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Does a blood test show all infections?

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.

everydayhealth.com - Things Your Doctor Won't Tell You About Your Blood Test Results

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2. ‘Normal’ May Differ Between Men and Women

If you compare your blood test results with those of someone of the opposite sex, you may be surprised to find differences. For example, the normal reference range for the number of red blood cells in a complete blood count is between 5 million and 6 million cells per microliter for a man, but for women, it’s between 4 million and 5 million, according to the NHLBI.

3. ‘Normal’ May or May Not Vary by Age

For some tests, such as the hemoglobin test, normal results vary by age. For children, for example, a hemoglobin level of about 11 to 13 grams/deciliter (g/dl) is normal, while for men, a value of about 14 to 17 g/dl is normal, and for women, 12 to 15 g/dl is normal. But for other tests, such as LDL cholesterol, a level less than 100 milligrams/deciliter (mg/dl) is considered optimal regardless of age. That being said, your age and other risk factors for heart disease may influence how your doctor reacts if your blood test shows a higher-than-optimal LDL cholesterol level. If you are a man over 45 or a woman over 55 and you have heart disease or diabetes, your doctor will likely advise you to take steps to lower your LDL cholesterol if it is higher than 100 mg/dl.

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 and ovarian cancer risk. Results are considered “positive” when the test finds the disease marker — DNA, antibody, or protein — that it is looking for. In these cases, a positive test result means you may have the disease or disorder or, in the case of infectious diseases, that you may have been exposed to it in the past.

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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 it was a disease marker or a risk factor for a health condition. When you’ve had a blood test to check for an infectious disease — an interferon gamma release assay for tuberculosis, 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 Are Possible

The first screening test for a condition often has to be confirmed 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, which is often accurate but rarely will cause a false positive (meaning the test result is positive, but the person does not actually have the disease). This can occur with certain tests that measure antibodies, since a person may have an immune condition (such as rheumatoid arthritis or multiple myeloma) that also produces antibodies and interferes with the test.

7. False-Negative Test Results Happen, Too

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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. Similarly, if you’re tested for Lyme disease within the first few weeks of an infection, your blood test is likely to come back negative because your body has not yet developed antibodies.

8. Test Values Can Be Different From Lab to Lab

Lab technicians’ reports compare your blood test results with 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 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 May Not Be Due to a Disease

A test result outside the normal range of expected lab values does not necessarily mean you have a disease or disorder. Test results can be abnormal for other reasons. If you had a fasting plasma glucose test and you ate something before the test, or were drinking alcohol the night before or taking certain medications, your results could be temporarily outside the normal ranges, but are not evidence of a disease. To avoid such problems, it is best to talk with your doctor before any lab tests about whether you need to make any special preparations before getting your blood drawn, such as fasting the night before.

10. Mistakes Happen

Although mix-ups of blood test samples are rare, they do happen. How your blood sample is handled before it's analyzed can affect results, too. For example, if the sample is collected in the wrong container, shaken inappropriately, or stored for too long or at the wrong temperature, you may get an erroneous result.

everydayhealth.com - Things Your Doctor Won't Tell You About Your Blood Test Results

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