Screening, Diagnosis and Monitoring

#Screening looks for possible disease

Screening is testing offered to a defined group of people who do not have symptoms of the condition being screened for. Its purpose is to identify people who may benefit from further assessment. A positive screening result generally indicates a need for evaluation, not a confirmed diagnosis.

Evidence for screening must go beyond showing that a test can find abnormalities. The overall pathway should offer benefits that outweigh harms, such as false alarms, unnecessary procedures or overdiagnosis. Overdiagnosis means finding a condition that would not have caused symptoms or harm during that person's lifetime.

#Diagnosis investigates a concern

Diagnostic testing investigates a suspected condition, often after symptoms, an examination finding or a screening result. The test is interpreted alongside information already available. Depending on the condition, diagnosis may rely on one highly informative finding or on several pieces of evidence considered together.

Evidence needs to show how well the test distinguishes relevant conditions in people similar to those being assessed. A study in a very different population may not describe performance in everyday practice. The suitability of the comparison method used to establish disease status also matters.

#Monitoring follows change

Monitoring uses repeated measurements to follow a known condition, treatment effects or possible complications. The central question is often whether a meaningful change has occurred. Consistent methods and timing can help distinguish a real change from ordinary biological fluctuation or variation introduced by measurement.

A monitoring test needs evidence that it measures change reliably and that the change has a useful interpretation. A value moving in a preferred direction does not always mean that health outcomes have improved. The same test may serve several purposes, but evidence for one purpose does not automatically support another.

#Common misunderstandings

Screening, diagnosis and monitoring are not interchangeable, even when they use the same test. A test that works well for investigating symptoms may not work equally well for screening people without symptoms. Its usefulness depends on who is tested, what question is being asked and what happens after the result.

A positive screening result does not necessarily mean disease is present, and a negative result does not guarantee its absence. Further assessment may be needed. Diagnostic tests also have limits: results are usually interpreted alongside symptoms, medical history and other findings rather than treated as answers on their own.

In monitoring, a changed result does not always mean a condition has improved or worsened. Natural variation, measurement differences and timing can affect results. More testing is not automatically better; it can create uncertainty or lead to unnecessary follow-up. Useful testing provides information that can meaningfully guide a decision.

#Questions worth asking a clinician

• Is this test intended to screen before symptoms, investigate a specific clinical concern, or monitor changes in an established condition?
• What evidence shows this test is accurate and useful for people like me in its intended screening, diagnostic or monitoring role?
• If my screening result suggests possible disease, which diagnostic tests would follow, and what harms could arise from that follow-up?
• If this diagnostic result conflicts with my symptoms or other findings, how would you decide whether further investigation is needed?
• When monitoring over time, how do you distinguish meaningful change from normal variation or measurement error, and what change would trigger action?