Screening for Adult Disease

Introduction

Medical screening is a public health activity that involves examining or testing asymptomatic, apparently healthy people to detect disease at an early stage. Measures can then be taken to prevent the disease (if there is a precursor stage), treat it early (hoping for improved cure rates), or at least offer treatment to delay advanced disease. For example, colonic screening can detect adenomas and carcinomas; removing adenomas prevents the well-recognised adenoma–carcinoma sequence, and actual cancers detected are often earlier and at a more curable stage. Unfortunately, for other cancers without an easily detected early precursor stage, such as breast or prostate, beneficial outcomes are elusive.

Screening can detect disorders that predispose to other diseases, for example, hypertension or elevated cholesterol levels, to ascertain people at increased risk of atherosclerotic heart disease and stroke. Screening is also useful for infection control, for example, preoperative screening of patients for methicillin-resistant Staphylococcus aureus (MRSA) carrier status to enable elimination therapy before operation.

An entire population can be screened ( mass screening ) but more usually, it is targeted at at-risk groups. Selection might be by age, gender or cardiovascular risk factors, for example ( Box 6.1 ). Opportunistic screening involves a more random approach, such as screening patients who happen to attend a particular clinic.

Assessing the Potential Benefits of Screening

Many lay people subscribe to the simplistic view that screening must be ‘a good thing’. These include the public, people associated with distressing diseases, populist politicians and people with vested financial interests. Poorly conceived screening, however, may consume massive resources to identify just a few new cases with little clinical benefit, for example, computed tomography (CT) scanning for lung cancer in low risk populations. Worse still, early diagnosis of a condition where early intervention brings no advantage may cause suffering. These people can be prematurely placed into an anxiety-provoking sick role and given unrealistic expectations. They may also be subjected to unnecessary treatments with potentially severe side-effects, for example, some breast or prostate cancers that might never progress to metastatic disease.

As with any public health measure, medical and social benefits accruing from any screening programme need to be rigorously evaluated and the process separated entirely from the incentive to screen for profit. Whole-body scanning by CT or magnetic resonance imaging (MRI) is currently strongly marketed on the basis that a scan will show unsuspected abnormalities and allow early treatment. Abnormalities are bound to be discovered by such extensive screening, but it is difficult to reliably determine which signify serious disease and which (if any) should be treated. Doctors should not perform unvalidated screening tests any more than they should use unproven drugs, and should resist patient pressure for inappropriate screening.

Criteria for an Effective Screening Programme

To initiate a new national screening programme, certain criteria must be fulfilled. There must be a perceived need in the medical community or in the wider public. Pilot studies are then carried out. If outcomes are promising, large scale prospective randomised controlled trials (RCTs) need to be performed, seeking robust evidence for implementation. This is critical because it is politically difficult to stop a screening programme, even when evidence shows little benefit, for example, some early breast screening programmes (Nordic Cochrane Collaboration 2001 and 2006).