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Interpreting diagnostic tests in practice
Audience: doctors-clinicians, students-learners ยท Review state: published
What a test result is
A test result is information that shifts the probability of a condition; it is rarely a verdict. The same value can mean different things in different patients because interpretation starts from how likely the condition was before testing.
This pre-test probability, set by history and examination, is the anchor of all test interpretation.
Accuracy has two directions
Sensitivity asks how often a test catches disease when it is present; specificity asks how often it correctly clears people without disease. Both are properties of the test, measured in study populations.
What a clinician usually needs is the other direction: given this result, how likely is disease? That depends on sensitivity and specificity together with pre-test probability. A positive result on a low-probability patient can still leave disease unlikely, and a negative result on a high-probability patient does not end the workup.
Reference ranges and their limits
A reference interval typically covers the central 95 percent of a healthy population, so about one in twenty healthy people falls outside it on any single test. Panels of many tests make at least one out-of-range result common in healthy people.
Units, assay methods and patient factors such as age, pregnancy and time of day move results. Compare like with like, and read trends rather than single values whenever possible.
Working habits that hold up
Order tests to answer a question whose answer would change management. Before acting on an unexpected result, consider repeating it or confirming it with a different method. When results and the clinical picture disagree, trust the disagreement as a signal to reassess both.
Educational examples like these teach interpretation method only. Real results belong to real patients, and interpretation in care belongs with the treating clinician and the laboratory that produced them.
Limitations
Educational method only. Not interpretation of any real result, and not authority for diagnosis or treatment decisions.