a method of diagnosing diseases and studying the physiological condition of a human organism based on the investigation of cell morphology and cytochemical reactions.

Cytological diagnosis is used in various branches of medicine. In oncology it is used to diagnose malignant and benign tumors, in mass screenings to detect the early stages of cancer and pre-cancerous diseases, and to observe the course of antitumor treatment. In hematology it is used to diagnose various diseases and evaluate therapy effectiveness, and in gynecology, to diagnose various types of cancer and to determine pregnancy and hormonal disturbances. It is also used to diagnose many diseases of the respiratory, digestive, urinary, and nervous systems and to evaluate the results of therapy.

Cytological examination is valuable in diagnosing various types of malignant tumors, in assessing the extent of malignancy, and in determining the type of tumor tissue involved; the histological form and size of a tumor are established in 70–85 percent of the cancer cases. Various criteria have been developed for the cytological diagnosis of diseases of the hematopoietic and reticulo-endothelial systems, some diseases of the stomach and kidneys, tuberculosis, skin diseases, and a number of other diseases. When required, cytological diagnosis can be made quickly. Cytological methods are often combined with histological methods.

Cells for examination are obtained by a variety of methods. In the exfoliative method, cells that are sloughed off naturally in the normal fluids of the body, such as blood, are studied, as well as cells that are found in pathological discharges, such as sputum, or that are artificially separated by washing and desquamation. In another method, the material is extracted with a fine needle through a puncture (exploratory puncture). The introduction of endoscopy into clinical practice has led to the widespread use of the biopsy method of cytological diagnosis.

The specimen is prepared on slides for cytological examination. Various techniques are used, depending on the purpose of the examination, for example, microscopy of material in the normal state or of fixed and stained material, using standard staining techniques, and phase-contrast microscopy, ultraviolet microscopy, and fluorescence microscopy (with fluorochromes used to stain the preparations). A number of cytochemical methods have found practical applications. Special methods are also used in research, including autoradiography, immunocytochemistry, spectrophotometry, fluorimetry, electron microscopy, and tissue culture.