Antioxidant assays in biological fluids are used to detect and monitor various diseases. Biological fluids such as plasma, blood, serum, saliva, and urine are valuable tools for measurement of antioxidant level by potential antioxidant assay. These assays usually measure the total antioxidant capacity (TAC) or antioxidant activity of certain antioxidants. When the excessive production of free radicals suppresses the natural antioxidant defence system of the body, then the oxidative stress takes place. The highly reactive molecules of free radicals cause destruction of cell and develop chronic diseases. This review will investigate the application of antioxidant assays to detect various diseases. Antioxidant assays such as ferric reducing antioxidant power (FRAP), cupric reducing antioxidant capacity (CUPRAC), oxygen radical absorbance capacity (ORAC), etc., are commonly used for measurement of overall antioxidant activity. These assays show how the body neutralizes the free radicals by its own ability. Specific antioxidants such as glutathione or Vitamin-C provide specific information about potential deficiencies. The conjugation of antioxidant capacity with other disease markers can examine how oxidative stress plays a role in the emergence of specific diseases. Low blood antioxidant levels indicate a possible connection between the development of cardiovascular disease and oxidative stress. Though the antioxidant assays are promising for disease detection, they also have some limitations. These assays can be affected by various factors like diet, medications, environment, sample collection procedure, etc. Furthermore, a single antioxidant test or low antioxidant capacity may not be able to diagnose a specific disease to the individuals. So, with antioxidant assays other factors should be considered for disease management.