To screen or not to screen? That is not the question.

The question is not whether to screen, it is why, what, where, when, how, and how much, how often, and at what cost for what benefit.

Patients and physicians must and do screen. The issue is cautious appropriateness. Self-screening by patients is easy, free, and fundamentally harmless. Look at your skin for potential melanomas, be alert to warning symptoms of a stroke, learn the early signs of alcohol dependence, observe your urine for gross blood.

These are observations that have almost no downsides and could trigger life saving interventions. But when the American Medical Marketing Machine (AMMM) starts screening campaigns, watch out. Both the well intended zeal of the advocacy groups and the ambitious avarice of the suppliers and providers can wreak real havoc, especially when they combine forces.

Is the benefit to individuals or the public going to be worth the harm to individuals and the costs to whomsoever pays the bills? Case in point: lung cancer.

The number one cancer killer in America. A really big deal. Caught late; usually kills; caught early; also often kills. How could even earlier change that equation? What are the downsides to screening for it?

Five investigators at the National Cancer Institute in 2010 reported in the Annals of Internal Medicine the results of a randomized, controlled clinical trial using low dose computed tomography (CT) versus chest x-ray on more than 3,000 current or past 30 pack-year smokers ages 55 to 74 with no history of lung cancer.

The cumulative risk outcome of a false-positive after one annual screen with CT was 21% and after two, 33%; false positive rates after chest x-ray were 9% and 15% at one and two years.

Not trivial results, and they often triggered an unnecessary and potentially hazardous invasive procedure, not to mention the hazard of the radiation itself.

Good things can happen after screening. But so can bad. A false positive means you found something that was not there; a false negative means something was there and you did not find it; a misidentification means you found something that was there but you called it the wrong thing.

Those are all bad. It is a little like in football; you throw a forward pass; three things can happen, but two of them are bad.

So, sure, screen; but remember Hippocrates. First, do no harm.

George Lundberg is a MedPage Today Editor-at-Large and former editor of the Journal of the American Medical Association.