Screening tests are designed to identify undetected medical problems in people who feel well and don't have any presenting symptoms. So, the main goal of screening tests is early diagnosis of the disease, so as to improve prognosis and increase success rate of the therapeutic measures. 

The need of screening tests is assessed according to many factors, such as personal or family history as well as presence of certain risk factors (obesity, smoking, hypertension, diabetes… etc.). 

To make it more obvious, let's discuss colorectal cancer as an example. According to medical studies, colorectal cancer develops very slowly over many years. Through these years, benign pre-cancerous polyps usually develop on the inner lining of the colon or rectum, without any symptoms at all.  

According to these facts, regular screening of colorectal cancer (by endoscopy) helps in detection and removal of these benign polyps, which in turn prevents development of colorectal cancer. In general, colorectal cancer screening is recommended for all men over 50 years (40 years for persons with positive family history). 

On the same context, the most significant screening tests that should be in consideration include the following: 

• Breast cancer screening (mammogram): it should be started at the age between 40 – 50 years old. 
• Skin cancer screening: it should be applied on people with excessive exposure to direct sunlight (like sailors) or in cases of formation of an abnormal mole. 
• Screening of lipid profile: for assessment of cholesterol and triglyceride levels which could be responsible for atherosclerosis and heart disease. 
• Screening of diabetes (fasting blood sugar): especially in obese people or who have positive family history.