Like most things in life, cholesterol measurements are not perfect. For one thing, in most labs, your LDL cholesterol is calculated based on the measurements of your total cholesterol, HDL, and triglyceride levels. As previously mentioned, if those levels are off, then your LDL measurement will be wrong as well.

A lot of things can affect a test’s sensitivity and accuracy, including the method used to prepare the sample, the purity of the chemicals added to it, the skills of the technician, and the quality of the machine used to analyze the sample. A good lab can measure the same blood sample repeatedly and not have the total cholesterol level vary by more than about 3 percent.

Although the lab should not have more than a 3 percent variation in measuring your cholesterol level, your personal level may vary a good deal more when different blood samples are compared. One study showed that total cholesterol levels can fluctuate by as much as 11 percent over the course of a year.

Researchers estimated that 60 percent of that variation was caused by biological fluctuations, and the remaining 40 percent by variations in the test itself. Changes in your average daily levels may reflect changes in your diet, smoking, illness, weight, exercise routine, or certain medications.

That same study found that triglyceride measurements can vary anywhere from 12.9 percent to 40.8 percent, and HDL levels from 3.6 to 12.4 percent. This is why it’s important to take more than one measurement if anything looks abnormal and not to overinterpret small changes in lipid values.

If you see a small rise or fall in your levels from year to year, don’t worry; it may not represent a real gain or loss. You can do some things to help control the biological variability part. First, try to have your blood drawn at the same time of day every time you have a test done, and follow a similar eating, exercising, and medication-taking pattern.

Some other things can also help increase the accuracy, including avoiding alcohol for a few days before the test, sitting for at least five minutes before your blood is taken, staying seated during the procedure, and mentioning to your doctor any fevers you’ve had recently.

As I already mentioned, if the results of one cholesterol test indicate that you need treatment, you should ask for another test a few weeks later to verify the results so that you don’t get put on a drug for life that you don’t really need to take.