finding in this trial -- were not lung cancer. Nonetheless, invasive procedures were needed to address 11 percent of those nodules, creating the associated risks of lung collapse, bleeding and death.

Also, the chest radiation incurred through such scans could potentially increase the risk of radiation-induced cancers. (This concern is somewhat tempered by the use of low-dose CT scans for screening.) Finally, some slow-growing lung cancers found by CT screenings may never have led to a problem.

That said, CT screening for lung cancer does improve overall mortality rates as well as mortality rates from lung cancer. But strict guidelines -- based on the 2011 study -- address who can be screened as part of their insurance coverage. The U.S. Preventive Services Task Force, the American Association for Thoracic Surgery and the American College of Chest Physicians recommend CT screening for those adults ages 55 to 79 who have a 30-pack year history of smoking and who are either current smokers or smokers who have stopped in the last 15 years. Medicare and other insurers follow these guidelines as well.

That's not to say you aren't at risk for lung cancer, but the benefits of lung cancer screening may not be greater than the risks.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)