Any sensible people will tell you that prevention is better than cure. I’ve heard it a million times. There was even a television show named “It‘s better to prevent”. Besides, nobody in their right mind doubt about the health benefits that preventive medicine has achieved promoting the improving of lifestyles, controlling environmental conditions or with vaccination programs. But, however, when it comes to screening programs, I’ll say that it’s not always so clear that it’s better to prevent and, at times, it’s better to do nothing for two reasons. First, because our resources are limited and all that we spend on screening will come from other needs that will have fewer resources. Second, because even if we do it out of the best of intentions, if we try to prevent indiscriminately we can cause more harm than good.
So, we’ll have to think if there’s justification for any screening strategy before implementing it. The diagnostic test with which we plan to screen must be simple, inexpensive, reliable and with good acceptability by the population. It is important not to forget that we are going to do the test to healthy individuals that may not want to be bothered. Furthermore, it’s rare that we can confirm the diagnosis with a single positive result, and test to confirm it surely will be more expensive and cumbersome, if not clearly invasive (imagine that the screening must be confirmed by a biopsy). We will have to consider the sensitivity and specificity of the test because, although we tolerate some number of false positive when screening, if the confirmation test is expensive or very cumbersome, it will be better that false positive are few, or screening won’t be cost-effective.
Moreover, for the screening being worth of doing, the preventable disease has to have a long preclinical phase. If it is not so, we’ll have little opportunity to detected it. The problem is, of course, that we are more interested in detecting the more severe diseases, and those often have shorter preclinical stages without symptoms.
Besides, who is going to be screened?. Everyone, you will tell me. The problem is that this is the most expensive option, especially considering that healthy people do not usually go to the doctor and you’ll have to actively recruit them if you want to do the screening (for their sake, of course). To those who are sick, but not so much, you´ll tell me then. Well, not a great deal because when they go to the doctor they are already out of the reach of prevention (they’re already sick). But we can take advantage of those who go to the doctor for other reasons, some of you could think. This is called opportunity screening, and is what sometimes is done for practical reasons. It’s cheaper, but the theoretical benefits of universal screening are lost. Screening a number as large as possible is of particular interest when we’re trying to detect risk factors (such as hypertension) since, in addition of the advantages of early treatment, we have the opportunity to do primary prevention, much cheaper and with better health results.
So, as we see, doing a screening can have many advantages, what is evident to everyone. The problem is that we rarely think about the damage we can cause with that way of prevention. How is it possible that early disease detection or the possibility of doing an early treatment could harm someone?. Let’s make some considerations.
The test may be painful (a shot) or be bothering (to gather up stools in a container for three days). But if you think this is bullshit, think about people who have a heart attack while doing a stress test, or which have an anaphylactic shock, not to speak about the Japanese who suffer a perforation during a colonoscopy. That’s a horse of a different color. Moreover, the mere prospect of screening can cause anxiety or stress in a healthy person who should not be worried about it.
And think about what will happen if the test is positive. Imagine that, to confirm the diagnosis, we have to do a colonoscopy or a chorionic biopsy, not to mention the patient anxiety until diagnosis is confirmed or ruled out. And although we confirm the diagnosis, the benefit may be limited: what is the benefit for an asymptomatic person to know that he has a disease, especially when there is no treatment or it’s not already the time to start it?. But the fact is that, although there’s a treatment, it may be also injurious. One very up-to-date example of that is the effect of a prophylactic prostatectomy for a low-grade carcinoma detected with PSA screening: the patient can suffer incontinency or impotency (or both of them) for being operating on a surgery that could be delayed for years.
Think always that the potential benefits of screening in general healthy population may be limited because of this reason: people are healthy. If there is the slighted damage that may arise from the strategy of early screening and treatment we should seriously consider whether it is worth performing the screening program.
So, when do we have to do the screening for a given disease?. First, when disease burden is worth of doing screening. Disease burden depends on the severity and prevalence. If a disease is very common but very benign, disease burden will be low and probably we’ll not be interested in screening. In the event that it is very rare it should be neither worth of doing screening, unless the disease is severe and has a very effective treatment to prevent its complications. An example could be the screening of hyperphenylalaninemia in newborns.
Second, we need to have a proper test with the mentioned characteristics, especially the fact that number of false positives is not too high to avoid to have to confirm the diagnostic in too many healthy people, making a ruinous business.
Third, there has to be an early treatment that, also, has to be more effective than usual treatment at the onset of symptomatology. Of course, we must also have the resources to perform this treatment.
Fourth, both the screening test and the treatment arising from the positive result must be safe. Otherwise, we could do more damage than that we want to avoid.
And fifth, we must balance costs and potential benefits of screening. Don’t forget that, although the test is not very expensive, we are going to do it in a lot of people, so we’ll have to spend a huge amount of money, which is rather scarce at this moment.
Finally, just say that any screening program must be supplemented with studies proving its effectiveness. This can be done by direct or indirect methods depending on if we are comparing the possibilities of to do or not to do the screening, or if we study and compare the different screening strategies. But that’s another story…