The test and condition pages on this site answer narrow questions: what one number means, or how one diagnosis is made. These guides do the opposite. They cover the situations where the useful question spans several tests at once, such as which screening is worth having at a given age, how to read a full report rather than a single line, what to do before a blood draw, or how the tests for one body system fit together.

Screening is not the same as diagnosis

Several of these guides deal with screening, and the distinction matters. A diagnostic test is done because there is a reason to suspect something. A screening test is done in people without symptoms, in the hope of catching a problem early. Screening tests are therefore deliberately tuned to miss as little as possible, which means they produce more false positives, and an abnormal screening result is usually the start of an investigation rather than an answer.

This is also why more screening is not automatically better. Testing without an indication produces incidental findings that lead to further tests, cost and worry, often with no benefit to health. Where guidance recommends against a test in the general population, we say so rather than leaving it out.

Recommendations vary by country

Screening intervals, age thresholds and even the units results are reported in differ between countries. Cholesterol is reported in mg/dL in some health systems and mmol/L in others; PSA screening is offered routinely in some countries and discouraged in others. Where a recommendation is specific to one health system, the guide says which one. Your own national guidance and your doctor's judgement take precedence over anything written here.

Medical disclaimer: This page is for general educational purposes only and does not constitute medical advice. Reference ranges vary between laboratories, and results must be interpreted alongside your symptoms and history. Always consult a qualified healthcare professional for diagnosis and treatment. Read our full disclaimer.