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Every time we go in for a physical we get blood tests, but our doctors don't always tell us everything we need to know about the results. Here are 10 things your doctor may not take time to tell you abour laboratory testing.

Just about all of us get periodic blood tests at the doctor's office. But most of us don't get all the information we need to make the best use of our lab numbers. Here are 10 things about blood work that most patients need to know.

1. Different labs get different numbers when testing the same blood sample

In the United States, about 90% of doctor offices that don't do blood work in-house send off samples to Lab Corp, a giant testing corporation that has strict standardization procedures to make sure blood work values are measured consistently.

If your blood work is done at another facility, however, for instance, in the doctor's own lab or in a hospital, the lab techs may use  different standardization procedures that result in different lab values.

Laboratory reports are supposed to be interpreted with the help of reference ranges provided with the report, but most of us, and most of our doctors, focus on the numbers themselves. Variation in standardization procedures is particularly prone to make a different in LDL cholesterol measurements, in thyroid hormone measurements, and in HbA1C for diabetics.

2. When you go in for a blood draw for a fasting blood test, it is very important that you have actually fasted

Or at least that you are honest about whether you have eaten or not. Blood glucose levels and blood lipid levels, especially triglycerides, can vary tremendously when you have eaten. Sometimes you will be asked simply to be sure not to eat anything or drink anything other than water after 8 pm the evening before the morning you come in for your test, so that you have not had any food for at least 12 hours, but sometimes you may be asked to fast for as long as 24 hours.

3. Post-prandial blood tests (blood tests that are done after you eat) are especially useful in detecting type 2 diabetes in its earliest stages

Sometimes it is important that you get certain blood tests when you aren't fasting. In the early stages of type 2 diabetes, the pancreas does not make enough insulin to clear glucose from the bloodstream in the first two to five hours after a meal, but it does make enough insulin to get blood sugar levels back down to normal by the next morning.

Post-prandial blood glucose testing serves as a kind of early warning signal that pre-diabetes is soon to progress to full-blown diabetes.

Ask for post-prandial blood sugar testing if you know you are at high risk for diabetes. But make sure your doctor (and the laboratory technician) know whether you have eaten or not.

4. Coffee, Diet Coke, and other zero-calorie beverages, other than water, are not recommended before fasting blood tests

Drinking black coffee or zero-calorie soft drinks can interfere with testing for homocysteine (a measure of vascular inflammation), LDL cholesterol, triglycerides, and creatinine (a measurement of kidney function). 

5. Blood testing should not hurt

A skilled phlebotomist (the technician who draws your blood) should be able to find a vein by palpating (feeling) the fold in your arm, and draw blood with just one stick. You should feel little or not pain. If your phlebotomist cannot find a vein after two attempts, consider asking another phlebotomist to do the draw.

Continue reading after recommendations

  • Liang WW. Seasonal changes in preprandial glucose, A1C, and blood pressure in diabetic patients. Diabetes Care. 2007 Oct
  • 30(10):2501-2. Epub 2007 Jun 22.
  • Kim TH, Kim KW, Ahn HY, Choi HS, Won H, Choi Y, Cho SW, Moon JH, Yi KH, Park do J, Park KS, Jang HC, Kim SY, Park YJ. Effect of seasonal changes on the transition between subclinical hypothyroid and euthyroid status. J Clin Endocrinol Metab. 2013 Aug. 98(8):3420-9. doi: 10.1210/jc.2013-1607. Epub 2013 Jun 14. PMID: 23771919
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