Following the withdrawal of blood, patients may resume medications and food intake according to their physician’s orders. Immediately after the blood sample is taken, the test tubes are labeled with the date and the patient’s name, and they are sent to the laboratory for testing. Results are usually returned within 24 hours, depending on which tests need to be performed. Based on the test results, additional blood tests may be ordered.

Test results are always evaluated in relation to the “normal range” for that test. The range of values considered to be normal is the range of test results from the blood of normal, active healthy people. When someone has a disease or health problem, his or her blood test results may be higher or lower than normal – or “outside of the normal range.” When a physician sees that a blood test is outside of the normal range, he or she may order a repeat test to verify the results or additional tests to determine the underlying causes behind the abnormality.

Normal ranges for some tests may vary slightly from lab to lab, especially between labs that use machines to perform blood tests and those that perform the tests by hand. Frequently, results of a patient’s blood test are compared to another “known” blood sample taken from a healthy individual that is run at the same time and is designated as the “normal control.” When the “normal control” sample falls within the normal range, or reaches a specific “known” measurement, the laboratory results confirm that the blood test has been carried out accurately.