Last fall, we held a health fair at our office. One employee who had recently had her annual physical decided to have her cholesterol levels measured again at the fair. To her surprise and dismay—the reading on the second cholesterol test was almost 20% higher than the first. Which results should she believe?

I recounted this incident to Kandice KottkeMarchant, MD, PhD, clinical pathologist at the renowned Cleveland Clinic. She told me that many factors can influence test results. These include diet, medications, differences between laboratories, even season (cholesterol counts tend to increase in winter). Patients can't control all of these factors—but there are ways to increase lab test accuracy. Steps to take in advance.

  • Make sure the lab is accredited. The physician and the insurance carrier dictate where a test is performed. Labs are typically accredited by reputable organizations, but it is a good idea to check that the lab has accreditation.
  • Use the same lab every time, if possible, since there can be significant testing differences between laboratories. In my colleague's case, only the first test actually involved lab work. The health-fair test was the "instant" kind-50 disparate results were more likely.
  • Follow all instructions—for example, eating a meal exactly two hours before a twohour postprandial blood sugar test or abstaining from sexual intercourse for two days prior to a prostate-specific antigen (PSA) test.
  • Review your medications, dietary supplements and herbal remedies with your doctor. These can affect lab test results. For example, high levels of vitamin C interfere with certain tests...birth control pills can increase blood sugar...some antidepressants and blood pressure medications can decrease blood sugar. Generally, these factors can be taken into account when interpreting the test results, but your doctor may advise a change in your regimen prior to the test.
  • If you have experienced significant stress recently or exercised strenuously, tell your doctor. Those factors can increase blood levels of C-reactive protein (CRP), an inflammation indicator associated with heart disease.
  • To avoid bacterial contamination, clean yourself with soap and water before giving a urine sample, or particularly for women, use the special wipes available at testing facilities.

Dr. Kottke-Marchant warns that patients should keep a watchful eye during a test, too. For example...

  • If blood flows too slowly as it is drawn, clots may form, invalidating the results for some tests, especially hematology and coagulation tests (drawn in tubes with purple or light blue tops). A simple tilt of the tube back and forth may detect a clotted sample.
  • Check that the sample is labeled accurately. Confirm your name, type of test, date drawn and, if at a hospital, patient ID number.

Treatment decisions generally should not be made on the basis of a single out-of-range reading, says Dr. Kottke-Marchant. A second test, drawn from a new sample, helps to confirm whether an out-of-range result is in line with prior results.

Sometimes trends can be more important than absolute numbers. A Johns Hopkins University study, for example, suggests that the annual rate of increase in PSA may be more significant than simply looking at whether the numbers are within the normal range.

As for my colleague-after her elevated cholesterol readings, her physician decided to change her cholesterol-lowering medication. Her total cholesterol count is now lower, and she expects it to continue to drop. She is delighted - and so am I!

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