LDL-3 is one of the subfractions of Low-Density Lipoprotein (LDL) — often referred to as “bad cholesterol.” LDL particles transport cholesterol through your bloodstream, but not all LDL particles are the same.

LDL is made up of different particle sizes and densities, classified from LDL-1 (large, buoyant) to LDL-7 (small, dense).

• Larger LDL particles (LDL-1, LDL-2) are considered less harmful.

• Smaller, denser LDL particles (LDL-3 and beyond) are more atherogenic, meaning they are more likely to promote plaque buildup in arteries.

Elevated LDL-3 levels suggest a shift toward smaller, denser LDL particles, which are more easily oxidized and can penetrate the artery wall — a key early step in atherosclerosis.

LDL-4 is one of the small, dense subfractions of Low-Density Lipoprotein (LDL) — the lipoprotein commonly referred to as “bad cholesterol.”

LDL particles vary in size and density, ranging from LDL-1 (large and buoyant) to LDL-7 (very small and dense).

• Larger LDL particles (LDL-1, LDL-2) are considered less harmful.

• Smaller, denser LDL particles (LDL-3, LDL-4, and beyond) are more atherogenic, meaning they have a greater tendency to promote plaque buildup in the arteries.

When LDL-4 is elevated, it indicates a shift toward a more atherogenic (plaque-forming) lipid profile, even if total LDL cholesterol appears normal.

Lipoprotein-associated phospholipase A2 (Lp-PLA2), also known as platelet activating factor Acetylhydrolase, is an inflammatory enzyme that circulates bound mainly to low density lipoproteins and has been found to be localized and enriched in atherosclerotic plaques.

The LP-IR Score (Lipoprotein Insulin Resistance Score) is a blood test that measures insulin resistance using NMR (nuclear magnetic resonance) spectroscopy analysis of lipoprotein particle sizes and concentrations. The optimal score is less than 25. Scores of 25–45 are borderline; scores above 45 indicate increasing insulin resistance. Unlike fasting glucose or HOMA-IR, the LP-IR Score can detect insulin resistance earlier by measuring the lipoprotein changes that precede blood sugar abnormalities.

Lupus anticoagulants are antibodies against substances in the lining of cells. These substances prevent blood clotting in a test tube.

Luteinizing hormone (LH) is the pituitary's signal to the ovaries or testes. In women, its mid-cycle surge triggers ovulation; in men, it drives testosterone production. A high or low LH number does not show whether the signal worked. Interpret it using the applicable sex-, age-, and cycle-specific range, then add FSH, testosterone or estradiol, and physiologic context.

Luteinizing Hormone, Human (hLH), plays a crucial role in the female reproductive system and is a key marker on a female hormone panel. This hormone, produced by the pituitary gland, is involved in regulating the menstrual cycle and ovulation. In the first half of the menstrual cycle, hLH levels gradually increase, leading to a significant surge just before ovulation. This surge is critical as it triggers the release of an egg from the ovary, making it a pivotal moment for fertility. After ovulation, if the egg is not fertilized, hLH levels decrease, and the menstrual cycle continues.

Lyme Ab (IgG) measures immunoglobulin G antibodies produced by the immune system in response to infection with Borrelia burgdorferi, the bacterium that causes Lyme disease. These antibodies typically develop several weeks after infection and may remain detectable for months or even years. A positive Lyme IgG result suggests previous exposure or later-stage infection, while a negative result may indicate no prior infection or testing performed too early in the disease course. This marker is often evaluated together with Lyme IgM antibodies and confirmatory tests to help determine whether symptoms may be related to Lyme disease.

Lyme Ab (IgM) measures immunoglobulin M antibodies produced by the immune system shortly after infection with Borrelia burgdorferi, the bacterium responsible for Lyme disease. IgM antibodies typically appear within the first few weeks following a tick bite and may indicate a recent or active infection. This test is usually interpreted together with Lyme IgG antibodies and clinical symptoms to help determine whether Lyme disease may be present.

Two types of antibodies are detected in the Western blot test.

IgG antibodies are a sign of an older infection. IgM antibodies in contrast reflect a relatively recent infection.

Two types of antibodies are detected in the Western blot test.

IgM antibodies reflect a relatively recent infection. IgG antibodies in contrast are a sign of an older infection.