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Blood Tests

Blood tests help doctors check for certain diseases and conditions. They also help check the function of your organs and show how well treatments are working.

PubMed Health Glossary
(Source: NIH - National Heart, Lung, and Blood Institute)

Blood Tests

Blood tests help doctors check for certain diseases and conditions. They also help check the function of your organs and show how well treatments are working.

Specifically, blood tests can help doctors:

  • Evaluate how well organs - such as the kidneys, liver, thyroid, and heart - are working
  • Diagnose diseases and conditions such as cancer, HIV/AIDS, diabetes, anemia (uh-NEE-me-eh), and coronary heart disease
  • Find out whether you have risk factors for heart disease
  • Check whether medicines you're taking are working
  • Assess how well your blood is clotting


Blood tests are very common. When you have routine checkups, your doctor may recommend blood tests to see how your body is working.

Many blood tests don't require any special preparations. For some, you may need to fast (not eat any food) for 8 to 12 hours before the test. Your doctor will let you know how to prepare for blood tests.

During a... Read more about Blood Tests

What works? Research summarized

Evidence reviews

Screening for Colorectal Cancer: An Updated Systematic Review [Internet]

We conducted a systematic review of five key questions to assist the U.S. Preventive Services Task Force (USPSTF) in updating its 2002 recommendation for colorectal cancer (CRC) screening in average-risk adults aged 50 years or older using home fecal occult blood testing (FOBT), flexible sigmoidoscopy (FS), FS and FOBT, colonoscopy, or double-contrast barium enema (DCBE). Key questions for this updated review primarily focused on evidence gaps from the previous review: 1) the accuracy (one-time test performance characteristics) and potential harms of newer CRC screening tests—fecal immunochemical tests (FIT), high-sensitivity FOBT, fecal DNA testing, and CT colonography (CTC)—as possible substitutes for currently recommended CRC screening modalities; 2) updating of evidence on the impact of CRC screening on mortality and to estimate the accuracy and harms of colonoscopy and FS in the community setting. A concurrent decision analysis done by others addressed screening program performance, and compared the life-years gained using different CRC screening tests, test intervals, and stopping ages.

Immunochemical versus guaiac fecal occult blood tests

Bibliographic details: Piper M A.  Immunochemical versus guaiac fecal occult blood tests. Chicago, IL, USA: Blue Cross and Blue Shield Association, Technology Evaluation Center. TEC Assessment Program; 19(5). 2004

What is the Value of Routinely Testing Full Blood Count, Electrolytes and Urea, and Pulmonary Function Tests Before Elective Surgery in Patients with No Apparent Clinical Indication and in Subgroups of Patients with Common Comorbidities: A Systematic Review of the Clinical and Cost-Effective Literature

The evidence base which supported the National Institute for Health and Clinical Excellence (NICE) published Clinical Guideline 3 was limited and 50% was graded as amber. However, the use of tests as part of pre-operative work-up remains a low-cost but high-volume activity within the NHS, with substantial resource implications. The objective of this study was to identify, evaluate and synthesise the published evidence on the clinical effectiveness and cost-effectiveness of the routine use of three tests, full blood counts (FBCs), urea and electrolytes tests (U&Es) and pulmonary function tests, in the pre-operative work-up of otherwise healthy patients undergoing minor or intermediate surgery in the NHS.

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Summaries for consumers

Bowel cancer: Testing for occult blood in the stool

Stool tests can find traces of blood that may be caused by tumors or polyps. A positive test should be followed up by a colonoscopy to find out whether a tumor or an early stage of a tumor really is the cause. It has been proven that the stool test can then lower the risk of dying of bowel cancer.Bowel cancer and advanced bowel polyps can cause bleeding in the bowel. Stool tests – also called stool blood test, or fecal occult blood tests (FOBT) – can detect these traces of blood. But they do not provide a definite answer because only few tumors and polyps leave traces of blood in the stool. There are also harmless causes of blood in the stool, so stool tests are more of a preliminary test. If the results are abnormal, you will be referred to have a colonoscopy to find out the cause. Both the stool test and the following colonoscopy are covered by statutory health insurance in Germany for people aged 50 and above.Stool tests only provide an opportunity to find polyps or bowel cancer early if a bowel endoscopy is performed after abnormal findings in the stool test. So it only makes sense to do a stool test if you are also prepared to have an endoscopy of the bowel if necessary.Chemical stool tests are the most commonly used. These tests use a chemical reaction to detect traces of hemoglobin, a protein in red blood cells. They are the only stool tests covered by statutory health insurance in Germany. That is why we will look at how these tests work in more detail. Three chemical stool tests are used for screening tests covered by statutory health insurance in Germany: Hämoccult, HemoFec and HemoCare.

Understanding tests used to detect bone problems

Just like other tissues and organs in our body, bones can be affected by medical conditions too. These include things like fractures, signs of wear and tear, inflammations and cancer. Injuries and fractures are common in younger people. As we grow older, diseases like osteoporosis and osteoarthritis are more likely to develop. Various tests and examinations can be used to find out what is causing problems like pain or difficulties moving.

Thyroid function tests

The thyroid is a vitally important hormonal gland, which mainly works for the body’s metabolism. It is located in the front part of the neck below the voice box and is butterfly-shaped. The functions of the thyroid gland include the production of the thyroid hormones triiodothyronine (T3) and tetraiodothyronine, also called thyroxine (T4).

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Terms to know

A tissue with red blood cells, white blood cells, platelets, and other substances suspended in fluid called plasma. Blood takes oxygen and nutrients to the tissues, and carries away wastes.
Blood Plasma
The clear, yellowish, fluid part of the blood that carries the blood cells. The proteins that form blood clots are in plasma.
Bone Marrow
The soft, sponge-like tissue in the center of most bones. It produces white blood cells, red blood cells, and platelets.
Protein made by the body that brings about a chemical reaction - for example, the enzymes produced by the gut to aid digestion.
A measure that tells what portion of a blood sample consists of red blood cells. Low hematocrit suggests anemia or massive blood loss.
A protein inside red blood cells that carries oxygen from the lungs to tissues and organs in the body and carries carbon dioxide back to the lungs.
Platelets (Thrombocytes)
A tiny piece of cell that is made by breaking off of a large cell in the bone marrow. Platelets are found in the blood and spleen. They help form blood clots to slow or stop bleeding, and to help wounds heal. Also called thrombocyte.
Red Blood Cells (Erythrocytes)
A cell that carries oxygen to all parts of the body.
One of the major forms of fat that is produced in the liver and found in the blood.
White Blood Cells (Leukocytes)
A type of immune cell. Most white blood cells are made in the bone marrow and are found in the blood and lymph tissue. White blood cells help the body fight infections and other diseases. Granulocytes, monocytes, and lymphocytes are white blood cells.

More about Blood Tests

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Other terms to know: See all 10
Blood, Blood Plasma, Bone Marrow

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