1) As we know that blood is a fluid connective tissue , it freely flows from one part to the other part of the body .
2) Any pathogenic organism which enters our body will be in the body stream and can be detected by testing the blood sample .
3) We know that blood transports gases , digested food , hormones and various waste materials from different parts of the body . Hence it is very easy to check all these components levels whether they are in normal or abnormal level , just by taking the blood sample testing .
4) So, doctors prefer to go for a blood test before diagnising any disease . Complete Blood Picture (CBP) gives the ampul information about the patient and helps in diagnising the disease .