LabMedicine, 2009:40, 105-113.

According to Dr. Michael Laposata, the medical specialty that nearly every practicing physician relies on every day, for which training in many medical schools is limited to no more than a scattered few lectures throughout the entire curriculum, is “laboratory medicine.” The importance of understanding the principles for selecting and ordering the most rational laboratory test(s) on a specific patient is heightened in the current age of managed care, medical necessity, and outcome-oriented medicine. The days of a “shotgun approach” to ordering laboratory tests has, of necessity, been replaced by a “rifle” (or targeted) approach based on an understanding of the test’s diagnostic performance and the major “legitimate” reasons for ordering a laboratory test. Such an understanding is critical to good laboratory practice and patient outcomes.

The purpose of this CE Update is to discuss the laboratory testing cycle and its importance in diagnostic decision making. This discussion will begin with some general comments about approaches to ordering clinical laboratory tests, followed by “real-world” examples to illustrate these approaches. We will then review the important diagnostic performance characteristics of laboratory tests, how they are calculated, and a principal tool (ie, receiver-operator characteristic [ROC] curves) used to assess the diagnostic accuracy of a laboratory test at specific cutoff values for the test. We will then discuss how laboratory tests are interpreted using a reference interval and its limitations, followed by some brief remarks about the concepts critical difference and neural network.

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The various multiparameter blood chemistry and hematology profiles offered by most labs represent an economical way by which a large amount of information concerning a patient’s physiologic status can be made available to the physician. The purpose of this monograph is to serve as a reference for the interpretation of abnormalities of each of the parameters.

Reference ranges (“normal ranges”)
Because reference ranges (except for some lipid studies) are typically defined as the range of values of the median 95% of the healthy population, it is unlikely that a given specimen, even from a healthy patient, will show “normal” values for all the tests in a lengthy profile. Therefore, caution should be exercised to prevent overreaction to miscellaneous, mild abnormalities without clinical correlate.

Units of measurement: America against the world
American labs use a different version of the metric system than does most of the rest of the world, which uses the Système Internationale (SI). In some cases translation between the two systems is easy, but the difference between the two is most pronounced in measurement of chemical concentration. The American system generally uses mass per unit volume, while SI uses moles per unit volume. Since mass per mole varies with the molecular weight of the analyte, conversion between American and SI units requires many different conversion factors. Where appropriate, in this paper SI units are given after American units. Dennis Jay, PhD, has kindly made available an online converter between SI and conventional units: