LDL on a blood test stands for low-density lipoprotein, a type of cholesterol your body uses to deliver fats to cells throughout your body. It’s often called “bad” cholesterol because high levels increase your risk of heart disease. An optimal LDL level is below 100 mg/dL, while anything above 160 mg/dL is considered high.
What LDL Actually Does in Your Body
LDL particles are tiny transport vehicles that carry cholesterol through your bloodstream to cells that need it. Each particle contains a single protein on its surface that acts like a docking key, allowing it to latch onto receptors on your cells and deliver its cargo. Inside, each particle holds roughly 3,000 to 6,000 fat molecules wrapped in a protective shell.
Your cells genuinely need this cholesterol. It’s a building block for cell membranes, hormones, and vitamin D. The problem isn’t LDL itself. It’s what happens when there’s too much of it circulating in your blood with nowhere to go.
Why High LDL Damages Your Arteries
When LDL levels stay elevated, excess particles begin slipping into the walls of your arteries. Once inside, they get stuck. Molecules in the artery wall act almost like flypaper, binding to LDL particles and preventing them from passing back out. The longer these particles sit trapped in the artery wall, the more they undergo a chemical change called oxidation, essentially going rancid.
Your immune system treats these oxidized particles as a threat. White blood cells rush in and start swallowing the damaged LDL, bloating into what scientists call foam cells. These foam cells pile up along the artery wall, forming fatty streaks visible even in young adults. Over time, smooth muscle cells migrate over the growing deposit and form a tough fibrous cap, creating a mature plaque that narrows the artery and restricts blood flow. If that cap ruptures, it can trigger a blood clot, which is how heart attacks and strokes happen.
How to Read Your LDL Number
Your LDL result is reported in milligrams per deciliter (mg/dL). Here’s what the standard ranges mean:
- Below 100 mg/dL: Optimal
- 100 to 129 mg/dL: Near optimal
- 130 to 159 mg/dL: Borderline high
- 160 to 189 mg/dL: High
- 190 mg/dL and above: Very high
These ranges are general guidelines. Your doctor may set a lower target if you have other risk factors like diabetes, high blood pressure, or a family history of early heart disease. For people who already have heart disease, guidelines often push for LDL well below 100 mg/dL.
An LDL level above 190 mg/dL in adults, especially without an obvious lifestyle explanation, is one of the hallmark signs of familial hypercholesterolemia, a genetic condition affecting roughly 1 in 250 people. In children, that threshold drops to 160 mg/dL. If your number is in this range and heart disease runs in your family, genetic factors are worth discussing with your doctor.
How Labs Calculate Your LDL
Most labs don’t measure LDL directly. Instead, they measure your total cholesterol, HDL cholesterol, and triglycerides, then plug those numbers into a formula developed in the 1970s called the Friedewald equation. It works by subtracting your HDL and an estimate of your triglyceride-carried cholesterol from the total.
This formula is generally accurate for most people, but it tends to underestimate LDL when your actual level is below 100 mg/dL, particularly if your triglycerides run above 150 mg/dL. When triglycerides exceed 400 mg/dL, the calculation becomes unreliable enough that labs typically switch to a direct chemical measurement instead. Non-fasting blood draws can also reduce the formula’s accuracy, since eating raises triglyceride levels temporarily.
Do You Need to Fast Before the Test?
Traditional guidelines recommend fasting for at least 8 hours before a cholesterol test. In practice, the evidence behind this requirement is weaker than most people assume, and many experts now consider non-fasting tests acceptable for routine screening. The difference matters most when triglycerides are very high or when you’re already on cholesterol-lowering medication. In those situations, a fasting draw gives more reliable LDL and triglyceride numbers. If your doctor hasn’t specified, it’s reasonable to fast, but a non-fasting result is still useful for general screening.
LDL Particle Size Matters Too
Not all LDL particles are equally harmful. LDL comes in a range of sizes, from large and buoyant to small and dense. The smaller, denser particles appear to be significantly more dangerous. They penetrate artery walls more easily and are more prone to oxidation once trapped inside.
About 30 to 40% of people with heart disease have an abundance of small, dense LDL. In large studies, people with predominantly small LDL particles had a roughly threefold increase in heart disease risk, even after accounting for their total LDL level, HDL, and triglycerides. In people who already have heart disease, the concentration of the smallest LDL particles is one of the strongest predictors of worsening artery narrowing. Standard lipid panels don’t measure particle size, but advanced testing is available if your doctor suspects it’s relevant.
Non-HDL Cholesterol: A Broader Picture
You may notice another number on your results: non-HDL cholesterol. This is simply your total cholesterol minus your HDL, and it captures all the cholesterol carried by particles that can contribute to plaque, not just LDL. It includes cholesterol from triglyceride-rich particles that standard LDL testing misses.
For most people, LDL and non-HDL cholesterol track closely. But in some cases, particularly in people already taking cholesterol-lowering medication, non-HDL cholesterol does a better job of reflecting remaining cardiovascular risk. Guidelines increasingly recognize non-HDL cholesterol as a more complete snapshot of your atherogenic cholesterol burden. You can calculate it yourself from a standard lipid panel, and it doesn’t require fasting to be accurate.
How Diet and Lifestyle Affect LDL
Dietary changes can meaningfully lower LDL, though the effect varies from person to person. Cutting saturated fat intake to less than 7% of daily calories (roughly 15 grams on a 2,000-calorie diet) typically reduces LDL by 8 to 10%. Adding plant sterols, found in fortified foods and some supplements, at about 2 grams per day can lower LDL by another 5 to 15%.
Soluble fiber from foods like oats, beans, and barley binds cholesterol in the gut before it can be absorbed. Regular aerobic exercise raises HDL and can modestly reduce LDL, while losing excess weight improves nearly every lipid marker. These changes work best in combination. Someone who overhauls their diet, starts exercising, and loses weight can sometimes bring borderline-high LDL back into the optimal range without medication, though people with genetically driven high cholesterol often need additional treatment regardless of lifestyle.