Diagnostic accuracy has been greatly enhanced by the sensitive and specific tests available in modern clinical laboratories and the rapid advances in clinical imaging. However, it is essential to recognize that proper use of the laboratory and imaging modalities is dependent upon accurate clinical hypotheses generated by the clinician at the bedside. Laboratory tests and diagnostic imaging can provide reliable and valid answers to well-conceived clinical questions, but they are also liable to over interpretation and can be quite misleading if not interpreted in the clinical context as answers to particular questions. Beyond a few screening tests, these studies should be used to test the physiologic and diagnostic hypotheses generated during the history and physical examination. The laboratory and the radiology suite are not the places to go looking for ideas; they are the place to test your ideas. If you are unable to generate testable hypotheses after the history, physical examination, and screening tests, it will be more useful to seek consultation than to begin an undirected series of laboratory and radiologic studies.
A full discussion of the proper use of diagnostic tests is beyond the scope of this text. The reader is referred to Clinical Epidemiology: The Essentials [Fletcher RH, Fletcher SW, Fletcher GS. Clinical Epidemiology, the Essentials. 5th ed. Baltimore, MD: Lipincott, Williams & Wilkens; 2012.] and is especially valuable. The reader is advised to read this text and incorporate the principles of clinical epidemiology into your everyday practice.
Principles of Laboratory Testing
Laboratory testing is principally done for two reasons: (1) to obtain information that cannot be determined clinically, but which is often important in forming hypotheses and (2) to test hypotheses.
Tests in the first category are those commonly described as “routine” testing and include serum electrolytes, blood urea nitrogen, creatinine, complete blood counts, urinalysis, and, less commonly, transaminases and erythrocyte sedimentation rate or C-reactive protein. Some, or all, of these tests are performed in patients with significant illness in order to help the clinician identify significant abnormalities in major organ function or laboratory signs of inflammation or infection.
Tests in the second category are innumerable, and more are being developed as you read. They are used to identify specific abnormalities and diseases. The diagnostic performance of these tests is highly dependent upon the patient population tested. Tests in this category are most useful when the diagnosis in question is in the mid-range of probability on the basis of your clinical assessment—that is, the probability for the disease being present is roughly between 20% and 80%.
To understand why this is so, it is necessary to understand the measures of test performance and how interpretation is dependent on both the diagnostic criteria for a disease or condition and the pretest probability that the disease is present.
Principles of Testing for Disease