Diagnostic Tests

Quality and Effectiveness

The quality and effectiveness of a diagnostic test is often measured by its accuracy. The accuracy of a test can be evaluated through four basic factors: sensitivity, specificity, positive, and negative predictive values. The sensitivity of a test is the probability that the test results reveal accurate diagnosis of an existing disease. The sensitivity is the proportion of accurate diagnoses correctly identified by the test. The specificity of a test is the probability that the test results will accurately identify individuals without the targeted disease. The positive predictive value of a test measures the percentage of individuals with a positive test result who actually have the disease. The negative predictive value measures the percentage of individuals with a negative test that do not have the disease.

Types of Medical Tests

Analysis of body fluids commonly includes of tests of the blood, urine, as well as the fluid surrounding the spinal cord and brain (cerebrospinal fluid). Less commonly, fluids such as sweat, saliva, or fluid from the digestive tract are assessed. Body fluid analysis can be used for many different purposes, including determining whether a person has an infection to determining whether a person has cystic fibrosis. Imaging provides a picture of the inside of the body. The most common test in this category is the X-ray. Some other commonly used examples are ultrasound, radioisotope (nuclear) scans, computed tomography (CT) scans, magnetic resonance imaging (MRI), and positron emission tomography (PET) scans. The test is selected based on what the examiner is wishing to determine, and the cost of each exam varies considerably.

Endoscopy is a test that looks inside the body. An endoscope is a long flexible tube that has a camera and light inside it, permitting a physician to view images on a monitor during the procedure. An endoscope may be passed through the mouth to look at the esophagus or stomach (esophagogastroduodenoscopy), or can be passed through the rectum to look in the large intestine (colonoscopy). A newer endoscopic test consists of swallowing an extremely small capsule that contains a camera, and that takes pictures, permitting observation of areas without use of anesthesia.

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