Your Blood Never Lies: You Can Use Simple Laboratory Tests to Make Informed Health Decisions

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.

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.

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). 

See Also: Blood Tests: What Do They Tell?

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.

Five More Important Facts About Blood Work

6. Make sure you are adequately hydrated, that you have drunk enough water, before you report for your blood test

Especially if it usually takes more than one poke to draw blood from your veins, it is important to be adequately hydrated before you report for your blood work. Usually it is a good idea to drink additional water about an hour before you go to the lab. The additional fluid increases the volume of blood flowing through your veins and makes them easier for the phlebotomist to see.

7. If you are taking a blood thinner, let the technician know before your blood draw

Medications such as Coumadin (warfarin), Plavix (clopidrogel), Brilinta (ticagrelor), and Xarelto (rivaroxaban), along with many other "blood thinners," increase the probability of bleeding, even from the tiny prick needed for a blood draw. To keep from bleeding on clothes and furniture, your lab tech will need to have cotton gauze and tape on hand to put pressure on the wound.

They will place a cotton swab on the wound and wrap the the swab with tape; you remove the tape 30 minutes to an hour later.

8. Some readings will vary seasonally

Fasting glucose levels, HbA1C, and LDL cholesterol tend to be higher in the winter. Fasting glucose levels may be abnormally low when a blood test is conducted during the first few days of a warm up after a long period of cold weather.

Vitamin D levels may fall during the winter, or if you live in a location where hot summers keep you inside near the air conditioner, during the summer.

9. Dieting to lose weight can make your computed LDL numbers go up

Most laboratories don't actually measure LDL cholesterol. Instead, the estimate LDL cholesterol as 1/5 of the total blood lipids that aren't triglycerides or HDL cholesterol. The other 4/5 of those unmeasured blood lipids is VLDL cholesterol. If you diet to lose weight, or, if you are a diabetic, you start getting better control over your blood sugar levels, your body will make fewer triglycerides. This means there will be more VLDL + LDL cholesterol, but it does not necessarily mean that you will have more LDL cholesterol. The percentages may change--but the tests don't detect this.

See Also: Can A Simple Blood Test Detect Cancer?

10. You don't have to have your doctor's permission to get blood tests

In most states of the United States, in the UK, and in Japan, you can order your own blood tests. This kind of testing can provide valuable information about whether you would benefit from taking nutritional supplements, or it might help you know how well your medications or health routines are working between doctor visits. You should not diagnose yourself on the basis of your lab numbers, but self-directed testing can provide you with the right questions to ask your doctor early enough to get effective treatment.