Diagnostic Agents

chemical substances used to reveal, pinpoint, and define the localization of a pathological process.

The principal diagnostic agents are X-ray-contrast preparations, radioactive isotopes, and dyes. X-ray-contrast preparations are substances that possess the property of absorbing and retaining X-radiation more weakly or more strongly than the tissues of the body; introduction of these substances into the hollow organs makes it possible under X-ray examination to get an idea of the configuration of the organ, its volume, the character of its inner surfaces, and the presence of pathological changes. Used as X-ray-contrast diagnostic agents are gaseous substances (air, oxygen, nitrous oxide, carbon dioxide), which absorb X-rays more weakly than do body tissues; fluid substances, such as iodine-containing preparations in the form of aqueous (Diodrast, Urokon Sodium, Sergosin) and oily (propyliodone) solutions or suspensions; and substances administered orally in the form of powders and suspensions (Chlografin). An aqueous suspension of barium sulfate is used for X-ray examination of the gastrointestinal tract. X-ray-contrast agents are introduced directly into the organ to be examined (for the esophagus, stomach, intestine, bronchi, uterus, heart), orally or intravenously in examining the liver, bile ducts, and kidneys, or intraarterially in contrast examination of the blood vessels (angiography) and heart.

The use of radioactive substances (isotopes or tagged compounds) for diagnosis is based on the principle of recording the rays (mainly gamma rays) emitted by them when they are introduced into the body. Isotopes do not differ from nonradioactive substances in their chemical properties; they play the same role in the life processes of the body. The functional capability of a given organ may be judged by the intensity of the isotope uptake by that organ or tissue. Suitable for diagnosis are radioactive isotopes that have i- or hard β-radiation, which have a short halflife and do not yield long-lived daughter products. The radioactive isotopes of sodium, phosphorus, iodine, gold, iron, copper, potassium, and arsenic are widely used. In addition to radioactive substances in the ionic state, complex organic and inorganic compounds that are tagged with radioactive isotopes are also used (for example, diiodofluorescein, serum albumen, and rose bengal).

The diagnostic dyes include indigocarmine and fluorescein. Indigocarmine, for example, is used to elucidate the functional condition of the kidneys. The preparation is injected intravenously and then the speed and quantity of dye discharged from the ureters are determined visually by cystos-copy.

Diagnostic agents are used in doses that are harmless to the body; they are isotonic in relation to the body fluids and mix well with them, accumulating selectively in the appropriate organs, and they are readily and completely eliminated from the body in unaltered form.