laboratory test
Meaning of laboratory test: A reference range shows what is usual in a healthy population, not what suits one person, so a flagged result is not automatically abnormal.
Definition of laboratory test
A laboratory test is an examination of a sample taken from the body, such as blood, urine, stool, tissue or another fluid, to measure something that cannot be judged from the outside. Samples are collected in a clinical setting and analyzed using specialized methods, and results are read against a reference range together with the rest of the clinical picture.
What are they used for?
| Purpose | What the result is used to do |
|---|---|
| Screening | Look for a condition in someone with no symptoms |
| Diagnosis | Confirm or rule out an explanation for symptoms |
| Monitoring | Track a long term disease, or the effect of treatment, over time |
| Safety | Detect harm from a drug before it produces symptoms |
| Severity and planning | Judge how far a condition has progressed and how it may behave |
The same measurement can serve different purposes on different days, and the purpose changes how the number is read. A value that would prompt investigation in a screening context may be entirely expected in someone already known to have the condition.
How is a result interpreted?
Almost every numerical result arrives with a reference range, and the way that range is built explains most of the confusion surrounding results. It is the span covering the middle portion of an apparently healthy reference group, which means a small share of perfectly well people fall outside it by definition. Run enough tests on a healthy person and one will come back flagged. Ranges also shift with age, sex and pregnancy, and sometimes with the method a particular laboratory uses, so a figure means little without the range printed beside it. Trend usually carries more information than a single value, since a result drifting steadily across the range says more than one reading sitting at its edge. Every test also has a sensitivity and a specificity, so a negative result lowers the chance of a condition without abolishing it, and the weight a positive carries depends on how likely the condition was before the sample was taken.
What can go wrong before the analysis?
Most errors happen before a sample reaches the analyzer rather than inside it. Blood drawn into the wrong tube, shaken so the red cells rupture, left too long before processing, or taken from the arm receiving an infusion can all produce a figure reflecting the handling instead of the person. Timing matters too, since some substances vary through the day and others depend on when the last dose or the last meal fell. Mislabeling is the most serious error of all, because the result looks entirely plausible while belonging to someone else. These are the reasons an unexpected result is often repeated before anyone acts on it. A test of this kind is also distinct from a laboratory study, which investigates cells, tissue or organisms in research rather than reporting on one individual.
Used in a sentence
A laboratory test was ordered to check the patient's kidney function before starting the new medication.
Good to know
A single laboratory result is read against the reference range for that specific test and alongside other findings, since a value marginally outside the range is common in healthy people and does not by itself indicate disease.