A comment on the value of screening1


  1. This blog post was originally published 14th February 2014 at http://aheblog.com/2014/02/14/a-comment-on-the-value-of-screening

Everybody’s talking about screening again, with good reason. Research seems to suggest that screening for breast cancer, using mammography, is not effective (let alone cost-effective)(Miller 2014). Here I present a view on the value of screening; the validity of which I am yet to fully convince myself.

Why screening?

The point of screening is the early detection of disease, or the identification of increased risk of disease, in people without symptoms. It is based on the idea that commencing treatment or care at an earlier stage of disease can be beneficial to an individual’s health or well-being in the long-term, including life-extension. Read that sentence again – it’s important. So, the potential value of screening is commensurable to the incremental value of early treatment or care over and above treatment or care that begins with the onset of symptoms. However, as I suggest below, a screening trial is not the best way to capture this.

The value of treatment for disease is clear. Treatment can directly improve health and well-being, and this is something to which people attach value. The same cannot be said of screening, which in and of itself is not health-improving. I believe that we need to stop valuing screening in terms of its ability to extend life and quality of life. It may serve to give people peace of mind, but this is not something that we routinely consider in the evaluation of screening. The true value of screening lies in the extent to which it provides us with new knowledge about a patient; a screening intervention with better sensitivity and specificity is of greater value. It seems perverse to suppose that an intervention that provides us with information can be ‘ineffective’ in terms of health outcomes, or even harmful. The error is surely in our use of that information.

The redundancy of screening trials

To capture the value of screening we do the same as for any other health care intervention; we carry out an RCT. It appears to me that screening trials are unnecessary, of little value and potentially dangerous. This may be a foolhardy suggestion given that my employment depends upon a screening trial, but I’ll go on.

I’ll tackle my three claims in reverse order. Firstly, screening trials are potentially dangerous. This is simply because screening is potentially dangerous. As we’ve already established, the value of screening is derived from the value of treatment for the disease for which an individual is being screened. If a person is screened positive, they may be able to receive treatment earlier than they otherwise would. However, it seems unlikely to me that the treatment they will receive has been evaluated in asymptomatic individuals like themselves. Trials of pharmaceuticals, surgery or other health care interventions are invariably carried out in populations with symptomatic disease. Therefore, our understanding of the benefits, costs and side-effects of the treatment may not be relevant to individuals receiving treatment after being screened positive. The effect in this population may be detrimental.

Secondly, screening trials may be of little value. Given that the value of screening is derived from the effectiveness of treatment, a change in the cost-effectiveness of the best available treatment for a disease will render the ‘effectiveness’ results from the screening trial redundant. Studies of screening no doubt provide other important data and findings, but I do not believe ‘effectiveness’ to be one of them…(please don’t fire me).

Thirdly, trials of the effectiveness of screening are unnecessary. Again because the value of screening is derived from treatment. Armed with information about the effectiveness of treatment, screening uptake and the sensitivity and specificity of the screening method, we can accurately model the ‘effectiveness’ of screening. A trial of the effectiveness provides no new information.

The solution

It seems to me that the problems set out above can be addressed by making changes to the way we carry out trials of treatments for diseases that have an asymptomatic state. Trials of treatments for such diseases should recruit asymptomatic individuals and subject all participants to a screening programme. Upon detection of disease (with any necessary decision rules and stratifications) individuals can be given the treatment that is being evaluated. Then, at the end of the trial, the cost-effectiveness of the treatment should be reported as a function of disease progression. In some cases disease progression will be categorised — for example into asymptomatic/symptomatic — while in others it will be a function of some other indicator, such as tumour size. Using this information it would be simple to elicit the ‘value’ of a screening programme. There will be 3 possible results: i) commencement of treatment is not cost-effective at any stage of disease, in which case screening is not effective; ii) commencing treatment in the asymptomatic stages of disease is no more cost-effective than commencing treatment once symptoms are identified, in which case screening is not effective; or iii) there is a stage of pre-symptomatic disease at which the commencement of treatment provides benefits over and above waiting to start treatment once symptoms develop, in which case screening is effective. It is only in the third scenario that the information provided by screening can be harnessed for health benefit. Using this information it would be possible to implement a screening programme that maximises cost-effectiveness, without carrying out a trial of the ‘effectiveness’ of screening.

No doubt there are problems with this suggested solution. However, the dramatic inconsistency in the findings of screening trials, and the resulting scepticism amongst clinicians and decision makers, suggests that something is wrong with the way we currently think about the value of screening. It’s time for a rethink.