What does “accurate” mean?
Accuracy is agreement with the correct result, but test performance has several dimensions. Sensitivity concerns detecting a target when it is present; specificity concerns avoiding a positive when the target is absent. Precision concerns repeatability. Performance also depends on the specimen, cutoff, operator, storage, and population.
Why use screening and confirmation?
Screening methods are efficient and may detect a class of related substances. Confirmation uses a more specific analytical method to identify and often quantify selected analytes. A screen and confirmation can use different cutoffs and analyte targets, so their results should not be compared as if they were identical tests.
What is a false positive?
A false positive occurs when a test indicates the target condition but the accepted reference method does not confirm it. Cross-reactivity, contamination, clerical error, or incorrect interpretation can contribute. A legitimate confirmed medication finding is not necessarily a laboratory false positive; it may be a true analytical result with an authorized explanation.
What is a false negative?
A false negative occurs when the target is present but the test reports negative. Possible explanations include testing outside the window, a cutoff above the concentration, the wrong panel, degradation, dilution, or a method that does not detect a particular drug well.
Does concentration show dose or impairment?
Usually not without substantial additional context. Urine concentration is affected by hydration and excretion. Hair can reflect external and biological factors. Oral fluid and blood may be closer to recent exposure for some drugs, but interpretation remains substance- and method-specific.
What do invalid, adulterated, substituted, and dilute mean?
These are specimen-validity classifications under defined programs. They are not interchangeable and should not be guessed from appearance. Review the laboratory report and applicable rule. A dilute specimen can be negative or positive and may trigger recollection under some policies.
Can laboratories make errors?
Yes, although quality systems, chain of custody, calibration, controls, proficiency testing, review, and confirmation reduce risk. When the stakes are high, obtain the full record, verify identifiers and cutoffs, and use the program’s retest or review procedure.
Should I act on a home-test result?
Follow the instructions and treat a nonnegative home screen as preliminary. FDA advises laboratory confirmation before serious action. Do not start, stop, or change medication based on a home drug test without speaking to a clinician.
Related: Detection and Accuracy, screening versus confirmation, and the complete FAQ.