Lipid profiles: What the numbers mean
Cholesterol analysis is a commonly requested test in primary care, and one that usually falls to the practice nurse who may also be involved in reviewing the results. But what is the significance of the numbers for health and disease?
It is estimated that in recent years there has been a more than 15-fold rise in the overall number of lipid tests requested.1 Furthermore, with recent guidelines recommending that all people aged between 40 and 70 or more should have a routine cardiovascular risk assessment, which includes cholesterol analysis, the number of tests performed is likely to increase further. Many practice nurses are involved in carrying out these risk assessments and, for many, reviewing the related blood results is part of the role. Lipid results look at a number of parameters, which can be confusing. This article will look at the measurements reported and their significance for health and disease and, with a better understanding of what they mean, the task of reviewing results might be a little easier.
THE ROLE OF CHOLESTEROL
Some cholesterol is needed to help with a variety of body processes. It is necessary for the formation of the outer layer of cells, the formation of bile acids that aid the digestion of food in the intestine and it also has a role in the formation of Vitamin D and sex hormones, (oestrogen in women and testosterone in men).
The body obtains cholesterol via two methods: from the liver, which synthesises about 75% of cholesterol, and from food, which accounts for the remaining 25%.2 Common food sources include meat, milk, eggs, and cheese.
LIPID PROFILES
A full lipid profile typically measures cholesterol, high density lipoprotein (HDL), and triglyceride levels. (See Box1 for normal values).
Cholesterol: acceptable levels
Blood cholesterol is measured in millimoles per litre of blood, (mmol/l). Current guidelines recommend that for healthy adults levels should be 5mmol/l or less and 4mmol/l or less for those at high risk of developing CVD.
Abnormal results: why the concern?
Raised cholesterol levels are regarded as a key modifiable risk factor for cardiovascular disease (CVD) and frequently occur alongside other potentially damaging factors such as obesity, poor diet, sedentary lifestyle, smoking, diabetes, and a family history of heart disease. (Box 2) Is estimated that over 50% of CVD in developed countries is due to abnormal lipid levels, and in the UK alone, in 2011, around 50% of adults have a cholesterol level above 5mmol/l.3,4
Disease risk
Abnormal lipid levels are associated with the risk of a number of conditions in addition to coronary heart disease. (see Box 3)
HIGH DENSITY LIPOPROTEINS (HDL)
HDL (often referred to as good cholesterol) is synthesised and secreted by the liver and small intestine. It travels in the circulation and is able to pick up extra cholesterol from the cells and tissues and transport it back to the liver, where it can be converted into bile.
Significance
In contrast to cholesterol levels, high levels of HDL are beneficial while low levels are associated with adverse health effects. Low levels of HDL cholesterol are now recognised as an independent risk factor for CVD, and raising levels of HDL is a major treatment strategy for regressing atherosclerosis and enhancing CVD risk reduction.5
TRIGLYCERIDES
Triglycerides are the main type of fat transported by the body and are released into the bloodstream when dietary fat is digested, where they provide an energy source. Triglycerides are also manufactured by the liver and any transported around the body are either used as an energy source or stored as fat. The liver is also able to change some triglyceride into cholesterol, and can also convert any source of excess calories into triglycerides when needed.6
Significance
High levels of triglycerides (hypertriglyceridemia) are often associated with other lipid abnormalities, e.g. low HDL levels, and are frequently found in patients with other conditions such as obesity and/or hypertension, which are also linked to the development of CVD. Hypertriglyceridemia is also a risk factor for pancreatitis and is estimated to account for 1 to 4% of cases of acute pancreatitis.7
LOW DENSITY LIPOPROTEIN (LDL)
Low density lipoprotein (LDL) is the main carrier of plasma cholesterol and a major component of atherosclerotic plaques.8 It is therefore concluded that lowering LDL cholesterol will reduce coronary events and mortality from coronary artery disease.9 LDL testing is often not performed in the laboratory as it is time consuming and expensive, and requires specialised equipment but it can be calculated from quantitative measurement of cholesterol, HDL and triglycerides.10 Box 4 shows a method of calculating LDL using the Friedewald method. However, while this equation is excellent for LDL calculation if triglyceride is less than 200 mg/dl, the accuracy is thought to decline when triglyceride level is over 200 mg/dl.11
RATIOS
In an attempt to improve the accuracy of lipid profiles in predicting cardiovascular risk a number of ratios are now under scrutiny. Labelled ‘atherogenic indices’ the total/high-density lipoprotein (HDL) cholesterol ratio, and the LDL/HDL cholesterol ratio are two important indicators of vascular risk, the predictive value of which is greater than the isolated parameters.12 Individuals with a high total/HDL cholesterol or LDL/HDL cholesterol ratio have a greater CVD risk because of the imbalance between atherogenic and protective lipoproteins.12
SELF HELP MEASURES OR MEDICATION?
Encouraging patients to make healthy lifestyle choices is a key component of health promotion and an everyday part of the practice nurse role and it is now known that a number of lifestyle changes can affect HDL levels. HDL is modestly increased by smoking cessation (up to 20%), weight loss, and aerobic exercise.13 Obese patients will benefit greatly from the combination of exercise and weight loss, and the greater the weight loss the greater the elevation in HDL levels, and similarly the more the person exercises the more the HDL will increase.13 It has also been shown that moderate alcohol ingestion can also raise serum HDL and is associated with reduced risk for acute cardiovascular events.14 Changes to food intake, such as adopting a healthier diet rich in fruit, vegetables, whole grains foods, olive oil, and legumes (Meditteranean diet) is also associated with improved HDL levels.15 With an optimal diet and exercise regimen, it is possible to lower total and LDL cholesterol by 10% to 15%.2 Unfortunately, many patients will also require medication and there are several types available. Statins are frequently prescribed although alternatives are available which can be used as an alternative in the event of side effects or in addition to if cholesterol remains elevated. Available alternatives include ezetimibe, which blocks cholesterol absorption in the intestines and lowers LDL, nicotinic acid (niacin), which lowers LDL and raises HDL, and fibrates, which help the body lower triglycerides and raise HDL.2 Clinical trials in patients with and without CVD have demonstrated consistently that statins reduce the relative risk of major coronary events by as much as 30%.16
CONCLUSION
Given the impact that abnormal cholesterol levels have on health, practice nurses can play an important role in addressing the issues but in order to do so need to be able to accurately interpret blood test results. When assessed correctly and when combined with effective treatment and lifestyle advice there is the opportunity to make a difference to CVD risk.
REFERENCES
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