Top 10 laboratory tests: blood will tell - includes related information - part 1

FDA Consumer, July-August, 1989 by Dixie Farley

TOP 10 LABORATORY TESTS:

The heart flutters, the palms moisten, and the patient looks away as the nurse draws blood from a vein.

Another medical test. Is it really necessary?

With today's seemingly endless rise in health-care costs, insurance providers and even health professionals are asking that question. Patients, too, should ask questions so they can be as informed as possible about whether a recommended test is safe and needed. That's the judgment of both the Health Industry Manufacturers Association, whose member companies make instruments and equipmen t used in the tests, and the College of American Pathologists, physicians who are ultimately responsible for interpreting test results. To help with that "informing," FDA Consumer is this month beginning a series of articles on the most common in vitro diagnostics, test on samples of tissue and fluids taken from the body.

The leading in vitro diagnostics happen to be blood tests. It's not hard to figure out why.

Always on the move throughout the body, blood takes oxygen, nutrients, and other essentials of life to every cell. It carries away wastes of cell metabolism, transports hormones from the glands to appropriate sites in other tissue, protects against foreign invaders, and helps maintain a safe temperature. Moreover, this giver and sustainer of life is a veritable barometer of health, which is why blood tests can provide abundant clues to the body's well-being. The top 10 tests, known as "chemistries," analyze blood for glucose, creatinine, blood urea nitrogen, lactic dehydrogenase, bilirubin, potassium, chloride, sodium, calcium, and carbon dioxide.

Nowadays, various "chemistries" from a single blood sample can be run simultaneously, thanks to automated instruments such as the sequential multiple analyzer and, in physicians' office laboratories, small table-top analyzers. Sometimes, samples are put on a plastic slide (dipstick) for insertion into a machine that "reads" the samples much like an instant camera takes a picture. Automated machines can be programmed to identify blood specimens, separate plasma from serum, select specified tests, add needed chemicals (which vary with the test), and quickly calculate results. A computer printout shows both the patient's test results and the normal range for each test. Abnormal levels may be flagged. These analyzers and most other laboratory equipment are regulated as medical devices by the Food and Drug Administration's Center for Devices and Radiological Health, whereas laboratories themselves are the province of the Health Care Financing Administration. (See "Watching the Testers," page 23).

Blood chemistries are performed on plasma (the fluid part of blood) or serum (the watery portion of clotted blood). As a rule, the blood sample--a teaspoon or so--is taken from an arm vein. When this isn't practical or when only a few drops are needed for one or two tests, the sample may be drawn from a finger, ear, big toe, or heel. Some tests require special directions for the patient, such as fasting, that must be carefully followed so as not to skew results. It's also important to report any medications being taken, as certain drugs can distort the findings.

Depending on the laboratory equipment and the sex and age of the patient, the normal range of a substance in the sample can vary. Thus, the "normals" listed for the following tests are only examples, taken from Clinical Laboratory Tests, by Kathleen Treseler, a nurse educator at Seattle University. The way the tiny measurements are expressed differs from test to test. These terms are described in "Terminology of Lab Testing," page 26.

Glucose and the Fuel System

Measurement: The amount of glucose in serum.

Normal range: 60 to 110 milligrams per deciliter (mg/dL).

Use: To determine whether the body is regulating glucose properly. The test is vital for diabetics.

Comments: Several organ systems help make glucose--a type of sugar that is the body's main fuel--available for use. The small intestine, for instance, absorbs different types of sugar taken in from a meal. Glucose enters the bloodstream immediately; other sugars must be converted into glucose by the gut or liver. The liver also converts proteins and other nutrients into glycogen (a carbohydrate) and fat for storage and, when necessary, changes, the stored nutrients back into glucose. Glucose regulation is affected by various hormones, but the main influence comes from insulin, a hormone (secreted by the pancreas) that helps glucose enter cells where the body can use it.

The fasting glucose test shows roughly how well the body regulates glucose and, if there's a problem, what's likely to be causing it. The patient is told not to eat or drink anything but water for eight hour before the blood sample is to be drawn. Often, a two-hour, after-meal test is used. It shows how well the body handles a measured amount of glucose. Usually, "fasting blood" provides a base. The patient is then given a measured amount (about 100 grams) of glucose. Exactly, two hours later, a second blood sample is taken to see whether the patient's glucose level has returned to normal.