MADISON, N.J., May 13, 2013 /PRNewswire/ -- Decades of declines in LDL cholesterol blood levels, a key marker of death risk from heart disease, abruptly ended in 2008, and may have stalled since, according to a multi-year, national study published in PLOS ONE. The study, by researchers at Quest Diagnostics (NYSE: DGX), examined low-density lipoprotein, or LDL, blood-serum cholesterol test results of nearly 105 million individual adult Americans of both genders in all 50 states and the District of Columbia from 2001-2011. The study is the largest of LDL cholesterol levels in an American population, and the first large-scale analysis to include data from recent years 2009-2011. "Our study suggests that significant improvements in heart disease risk through declines in LDL cholesterol blood levels over the past several decades came to an unexpected and sudden end in 2008," said investigator Robert Superko, M.D., medical director, cardiovascular disease, Quest Diagnostics. "The unprecedented scale of our data set should spur additional research to identify the cause or causes in order to prevent a possible reversal in years of gains in cardiovascular health in the U.S. population." The study found a net 13% decline in the annual mean LDL cholesterol level of the study population over the 11-year period. Between 2001 and 2008, the average age-adjusted mean LDL levels declined from about 120 mg/dL to 104.7 mg/dL, but plateaued at that level for the remainder of the study period. LDL levels of 100 mg/dL or lower are considered optimal by the American Heart Association. By 2011, about 46% of patients had achieved LDL levels lower than 100 mg/dL, while 6% of patients had LDL levels in the high-risk category of 160 mg/dL or higher. Blood cholesterol levels are the primary biomarker for cardiovascular disease, which accounts for one in every three deaths in America. High levels of LDL ("bad") cholesterol can cause arterial clogging, increasing the risk of stroke and heart disease. Treatments typically include lifestyle modification and therapy with lipid-lowering medications such as statins. Every 10 mg/dL decline in LDL is associated with an approximately 5-13% decline in major vascular disease events, such as strokes and mortality.