Chapter 18: Common Laboratory Tests

Introduction

This chapter discusses normal and pathologic values for commonly ordered tests of the blood (cells and chemistries), urine, cerebrospinal fluid (CSF), and serous fluids. The tests discussed are commonly used to formulate physiologic and diagnostic hypotheses. The much more numerous, specific tests used for confirming the diagnosis of a specific disease should not be used until a narrow differential diagnosis has been established. These more specific tests are not discussed here.

Laboratory tests are ordered for one of the four reasons:

1. Screening. A small number of tests have been demonstrated to find “silent” disease in the patient who has no symptoms or signs or specific risk factors for the disease. Common examples include testing for hemochromatosis with iron studies and lipids for hypercholesterolemia.

2. Case finding. Some tests are used to identify affected symptomatic individuals within specific at-risk populations. This differs from screening because a specific high-risk population, rather than the general population, has been selected for testing. An example is testing the children of patients with breast cancer related to the BRCA genes for this genetic abnormality.

3. Diagnosis. This is the use of tests to assist in making (or excluding) a diagnosis suggested by the patient’s symptoms and signs. See the discussion in Chapter 17 for a summary of the proper approach to diagnostic testing.

4. Monitoring. Tests are often used to monitor the progress of disease, response to therapy, or concentration of medication.

Many laboratory tests are used for more than one, or even for all, of these reasons, depending on the clinical situation. For example, blood glucose is used to screen for diabetes mellitus, to identify cases amongst obese patients with a family history of diabetes who are at high risk for diabetes, to confirm the diagnosis, and to monitor treatment in patients found to have the disease.

The decision as to which tests, if any, should be obtained routinely can be debated interminably. Certainly, the prevalence of the disease in the population of which your patient is a member should affect the selection [Sox HC. Probability theory in the use of diagnostic tests. An introduction to critical study of the literature. Ann Intern Med. 1986;104:60–66]. In addition to assisting in the diagnosis, quantitative test results help to grade the severity of the physiologic abnormalities and provide objective verification for the purposes of documentation. Tests and their usefulness continually change, so the clinician must keep abreast of current indications for and uses of tests available in his clinical laboratory. Consultation with the pathologist in charge of the clinical laboratory is often useful when questions arise.

The reference ranges are presented for purposes of illustration only. Because each clinical laboratory determines its own reference ranges, those we have listed are not intended to be definitive.

Many organizations, including the American Medical Association, have supported the proposal of the American National Metric Council to ...