Do we need a fasting lipid profile to assess cardiovascular risk?

Abstract

How often do our patients fail to have lipid determinations because they feel unable to come for fasting blood drawing? We do not know their lipid status, and they become frustrated at not complying with the seemingly simple directions. There is, moreover, evidence that “patients on antidiabetic medications are at risk for developing hypoglycemia while fasting for laboratory testing [with] potential serious harm”.1 We should ask: Where does the “fasting” status requirement come from? Is it justifiable? Total and low-density lipoprotein cholesterol (LDL-C) levels are only minimally increased by recent food intake. LDL-C has a very long half-life (2–4 days) in circulation.2 The issue appears to be the clinical laboratory estimation of LDL-C from the Friedewald formula: LDL-C = total cholesterol − (high-density lipoprotein cholesterol [HDL-C] + very low-density lipoprotein cholesterol [VLDL-C]), where VLDL-C is calculated by dividing the serum triglyceride (TG) level by five. Triglycerides are primarily carried in VLDL particles, and normally composed VLDL particles have a triglyceride mass five times greater than that of cholesterol, so TG/5 should be a reasonable equation to calculate VLDL-C.3 As triglyceride levels do increase in the postprandial state, when using the Friedewald formula, fasting is required. Advice from medical professionals over the years has been more than confusing. Just try your favorite web engine and search for the answers. Virtually all websites that patients can access recommend “fasting” before blood sampling for a “reliable” lipid profile (see, for example, http://www.ask.com, http://www.answers.yahoo.com, http://www.drug3k.com, http://www.wiki.answers.com, http://www.lowercholesterolcenter.com, http://www.livestrong.com). As if the term “fasting” were not vague enough, various “experts” then advise abstaining from food intake anywhere between 8 and 12 h (see, for example, http://www.webmd.com, http://www.health.harvard.com, http://www.nhs.uk, http://www.heart.org, http://www.mayoclinic.org). What is the patient to do? And, what is the principal reason one needs to obtain the “reliable” lipid profile in the first place? The answer for this is clear: first, to assess the magnitude of risk for future cardiovascular events and thus help to drive subsequent specific therapeutic choices for that particular individual; and second, for those already on a therapeutic regimen, to determine whether the goals (i.e. specific lipid particle concentration targets) are being reached. The currently accepted approach is that plasma cholesterol and triglycerides should be measured periodically in adults to assess the risk of coronary heart disease. “Total serum cholesterol and serum HDL cholesterol are relatively unaffected by eating. That is one of the reasons why the calculation of non-HDL cholesterol is a particularly useful tool …”.4 An interesting recent study asked: Among patients with type 2 diabetes, does it matter when the blood sample for lipid determination was drawn in assessing the risk of future cardiovascular events?5 The authors studied 1337 participants without known cardiovascular disease and not using statins at baseline in the European Prospective Investigation into Cancer and Nutrition (EPIC) in the Netherlands and Germany, followed for a mean 8-year period. Cardiovascular events were tracked using registries and follow-up questionnaires. Lipid concentrations were assessed across four categories of postprandial time (0–2, 2–4, 4–6, and >6 h). None of the associations between the triglyceride or HDL-C level, or the total cholesterol:HDL-C ratio, and subsequent cardiovascular events, were dependent on postprandial time, suggesting that non-fasting blood lipid concentrations are as good in the assessment of cardiovascular risk and prediction of cardiovascular events as fasting levels. Indeed, the American Heart Association now recommends initial screening with determination of non-fasting triglyceride concentrations.6 Normal levels are defined as those <200 mg/dL, with those having higher non-fasting levels potentially requiring further, fasting, triglyceride determination. Let us hope that these latest publications lessen the stress we impose on our patients and lead us to recommend the less burdensome timing of blood tests for lipid profile: any time!