When Screening Can Do Harm

Screening can improve health, but offering tests to large groups also creates predictable harms. Some people receive false reassurance or unnecessary follow up. Others have real conditions detected that would never have affected them. Understanding these differences supports informed choice without dismissing screening's benefits.

Harm can occur anywhere in the pathway Screening is an organised pathway involving invitation, testing, results, further assessment and sometimes treatment or surveillance. Harm can arise at any of these stages. A worthwhile programme does more good than harm overall. This does not mean every participant benefits or that nobody experiences harm. The type and frequency of harm differ between programmes. They depend on the target condition, population, test threshold, follow up procedures and available treatments. False positive results can start a testing cascade A false positive occurs when screening indicates a higher chance or positive finding, but the target condition is not present. It is not the same as overdiagnosis. False positives can cause worry, disrupted sleep, time away from work or caring responsibilities and repeated appointments. Some people undergo imaging, endoscopy or biopsy before receiving reassurance. Further procedures can cause pain, bleeding, infection or another complication. The exact risks depend on the screening programme and investigation used. False positives are not simply caused by screening tests being designed for high sensitivity. They also depend on specificity, thresholds and how common the target condition is in the screened population. Negative results can create false reassurance A false negative occurs when the target condition is present at screening but the pathway does not identify it. This can happen because no test detects every case. False reassurance occurs when someone interprets a screen negative result as proof that they cannot have the condition. They may then delay reporting symptoms or seeking assessment. A condition that genuinely develops after the screening test is not automatically a false negative. Screening provides information about a defined time and cannot guarantee future health. Do not wait for the next screening invitation when new, persistent or concerning symptoms develop. Screening is not a substitute for individual clinical assessment. Uncertain and inadequate results also have consequences An uncertain finding is a result whose importance is not clear. It may lead to repeat testing, specialist review or surveillance without providing an immediate answer. An inadequate result means the sample, image or examination could not be interpreted reliably. Repeating the test can cause inconvenience and anxiety even when no disease is eventually found. Uncertainty can continue for weeks, months or longer. Balanced information should explain this possibility before screening whenever it is relevant. Screening tests and follow up can cause direct harm The screening test itself may cause discomfort, pain, radiation exposure or another small physical risk. Follow up procedures can carry greater risks than the first test. For example, colonoscopy can cause bleeding or a tear in the bowel. A biopsy can cause bleeding, infection or damage at the sampled site. These examples do not mean that the procedures are generally unsafe. They show why programme decisions must include the complete pathway rather than test accuracy alone. Incidental findings can create another pathway An incidental finding is an unexpected abnormality outside the main target of the screening test. It is more common when imaging covers a wide body area or one test searches for several conditions. Some incidental findings lead to useful care. Others prove harmless after repeated scans, procedures or prolonged uncertainty. Programmes need clear policies for deciding which incidental findings should be reported and how they should be managed. Detecting every visible difference is not automatically beneficial. Overdiagnosis is a real diagnosis without future benefit Overdiagnosis means correctly diagnosing a condition through screening that would never have caused symptoms or harm during that person's lifetime. The diagnosis is real rather than false. This can occur when a condition grows very slowly, stops progressing or would be overtaken by another illness or cause of death. Overdiagnosis usually cannot be identified with certainty in one person when the condition is found. Researchers estimate it by comparing outcomes across screened and unscreened groups over time. A slow growing condition is not automatically overdiagnosed. Some slow conditions eventually cause harm, while some faster looking findings may still behave unpredictably. Overdiagnosis and overtreatment are different Overtreatment means giving treatment that provides no meaningful benefit or is more intensive than needed. Treatment can then cause side effects or complications without improving health. Overdiagnosis can lead to overtreatment, but treatment is not inevitable. Active surveillance may monitor selected conditions and delay treatment unless meaningful change occurs. Active surveillance can reduce treatment related harm, but it can still involve repeated tests, appointments and uncertainty. It also requires evidence based criteria and reliable follow up. Overtreatment can occur without overdiagnosis. For example, a clinically important condition may still receive a more intensive treatment than evidence or personal priorities justify. Screening burden affects many people Screening burden means the combined time, discomfort, worry, travel, administration and repeated follow up experienced across everyone offered or completing screening. Most participants will not have the target condition detected through that screening round. Their individual burden may be small, but it can become substantial across a large population. Being labelled with a risk state or abnormality can also affect wellbeing. Some people remain worried even after later tests are reassuring. Harms and benefits may be distributed unequally Access barriers can prevent some eligible people from receiving screening benefits. The same barriers can also delay follow up after an abnormal result, increasing uncertainty and possible harm. Programmes should examine language, disability, geography, deprivation and other barriers across the whole pathway. Offering the same appointment system to everyone does not guarantee equitable access. Opportunity cost means the staff, facilities and money used for one programme cannot be used elsewhere at the same time. Screening can cause wider harm when it overwhelms diagnostic or treatment services. Why recommendations change Screening recommendations can change when evidence, disease patterns, tests, treatments or service capacity change. Starting ages, intervals, thresholds and target groups may be adjusted. A change does not automatically prove that earlier guidance was careless or that the new policy is permanent. It shows that programmes need continuing evaluation against current evidence. People should receive balanced information about likely benefits, possible harms and important uncertainty. Accepting or declining an offered screening test remains a personal choice. Contact the screening service when a result or next step is unclear. Seek clinical assessment for symptoms rather than waiting for screening. This lesson explains general screening harms. It cannot estimate your personal chance of benefit or harm from a specific programme.

Screening harm is not one event. It can arise from false results, uncertainty, procedures, incidental findings, overdiagnosis, overtreatment, repeated monitoring and unequal access across the complete pathway.