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In general, laboratory tests are useful adjuncts in establishing a rheumatologic diagnosis but are not absolutely diagnostic of any specific disease. Two features of rheumatologic diseases contribute to the difficulties of interpreting laboratory tests. First, many rheumatic diseases are chronic systemic inflammatory diseases and, therefore, share many laboratory abnormalities with other such diseases, particularly chronic infections and malignancies. Second, the prevalence of certain rheumatologic diseases is low in most patient populations. Therefore, even if sensitivity and specificity of a test are high for a specific disease, the positive predictive value of the test may be low. Considering these statistical characteristics of laboratory tests can help the clinician interpret the data within the context of the clinical case.

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Statistical Characteristics of Laboratory Tests

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Appropriate use of laboratory tests requires awareness of the rates and causes of false-positive and false-negative test results (see box, Defining Statistical Characteristics of Laboratory Tests). The sensitivity of a test demonstrates the ability of the test to detect a patient with disease and is measured by the proportion of people with disease who have a positive test result. The specificity of a test demonstrates the ability of the test to avoid detecting patients without disease and is measured by the proportion of people without disease who have a negative test result. The usefulness of a laboratory test is best reflected in the positive predictive value, which determines the proportion of patients with a positive test result who truly have the disease. The positive predictive value of a test depends on the prevalence of the disease in the population being examined (or the pretest probability of disease); thus, even if the sensitivity and specificity of a test are 99%, the positive predictive value of the test can be low if the prevalence of disease in the population is extremely low. The negative predictive value of a test determines how many patients with a negative test result truly do not have the disease. The negative predictive value also depends on the prevalence of the disease. The generally low prevalence of rheumatologic disease in the overall population means that many rheumatologic laboratory tests will only have a high positive predictive value when the tests are selected on the basis of clinical presentations that are highly suggestive of a rheumatologic disorder, which increases the pretest probability of disease.

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Box 3‐A. Defining Statistical Characteristics of Laboratory Tests
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The American College of Rheumatology (ACR) has published guidelines on some immunologic tests following review of the literature. Studies were evaluated for quality, and likelihood ratios were calculated through summary of the “best quality” studies.

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The likelihood ratio (LR) is the likelihood that a given test result would be expected in a patient with the target disorder compared to the likelihood that that same result ...

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