Minor headache or incurable brain tumor?

A simple tension headache? Or the harbinger of a life-threatening brain tumor?

A prosaic pimple? Or the earliest sign of a potentially lethal bacterial infection?

It's always been challenging for healthcare consumers to navigate the shoals of legitimate concern and needless worry as they figure out when to rush to the doctor.

"It's tough stuff," Dr. Tom Delbanco, an internist at Beth Israel Deaconess Medical Center, acknowledged.

That's never been truer than today. There's more medical information available from more sources - of varying credibility - than ever before, and that information can both enlighten and frighten. At the same time, out-of-pocket medical expenses are rising, and the ranks of primary care physicians are shrinking, meaning that false alarms can have very real consequences for cash-strapped consumers and overworked doctors.

So how do you know when to summon the cavalry?

One of the best ways is strikingly simple: listen to the basics of your body. Temperature, weight, sexual interest, menstrual cycles - "all these things are pretty good barometers of good health, and when they go awry, it might be a soft alert that you should look into the reasons," said Delbanco. Still, he assures, usually the changes are not "associated with a terrible disease."

While there's no one-size-fits-all answer, there are some other broadly relevant guideposts. It often comes down to the severity of symptoms, their duration, and whether they progress. And history - both the distant past and more recent developments - can definitely be prologue when it comes to health. An episode of swine flu in a healthy person is something entirely different than the same viral illness in someone with a disease-fighting system hobbled by long-standing medical problems.

Sometimes, the best tools for understanding and reassurance are the simplest: The bathroom scale (Delbanco calls it "one of the best lab tests we have") to detect sudden weight loss or weight gain. The eye, to observe changes in color or shape of a skin lesion. The phone, to call a doctor or nurse who, in a brief chat, may be capable of turning down the drone of fear.

"I can offer a lot of reassurance over the phone," said Dr. Elizabeth Roth, an internist at Massachusetts General Hospital. "It's hard for people to come in, it's time consuming, it's expensive. We never want to bring people in unnecessarily. But there are some times when it's just not possible to say over the phone that a symptom doesn't warrant further evaluation.

"We're all subject to that, 'What if?' "

(Even health reporters aren't immune. Days after I'd written about a young man with the condition commonly known as Lou Gehrig's disease, I became utterly convinced that twitching leg muscles meant I had the disease too. Of course, I didn't.)

"It's important for everybody to realize that's a natural phenomenon," said Dr. David Elvin, medical director of Cambridge Family Health, a satellite of Cambridge Health Alliance. "Sometimes, patients will come in and say, 'Well, a friend of mine at work was just diagnosed with cancer and he didn't have symptoms, so I thought I should come in and be checked.' "

The arrival of swine flu in the past month offers a case study in how patients respond to health threats. With news of the novel virus blaring from headlines, sniffling patients streamed into clinics. More often than not, patients didn't have telltale flu symptoms - fever, body aches, chills - and instead were suffering from run-of-the-mill allergies or a cold.

Dr. Anita Barry, top disease detective at the Boston Public Health Commission, said when it comes to swine flu as well as the seasonal variety, it's important for patients to assess their overall health status as they consider whether a trip to the doctor is necessary. Pregnant women and patients with faulty hearts, ailing lungs, diabetes, and compromised immune systems all need to be particularly vigilant. "Those people," Barry said, "should call sooner rather than later."

Other patients should try symptomatic treatments first - more fluids and pain relievers. "If they don't have risk factors that put them at increased risk from influenza," said Dr. Larry Madoff, director of epidemiology and immunization at the Massachusetts Department of Public Health, "then they should stay home and take care of themselves."

But if they don't feel better after a few days, or they can't keep liquids and food down, or experience breathing or mental problems, then a visit to a clinic is clearly warranted. That's true, too, for other ailments that don't respond to home remedies.

When Dr. Raj Krishnamurthy pulls the night shift, calls inevitably arrive from patients frantic with concern. Heads ache, stomachs churn.

"So I ask: 'Can you function? What else is going on? Are you able to continue your bodily functions like eating and drinking?' " said Krishnamurthy, outpatient medical director at Boston Medical Center. "If you're able to do what you need to do and what tasks you have to do for the day, that's a good marker that it's not something emergent or serious."

And what's alarming?

"Things that are worrisome," Krishnamurthy said, "are those that are getting stronger and stronger over time, the intensity of the pain is getting worse and worse and worse."

Like, for example, a lump that keeps getting bigger. And when it comes to neurological pain and worries about a brain tumor? "Typically, the classic thing we think about in medicine is 'the worst headache in your life'," said Dr. Lisa Owens, medical director of the Brigham Primary Physicians group. If that accurately describes what someone is experiencing, a trip to the clinic is warranted; if it doesn't, try ibuprofen or another pain reliever.

Generally, patients should be concerned if they experience chest pain, shortness of breath, and the unexpected appearance of blood in stool or saliva. But even then, there are no foolproof rules.

"If a guy is 55 and he's a smoker and he's on high blood pressure medicine and he has cholesterol, if he's having chest pain, it's probably not going to be heartburn," said Dr. Raul Seballos, vice chairman of preventive medicine at the Cleveland Clinic. "But if the guy is 22 and he just spent a weekend drinking six beers with his buddies at a Red Sox game, he probably just has irritation."

Similarly, if a pimple sprouts and doesn't erupt into a rash, there's little reason to fret that it might be the first indication of a hard-to-treat bacterial infection known as MRSA. First detected in hospitals, the germ is now circulating more broadly and has found fertile terrain among abrasion-prone athletes.

"So if your son is a wrestler and he was just ground into the mat by his opponents and he comes back with a bunch of abscesses on his shoulder, you might want to be a little bit more concerned," said Dr. Stephen Erban, a general internist at UMass Memorial Medical Center.

For patients like Tom Hill, whose doctor is Delbanco of Beth Israel Deaconess, there is a recognition that their own aches and pains fall against a backdrop of a healthcare system struggling to tend to an aging population.

"On any given day there is, I figure, someone sicker than I am," Hill, a 61-year-old who lives in Cambridge, wrote in an e-mail. "So this is what I balance: The notion that probably it's OK to try to get well, tempered by asking myself how likely is it that I can get there on my own?"

Stephen Smith can be reached at stsmith@globe.com.