Interpreting Laboratory Tests

Elizabeth A. Warner and Arthur H. Herold

Chapter contents

The Concept of "Normal"

176

Glucose

191

Evaluating a Test's Performance Characteristics

177

Glycosylated Hemoglobin (Hemoglobin A1c)

192

Separating Diseased from Disease-Free Persons

177

Helicobacter pylori

192

Hepatitis Serology

192

Multiple Test Ordering

179

Human Immunodeficiency Virus

193

Considerations for Ordering Tests

179

Iron Studies

193

Albumin

180

Lipid Profile

194

Alkaline Phosphatase

180

Magnesium

195

Aminotransferases

180

Mononucleosis (Epstein-Barr Virus Infection)

195

Amylase and Lipase

181

Natriuretic Peptides (BNP and N-terminal pro-BNP)

196

Antinuclear Antibodies

181

Phosphorus

196

Bilirubin

182

Potassium

197

Blood Urea Nitrogen and Creatinine

183

Pregnancy Tests

197

Calcium

183

Prostate-Specific Antigen

197

Carcinoembryonic Antigen

183

Total Protein

198

Chloride

184

Rheumatoid Factor and Anti-Cyclic Citrullinated Peptide Antibodies

199

Coagulation Studies

185

Sodium

199

Cobalamin (Vitamin B12) and Folic Acid Deficiency

186

Streptococcal Testing

201

Complete Blood Count

186

Syphilis Testing

202

Carbon Dioxide or Bicarbonate

188

Thyroid Testing

202

C-Reactive Protein

189

Uric Acid

203

Erythrocyte Sedimentation Rate

189

Urine Drug Screens

203

Fecal Occult Blood Test

190

Vitamin D

204

The use of the clinical laboratory to evaluate patients for the presence or absence of disease transcends medical and surgical specialties. Physicians in all areas of medical practice are dependent on laboratory testing to arrive at a correct diagnosis. Because many factors increase the uncertainty associated with a test result, physicians need to understand the limitations of interpreting test results.

Clinical decision making using diagnostic laboratory testing is based on the assumption that a given test is accurate and precise. Diagnostic test accuracy is the ability of a test to distinguish patients with a disease from those who are disease free (Leeflang et al., 2008). Test accuracy is not necessarily fixed; accuracy may vary among patient populations and with different clinical conditions. Precision is a measure of the reproducibility of a test measurement when the same specimen is rechecked under the same circumstances. Sources of imprecision include biologic variability and analytic variability. Biologic variability is the variation in a test result in the same person at different times because of physiologic processes, constitutional factors, and extrinsic factors (McClatchey, 2002) (Table 15-1). Analytic variation refers to the variation in repeated tests on the same specimen and relates to analytic technique and specimen processing. With current technology, biologic variation plays a larger role than analytic variation in most laboratory tests.

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