Chapter adapted and updated, with permission, from Nicoll D et al. Guide to Diagnostic Tests, 7th ed. McGraw-Hill, 2017.
Table e2–2 lists the general characteristics of useful diagnostic tests. Most of the principles detailed below can be applied not only to laboratory and radiologic tests, but also to elements of the history and physical examination. An understanding of these characteristics is very helpful to the clinician when ordering and interpreting diagnostic tests.
Table e2–2.Properties of useful diagnostic tests. ||Download (.pdf) Table e2–2. Properties of useful diagnostic tests.
|1. Test methodology has been described in detail so that it can be accurately and reliably reproduced. |
|2. Test accuracy and precision have been determined. |
|3. The reference interval has been established appropriately. |
|4. Sensitivity and specificity have been reliably established by comparison with a gold standard. The evaluation has used a range of patients, including those who have different but commonly confused disorders and those with a spectrum of mild and severe, treated and untreated disease. The patient selection process has been adequately described so that results will not be generalized inappropriately. |
|5. Independent contribution to overall performance of a test panel has been confirmed if a test is advocated as part of a panel of tests. |
|6. Test is accessible and affordable. |
The accuracy of a laboratory test is its correspondence with or closeness to the true value. A test is deemed inaccurate when the result differs from the true value even though the results may be reproducible (Figure e2–1A); this is also called systematic error (or bias). For example, serum creatinine is commonly measured by a kinetic Jaffe method, which has a systematic error as large as 0.23 mg/dL (20.33 mcmol/L) when compared with the gold standard gas chromatography–isotope dilution mass spectrometry method. In the clinical laboratory, accuracy of tests is maximized by calibrating laboratory equipment with standard reference material and evaluated by participation in external proficiency testing programs (eg, proficiency testing program offered by the College of American Pathologists).
Relationship between accuracy and precision in diagnostic tests. The center of the target represents the true value of the substance being tested. A: A diagnostic test that is precise but inaccurate; repeated measurements yield very similar results, but all results are far from the true value. B: A test that is imprecise and inaccurate; repeated measurements yield widely different results, and the results are far from the true value. C: An ideal test that is both precise and accurate. (Reproduced, with permission, from Nicoll D et al. Guide to Diagnostic Tests, 7th ed. McGraw-Hill, 2017.)
Test precision is a measure of a test's reproducibility or repeatability when repeated on the same sample. If the same ...