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 in formulating 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 identify silent disease in patients without symptoms, signs, or specific risk factors for the disease, e.g., testing for hemochromatosis with iron studies and lipids for hypercholesterolemia.

2. Case finding. Some tests identify affected asymptomatic individuals within an at-risk population. This differs from screening because a high-risk population, rather than the general population, is selected for testing, e.g., testing the children of BRCA-related breast cancer patients for the genetic abnormality.

3. Diagnosis. Tests 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 diabetic patients.

Which tests, if any, to obtain routinely is debated interminably. Certainly, the prevalence of the disease in the population of which your patient is a member should affect test selection. In addition to assisting diagnosis, quantitative tests reflect the severity of physiologic abnormalities. Tests and their usefulness continually change, so the clinician must keep abreast of current indications for and uses of clinical laboratory tests. Consult with the pathologist in charge of the clinical laboratory when questions arise.

Reference ranges are only illustrative; each clinical laboratory determines its own reference ranges.

Many organizations, including the American Medical Association, have supported the proposal of the American National Metric Council to convert result reporting to Système International d’Unités (SI) units introduced in the mid-1980s. US physicians, laboratory staffs, and hospitals have been reluctant to convert to SI units. Indeed, excellent medical journals have chosen to use conventional units, or both. For the non-American reader’s convenience, we have included values with SI units in parentheses following the conventional units. Most of the laboratory reference values were adopted from Harrison’s Principles of Internal Medicine.

Each test is followed by a list of diseases and disorders associated with abnormal values. The lists of associated diseases, syndromes, and ...