Articles on Medical Diseases and Conditions

Tag: Sensitivity

  • Sensitivity and Specificity of Syphilis Tests

    Studies have been done in which duplicate samples from known syphilitic patients in various stages of their disease and also from normal persons were sent to various laboratories. Besides this, many reports have appeared from laboratories all over the world comparing one test with another in various clinical stages of syphilis, in nonsyphilitic diseases, and in apparently normal persons. These results are summarized in Table 15-3.

    Comparison of serologic tests for syphilis

    Table 15-3 Comparison of serologic tests for syphilis (approximate percentage reported reactive)*

    Table 15-3 demonstrates considerable variation in results. Several factors must be involved besides the inherent sensitivity and specificity of the individual tests themselves:

    1. Antibiotic treatment may cause some previously reactive syphilitic patients to become nonreactive.

    2. Some clinically normal persons may have unsuspected subclinical syphilis.

    3. True BFP reactions, either acute or chronic, must be taken into account.

    4. There is obvious variation in technique and ability between laboratories. Some laboratories introduce their own modifications into standard techniques.

    5. The time of appearance differs for the various antibodies. In general, the FTA-ABS test result becomes positive in significant numbers of patients in the middle or end of the primary stage, followed by the MHA and RPCF, and then the VDRL. All of these procedures usually give positive results in the secondary stage, and also probably in the early latent stage.

  • Sensitivity and Specificity

    Interpretation of laboratory test results is much more complicated than simply comparing the test result against a so-called normal range, labeling the test values normal or abnormal according to the normal range limits, and then fitting the result into patterns that indicate certain diseases. Certain basic considerations underlie interpretation of any test result and often are crucial when one decides whether a diagnosis can be made with reasonable certainty or whether a laboratory value should alter therapy.

    All laboratory tests have certain attributes. Sensitivity refers to the ability of the test to detect patients with some specific disease (i.e., how often false negative results are encountered). A test sensitivity of 90% for disease Z indicates that in 10% of patients with disease Z, thetest will not detect the disease. Specificity describes how well test abnormality is restricted to those persons who have the disease in question (i.e., how often false positive results are produced). A specificity of 90% for disease Z indicates that 10% of test results suggestive of disease Z will, in fact, not be due to disease Z.