Blood work helps identify risk factors, diagnose conditions and diseases

Your doctor may order a blood test if you have certain symptoms or conditions, or if disease, such as cancer, is suspected. A blood test can also help your doctor assess your body’s response to medication or treatment, identify risk factors for disease, and check the function of liver, kidneys and heart. Once the blood is drawn it goes to a laboratory for analysis.

Different types of blood tests and why they are done

Depending on what the doctor is looking for, your blood test may include:

  • Blood chemistry – may include analysis of blood glucose (sugar), cholesterol, protein, hormones, minerals and vitamins, to determine your general health
  • Blood smear – used when a complete blood count is inconclusive or abnormal, for closer examination of red and white blood cells and platelets, their size and activity
  • Complete blood count (CBC) – to look for signs of anemia, infection, or blood cancer, and to identify or monitor changes in blood count due to condition or treatment
  • Flow cytometry – uses a special instrument to assess samples of blood or bone marrow, analyzing blood cell size and shape, and determining whether certain biomarkers exist, particularly for blood cancer
  • Fluorescence in situ hybridization (FISH) – uses special dye to highlight genetic material in blood or bone marrow cells, which can help determine treatment approaches
  • Immunophenotyping – often a follow-up test to a CBC, used to distinguish lymphoma and leukemia cells, and help determine treatment
  • Karyotype test – to detect chromosomal changes or abnormalities that may indicate disease, genetic abnormality that can be passed on to children, or in pregnant women, to detect or determine birth defects
  • Lipoprotein panel – to check for risk of heart disease by measuring level of fats in blood (“bad” and “good” cholesterol and triglycerides)
  • Metabolic panel – to evaluate your heart, kidney or liver function by assessing blood glucose, calcium, electrolytes
  • Polymerase chain reaction (PCR) test – used to “amplify” small samplings of DNA for easier detection and analysis of individual genes
  • White blood cell (WBC) differential – often done in conjunction with CBC to determine whether types of cells are in normal proportion to one another and can further help diagnose, monitor and treat diseases and conditions

How a blood test is done

A blood sample is typically taken via a needle in the arm or in some cases by finger prick. There may be mild pain where the needle sticks, but the process is done very quickly by skilled lab workers. Blood tests are either “fasting” or “non-fasting,” meaning you may be asked to not eat or drink anything for 12 hours in advance of the test, or you may proceed to the test at any time, regardless of when you last ate or drank.

Understanding the results from a blood test

Once your blood has been sampled and analyzed, the results are provided to your doctor who will then advise you of the results and any next steps needed. In some cases, you may not get a call back if the result was “negative” (no signs of disease or cause for concern). A “positive” result means that the test indicates there is an abnormality of some sort. Your doctor will thoroughly explain this to you in language that is easy for you to understand.

There can sometimes be “false-positive” or “false-negative” results, meaning that the lab determined there is or is not anything abnormal, when in fact the opposite is true. This can happen as a result of blood contamination during processing, or simply not having enough of a sample to make a correct analysis, or because the underlying condition or disease hasn’t yet manifested in a way that blood sampling can detect it.

It is possible to also have an “abnormal” result, which can be caused, for example, by not meeting the fasting requirements before getting the blood test done