The Small Dense Low-Density Lipoprotein (sdLDL) test measures the concentration of small, dense particles of LDL cholesterol in the blood. These particles are considered more atherogenic than larger LDL particles, meaning they have a higher potential to contribute to the formation of arterial plaques and increase cardiovascular risk. This test is used to assess the risk of cardiovascular disease, particularly in individuals with normal or borderline cholesterol levels.