Frequency of examination
The frequency of screening and whether the tests are performed on specific occasions will depend on the conditions to be detected, the resources available, and on presenting opportunities. For situations where screening can be effective, the recommended frequency of examination varies with age and the natural history of the disease. Commonly, frequency of examination varies from annually to once in 3-5 years.
In occupational asthma, if sensitization to an inhaled agent in the workplace occurs, it is more likely in the early stages of employment and exposure. Hence the UK Health and Safety Executive (HSE) advice on lung function testing for workers exposed to asthmagens puts emphasis on greater frequency of screening during initial employment. For diseases with a long latent period between first exposure and subsequent health effects, there are no clinical reasons for advocating screening in the earlier years following initial exposure.
Advantages and disadvantages of health screening
Health screening may detect sentinel cases of disease in populations exposed to hazardous materials. The detection of these cases will signal the need for preventive measures. The principle behind instituting health screening is therefore appealing: detect disease early, and take preventive action. In practice, there are good examples where this is effective, e.g. screening for cardiovascular risk factors followed by measures to reduce risk. There are also examples where the benefits of screening are limited. Periodic chest x-rays for exposure to fibrogenic dusts such as silica particles or asbestos fibres may enable earlier detection of pulmonary fibrosis, but there may be little that can be done to halt the progression of the disease. Early detection of disease may produce an apparent gain in the duration of life between detection and death, although the mean age at death of those screened is not altered. All that screening does in these cases is to alert the screened individuals to the occurrence of disease, without necessarily affecting disease outcome. This has been suggested previously in a study of periodic chest x-rays in chromate-exposed workers.4 Earlier diagnosis as a result of screening can cause an improvement in survival time (interval from diagnosis to death), but this can be due to lead time and length time bias, instead of actual prolongation of life.5 In any case, early detection is part of secondary prevention. Prevention of occupational disease through reduction of exposure (primary prevention) is preferable.
Adverse effects of screening can arise with the indiscriminate use of screening tests. A false positive test result causes unnecessary anxiety and worry in the subject. A false positive test often leads to further investigations, with their associated risk of morbidity and further expense.
False negative results can provide false reassurance and cause complacency. They may be viewed as an ‘all-clear’ until the next round of screening and can even lead to the person ignoring future early warning symptoms.
For example, the exercise electrocardiogram (stress ECG), when used for screening coronary heart disease in an apparently healthy general population, has a PPV of <30%.6 Hence, seven of ten persons who are stress ECG positive are subjected to unnecessary and potentially harmful further investigations. In addition to causing anxiety, a false positive stress ECG may also have negative consequences for occupational and insurance eligibility, or other leisure opportunities. Even in cases of suspected coronary artery disease, the PPV for stress ECG is only 51%.7
There is an evolving range of opinions on screening of asymptomatic workers in occupational groups. Various organizations and expert groups have recommended exercise electrocardiography for job categories including airline pilots, firemen, police officers, bus and truck drivers, and railroad engineers. For athletes, there is a suggestion that stress ECG could be considered for younger (aged <45 years) asymptomatic individuals if they have multiple cardiovascular risk factors.8 The American College of Cardiology and the American Heart Association9 indicate that exercise testing in healthy asymptomatic persons is not recommended, but may be considered in occupations where public safety may be affected. In the UK, exercise tolerance testing may be indicated for applicants for a Group 2/Category C (large goods or public service vehicles) driving licence, if there is possible underlying cardiovascular disease.10 The US Preventive Services Task Force also recommend against routine screening with exercise ECG for asymptomatic adults. However, they also suggest that for people in certain occupations involving public safety, considerations other than benefit to the individual may influence the decision to perform screening.11 A review for this task force of the published evidence on ECG screening of asymptomatic adults concluded that ECG abnormalities are associated with a risk of cardiovascular events, but ‘the clinical implications of these findings are unclear’.12