Typical routine blood tests include the complete blood count, also called CBC, to measure your red and white blood cell numbers as well as hemoglobin and other numbers. This test can uncover anemia, infection and even cancer of the blood. Another common blood test is the basic metabolic panel to check your heart, kidney and liver function by looking at your blood glucose, calcium and electrolyte levels. And to check for heart disease risk, you may have a lipoprotein panel that measures levels of fats in your blood, like good cholesterol (HDL), bad cholesterol (LDL) and triglycerides.
Here are 10 things that your doctor may not tell you about results of blood tests like these, unless you ask.
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1. Doctors often skip the good news. Your doctor should discuss all blood test results with you, but often the rule is, “No news is good news." If your CBC, blood chemistry, and cholesterol results fall within normal ranges, the doctor’s office probably won’t reach out to you about your report, or may send you a copy with little or no explanation. Even if things appear normal, be sure to follow up and discuss your blood test with your doctor, nurse practitioner or nurse, recommends the National Heart, Lung and Blood Institute. Ask if there have been changes since the last test of the same type, and what those changes mean.
2. What’s considered “normal” differs between men and women. If you compare your blood test results with your partner’s, you might be surprised to find differences. For example, the normal reference range for the number of red blood cells in a complete blood count, or CBC test, is between 5 million and 6 million cells per microliter for a man. The normal range is lower for women before menopause, between 4 million and 5 million, likely because of blood loss during menstruation. 
3. Results can mean different things depending on your age. Normal levels of hemoglobin, part of the CBC test, vary by age — lower for children and higher for adults. For children, a hemoglobin level of 11 to 13 grams/deciliter is normal, while for men, a value of 13.5 to 17.5 gm/dl is normal, and it's 12 to 15.5 gm/dl  for adult women. Age matters for your cholesterol numbers too. While most people should aim for LDL cholesterol levels below 130 milligrams/dl, levels of 160 to 190 mg/dl or above point to a risk for heart disease and are riskier if you are a man over 45 or a woman over 55, notes the Mayo Clinic.
4. A “positive” test result may not be positive news.Some blood tests look for diseases by searching for molecular markers in your blood sample — among them the sickle cell anemia test, the HIV test, the hepatitis C test, and the BRCA1 or BRCA2 gene test for breast cancer and ovarian cancer risk. Results are considered “positive” when the test finds the disease marker — DNA or protein — that it is looking for. In these cases, a positive test result means you may have the disease or disorder or that you may have been exposed to it in the past. 
5. A “negative” test result is usually good news. Negative is not the same as bad when it comes to blood tests. A negative result means that the test did not detect what it was seeking, whether a disease marker or a risk factor for a health condition. When you’ve had a blood test to check for an infectious disease — a rapid blood test for hepatitis C, for example — getting back a negative result is good news — it means the test found no evidence of an infection.
6. False-positive test results happen more often than you might think. The first screening test for a condition often has to be checked by a second, more specific test to find out whether the results are accurate and meaningful for your health. An example is the rapid HIV test, for which false positives are common. In communities where about 1 percent are infected with the virus, two false positive HIV rapid test results are expected for every 10 true positive results, according to the U.S. Centers for Disease Control and Prevention.
7. False-negative test results happen too. Sometimes a test doesn’t pick up evidence of a disease or condition even though you actually do have it. For example, if you had a blood test for hepatitis C and the results came back negative, but you were exposed to the virus in the past few months, you could still have an infection and not realize it. Getting tested again is a good idea, recommended nurse and Everyday Health columnist Lucinda K. Porter, RN, if you think you were recently exposed to this infectious disease.
8. Test values can be different from lab to lab. Lab technicians’ reports compare your blood test results to a range that is considered normal for that laboratory. The lab’s reference range is based on test results from many people previously tested in that lab. This normal range may not be the same as another lab’s, notes the U.S. Food and Drug Administration, so don’t be surprised if you find that a prior blood test report varies from newer reports — the difference could be in the lab.
9. Abnormal results might not be due to a disease. A test result outside the normal range of expected lab values could lead to diagnosis of a disease or disorder. But test outcomes can also be abnormal for other reasons. If you had a blood glucose test and you ate something before the test, or were drinking alcohol the night before or taking certain medications, your result could be temporarily abnormal.
10. Mistakes happen. Although mix-ups of patient blood test samples are rare, they do happen, like in the case of an HIV patient's sample that was switched with another person's, accidentally, as reported on ABC News. How your blood sample is handled even before it's analyzed can affect results, too. For example, if the technician shakes your blood sample in the collection tube, blood cells break open, releasing their contents and potentially changing the test results.