How Do You Measure Cholesterol in Blood Tests?

During an annual physical exam, your doctor may take blood samples and order lab tests that measure everything from your blood sugar levels for signs of diabetes to your cholesterol levels for risks of heart disease. 

Unlike other components in your blood, such as white blood cells, cholesterol is not only produced by the body but also found in the foods we eat, from fast food meals to cheese and yogurt. This waxy, fat-like substance helps to make vitamins and hormones (such as estrogen and testosterone) as well as the protective outer layer of cells. But too much of the substance can clog your arteries and increase the risk of heart disease and stroke.

So, when taking a test to check for cholesterol, how is the substance measured? At the doctor’s office, your primary care physician will order a lab test called a lipid panel (or lipid profile) in which a medical professional known as a phlebotomist, who is trained to draw blood, will stick a needle in the vein of your arm and draw a few tubes’ worth of blood. These blood samples are labeled and usually sent off to a contract lab, which runs an analysis checking not just for your overall cholesterol but for other components as well. 

Once at the lab, medical devices called chemistry analyzers can calculate the amount of cholesterol in a blood sample. They measure the color of light emitted when certain chemicals react to the presence of cholesterol. The analyzers measure both free cholesterol, the active raw form that circulates through the blood, and cholesterol ester, an inactive form of cholesterol found in the outer layer in cells. 

Different chemical reactions can create the color change that happens in cholesterol measurements. Some tests use chemical enzymes known as cholesterol esterase and oxidase. In these tests, the cholesterol esterase reacts with the cholesterol ester to form free cholesterol. This free cholesterol joins the other free cholesterol already present in the blood, and the test measures the total cholesterol from these two sources. The free cholesterol then reacts with cholesterol oxidase to create byproducts including hydrogen peroxide. Hydrogen peroxide is easily detected by fluorescent probes, which are typically another chemical reagent added to the reaction mixture with the blood sample. The fluorescent probe undergoes a reaction with the hydrogen peroxide to produce a new chemical that gives off light, or fluoresces, at a specific wavelength. The color intensity correlates to how much cholesterol is present: The brighter the hue, the higher the concentration of cholesterol in the sample.

In larger hospital systems, thousands of samples per day are measured using larger-scale chemistry analyzers. In-person cholesterol tests can also be done at a diagnostic lab, a local pharmacy or a clinic. For this type of routine testing, the lab, clinic and pharmacy use the same blood draw method and may have small setups known as point-of-care analyzers to measure total cholesterol. The point-of-care analyzers use reactions similar to the larger-scale analyzers for detecting cholesterol. 

Cholesterol measurements need to be precise, especially since physicians and medical professionals use the numbers to make important medical decisions about a patient’s health. However, during the late 1960s, these measurements could vary widely from one lab to another. One clinical lab could get a cholesterol test result that read 200 milligrams per deciliter (generally considered a healthy level) while another lab down the hall could get a result of 250 milligrams per deciliter (usually considered a high level) from the same blood sample. Of course, both labs’ results should be around the same even if they are using different methods or equipment. Concerningly, such significantly different numbers could create a false sense of security in patients or lead doctors to prescribe unnecessary treatments. 

To help solve this problem of inconsistent measurements, the National Institute of Standards and Technology (NIST) developed a cholesterol standard reference material called SRM 911c Cholesterol, a tool to increase confidence in the results. The SRM was first introduced in 1967 and has been updated and recertified every few years since then. SRM 911c exists in a pure powdered form and is used to calibrate the methods used to measure total cholesterol. It can also serve as a benchmark to validate cholesterol results. Other labs that conduct research, such as the Centers for Disease Control and Prevention (CDC), also can use the SRM to check against their own in-house working standards.

Other standard reference materials developed later, such as SRM 1951c Lipids in Frozen Human Serum, came out of joint collaborations such as NIST’s lipid standardization program with the CDC. 

SRM 1951c provides an analysis of the total cholesterol like SRM 911c but also is used to validate clinical procedures and methods for measuring the components of cholesterol known as high-density lipoprotein (HDL) and low-density lipoprotein (LDL). HDL is known as the “good” cholesterol that helps protect your heart, and LDL is known as the “bad” cholesterol that can clog arteries. SRM 1951c also includes certified amounts of triglycerides, which are another type of fat found in blood.  

In addition to the familiar blood panels from a doctor’s office and hospitals, there are also cholesterol tests for personal use to monitor one’s own cholesterol levels at home. At-home test kits involve a lancet to prick your finger to draw blood, and paper test strips. The small drop of blood is placed on the strip, which contains chemical enzymes including cholesterol esterase, cholesterol oxidase and cholesterol peroxidase that cause a similar reaction as the methods mentioned above. 

A different type of test that’s noninvasive is skin testing, which involves placing a few drops of a test liquid on the palm of a hand. Enzymes in the liquid react to the presence of cholesterol on the skin, causing the liquid to change color. The intensity of the light is then be measured using a device called a spectrophotometer. This new type of test is still in its infancy. The U.S. Food and Drug Administration has approved at least one skin test, but this kind of test is generally a newer option that is not used as frequently compared to other cholesterol tests.

These at-home tests measure total cholesterol, which includes HDL, LDL and very-low-density lipoprotein (VLDL), but they don’t provide the specific cholesterol levels. Instead, they give a general range within which your cholesterol lies. Clinics, diagnostic labs and hospitals can also measure just your total cholesterol level, but because of their more precise methods the results are specific to a number rather than a general range. 

So, next time you sit down for a plate of scrambled eggs or take a bite of a cheeseburger, know that there are different ways to monitor your cholesterol levels at home. And when you visit your doctor’s office for your next exam, know there are standards that help ensure you can trust the results you’re getting from your doctor.