The vitamin D test is an immunoassay based on the principle of competitive binding. During the test, the mixture migrates upward on the membrane chromatographically by capillary action. The membrane is pre-coated with 25 (OH) D antigens on the test line of the strip. During the test, 25 (OH) D present in the sample will compete with 25 (OH) D on the test line for limited amount of anti-OH Vitamin D 25 antibodies in the conjugate. The higher the concentration of 25 (OH) D in the sample, the lighter the T line appears. The result will be read according to the color card supplied with the kit. To serve as a procedural control, a colored line will always appear in the control line region, indicating that the correct volume of sample has been added and that membrane elimination has occurred.