The test for LDL is used to predict your risk of developing heart disease. Of all the forms of cholesterol in the blood, the LDL cholesterol is considered the most important form in determining risk of heart disease. Treatment decisions are often based on LDL values.

LDL levels are ordered as part a lipid profile, along with total cholesterol, HDL, and triglycerides. This profile may be used as a screening test in a healthy person as part of a routine physical examination. A lipid profile may be requested on someone who has had a high screening cholesterol to see if the total cholesterol is high because of too much LDL.

Elevated levels of LDL indicate risk for heart disease. Treatment (with diet or drugs) for high LDL aims to lower LDL to a target value of less than 3 mmol/L.

This is especially important if you have other risk factors for heart disease. Risk factors include cigarette smoking, hypertension, low HDL (< 1 mmol/L), family history, age (male 55 or older; female 65 or older), being overweight, and failure to exercise regularly.

Measurement of LDL generally requires a 12-hour fast—meaning that you must not eat or drink anything that has calories for 12 hours before your blood is drawn. This is because LDL is usually calculated from the results of other tests, including triglycerides, that require fasting. This result may be reported as 'calculated LDL'. Some laboratories can measure LDL directly using a special technology and fasting is not necessary. This test is usually called 'direct LDL'.

LDL cholesterol should be measured when a person is healthy. LDL cholesterol is temporarily low during acute illness, immediately following a heart attack, or during stress (like from surgery or an accident). You should wait at least six weeks after any illness to have LDL cholesterol measured.

In women, cholesterol is high during pregnancy. Women should wait at least six weeks after the baby is born to have LDL cholesterol measured.