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Learning Objectives

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  1. To understand the concepts of sensitivity, specificity, predictive value, prevalence, and incidence.

  2. To learn the frequently encountered preanalytical variables that influence laboratory test results.

  3. To identify the well-known interferences in many of the laboratory tests.

  4. To understand the individual steps in specimen processing and handling.

  5. To understand the guidelines for appropriate selection of laboratory tests.

  6. To understand how cell injury and inflammation result in the generation of plasma markers of these processes.

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An understanding of the principles set forth in this chapter is essential for the appropriate selection of laboratory tests and the accurate interpretation of the test results.

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Analytical and Statistical Concepts in Data Analysis

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Ranges Used in the Interpretation of Test Results

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In clinical practice, the laboratory test result is typically placed alongside a range of values for that test. In most cases, this is the reference range, which is often considered to be the normal range. It is important to understand that individuals with values inside the reference range may have subclinical disease, despite the presence of an apparently normal value. The reference range is dependent on the instrument and reagent used to perform the test. The reference ranges are ideally established inside the laboratory where the test is being performed. Reference ranges supplied by instrument and reagent manufacturers are not likely to correspond perfectly to ranges generated within an individual laboratory. This is because the population used to establish the range by the manufacturer and/or the instruments and reagents used by the manufacturer are likely to be different from those in an individual clinical laboratory.

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The Reference Range

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To obtain a reference range, individuals without disease and on no medications donate samples for testing. A distribution of these values, which should be numerous enough to be statistically reliable, is plotted. The data are not always distributed in a Gaussian pattern. Therefore, statistical methods that are nonparametric are used to identify the central 95% of values. This range, representing the middle 95% of results, is the reference range. As an indication that being outside the reference range does not always reflect the disease, 5% of normal healthy, nonmedicated individuals who donated samples for the reference range determination now fall outside of what has become the reference range for the test.

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To obtain a reference range, individuals without disease and on no medications donate samples for testing. The middle 95% of results is the reference range.

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The Desirable Range

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Several decades ago, the results for cholesterol testing demonstrated that individuals eating a high-fat diet showed high cholesterol levels that were associated with atherosclerotic vascular disease. When these apparently healthy, nonmedicated individuals provided samples for reference range determinations, the central 95% of values from this population provided an inappropriately high reference range. Therefore, the use of the classical reference ...

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