Sam was in his early 40s, not much older than I was. He'd had a cough off and on for a few weeks and had come to see me for reassurance. He'd quit smoking a few years earlier and was more motivated than my average patient to make healthy lifestyle choices, such as a balanced diet and exercise.

My examination was not clearly pointing to a reason for his occasional dry cough, although I suspected a mild allergy or asthma-like syndrome. Sam's lungs sounded clear in my stethoscope in the exam room, his breathing an alto counterpoint to the strong bass rhythm of his heart.

Still, I didn't feel I could shrug off Sam's concern until I had gotten a chest X-ray. Certain that the result would be normal, I barely glanced at the film before turning to start a reassuring word. But wait -- that spot -- I looked back up at the film to confirm a small round spot about the size of a nickel in Sam's lung. Uh oh.

Sam asked me what it was. I'm not sure, I said honestly. It might be nothing. Although Sam had quit smoking when he was relatively young, there were no guarantees. And lung cancer could arise spontaneously, even in nonsmokers. I asked Sam if he had any old X-rays that we could compare with the one we had just gotten. Sometimes bacterial infections, such as tuberculosis, or fungal infections, such as coccidiomycosis or histoplasmosis (common in parts of California), can cause lung scarring that remains visible on X-ray long after the body has overcome the infection. Finding on a previous film a spot like the one he had now would be reassuring. Unfortunately, Sam didn't have any old X-rays that we could check, so after reviewing the diagnostic possibilities I referred him to a pulmonary specialist for further evaluation.

I was stunned to hear from the specialist a few weeks later. After a complete office evaluation, the specialist had sent Sam to a major medical center for a chest CT scan and biopsy of the lesion in his lung to rule out malignancy. Under anesthesia, during the biopsy, Sam died of cardiac arrest.

Like most "invasive" medical procedures, lung biopsies are not without risk, but the news still came as a shock. The worst news I had expected to hear was treatable cancer in my patient, not his sudden death. In the end, Sam was not diagnosed with cancer -- but he lost his life.

"Primum non nocere." First do no harm. A phrase invariably attributed to the most influential physician-writers from the ranks of medical history -- Hippocrates and Galen of ancient Greece and Rome or renowned Canadian physician/professor William Osler -- its meaning is ever more critical today, as new diagnostic procedures and treatments provide new hope for patients. And new risks.

In 1999, Dr. Elliott S. Fisher and Dr. H. Gilbert Welch published an article in the Journal of the American Medical Assn. on the potential negative effects of the growth in the use and capabilities of diagnostic tests, such as angiography, CT and PET scans and MRIs. As our technology grows, we are in many cases becoming able to diagnose potentially harmful conditions at an ever earlier stage. But some of the conditions picked up with more effective screening may not ever develop into life-threatening or even visible disease. The authors called these conditions "pseudodisease, a disease that would never become apparent to patients during their lifetime without the diagnostic test."

Microscopic studies of tissue specimens have shown that as many as 40% of women in their 40s have ductal carcinoma in situ of the breast (in which malignant changes have developed in cells that line the milk ducts but have not yet spread into the surrounding breast tissue), and 50% of men in their 60s have adenocarcinoma of the prostate (the most common type of prostate cancer). Yet, most of these men and women would not develop life-threatening or even symptomatic cancer in their lifetime.

The net effect of our improved diagnostic capabilities is often increased intervention. An abnormal finding on a screening test leads to further testing. In some cases, the in-depth evaluation identifies a dangerous condition and leads to helpful or life-saving treatment. But, in other cases, a false positive or randomly abnormal result may lead to unnecessary, and potentially risky, testing to rule out true disease or risk to health and life. Physical complications and injury, severe psychological stress and devastating financial impact may be the result.

As our capabilities for finding disease -- and pseudodisease -- grow, it becomes ever more challenging for doctors and patients to "do no harm." Research studies that assess the benefits and risks of screening procedures and treatment options can help us decide which efforts improve our patients' lives and health.

I always think of Sam whenever I put pen to paper to order a diagnostic test. I have taken an oath to care for my patients, and am grateful to have the benefits of science and technology to aid me in my service. And yet I can't help but worry, will my well-intentioned efforts do no harm?

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Dr. Linda Reid Chassiakos is director of the Klotz Student Health Center at Cal State Northridge and a clinical assistant professor of pediatrics at UCLA.