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Blood tests are done, at regular intervals, to check the overall physical well-being of a person. They help to predict/diagnose/monitor/prognosticate certain illnesses and to make informed decisions by a doctor in treating certain diseases.
Blood tests can be qualitative (just saying present or absent) or quantitative (saying the exact measurement of a substance). Blood tests can be specific to an organ (Thyroid function test) or to a overall well-being of the person (Lipid profile, C-reactive protein, etc.). Some of the blood tests are diagnostic in themselves and some aid in the process of diagnosis.

Blood tests are done on a small amount of blood taken from the veins of your arm or sometimes from a small prick on your finger. Some of the blood tests require fasting, avoiding certain food items, etc. This will be informed before the collection of the blood sample.

Types of blood tests:

1. Blood counts

 A Complete Blood Count (CBC) or hemogram is one of the common tests prescribed by doctors. This test helps to assess the counts of various blood cells like hemoglobin & Red blood cells (RBC, oxygen carrier in blood), White blood cells (WBC, Immune system cells), Platelets (fragments that help in clotting). A CBC helps the doctor in detecting conditions such as anemia, infections, clotting problems, immune system disorders, some allergies, blood cancers, etc.

2. Blood Chemistry tests 

These are a wide range of tests that include tests for metabolic substances (products that are derived from the breakdown of the food that we consume) like glucose, lipid profile, Calcium, electrolytes, etc. Also, the substances formed by organs like the liver and kidney, some of which are enzymes, waste products of the body’s metabolism, and proteins are measured in these tests. A kidney or liver function test would include a panel of tests that assess the functioning of the kidney or liver. A lipid profile test measures the levels of various fractions of fat in the blood like cholesterol, HDL (good cholesterol), LDL (bad cholesterol), etc. 

3. Immunoassays & Serological tests

Some of the blood tests are used to detect the presence or absence of bacterial/viral/parasitic biomarkers. Our body detects these foreign proteins of the microorganisms called Antigens and forms protective protein molecules called antibodies against them, to protect the body. These serological tests detect these proteins and help in the diagnosis and treatment of infections. Some tests detect certain proteins formed by our body against own proteins and are called autoantibodies like the Rheumatoid factor.

4. Specialized tests

Some of the blood tests detect the presence of cancer antigens or biomarkers (proteins formed by cancer cells) and in turn aid in detecting cancers. Some of the blood tests help us in detecting genetic abnormalities, like Karyotyping, Newborn screening tests, Fluorescent in-situ hybridization (FISH), etc. Some of the specialized blood tests like Hemoglobin or serum electrophoresis detect the presence of abnormal hemoglobin or proteins respectively, which cause conditions like sickle cell anemia, thalassemia, multiple myeloma, etc.

Based on your age, physical condition, and other requirements, your doctor will order some of these blood tests periodically. Persons aged >40years, persons suffering from chronic diseases like diabetes, hypercholesterolemia, etc. will require regular follow-up with these tests.