The NK (Natural Killer) cell test is a type of blood test that is performed to check the number and functioning of natural killer cells in the body. Natural Killer cells are a type of white blood cells that play a critical role in the immune system's response to viruses and various forms of cancer. The procedure begins with collecting a blood sample from the patient. The sample is then sent to a laboratory where it is analyzed to determine the percentage and absolute count of NK cells.

The NK cell count can be performed by using methods such as flow cytometry, a technique used to detect and measure physical and chemical characteristics of a population of cells or particles. The second part of the test assesses how well the NK cells are functioning. This is usually performed using a cytotoxicity test, wherein the NK cells are mixed with target cells that they should typically destroy. If the NK cells are functioning correctly, they will kill the target cells. The results of an NK cell test can help diagnose certain diseases and conditions, like immunodeficiencies or autoimmune diseases, and can be crucial in determining the course of treatment.