During a general check-up, most doctors recommend that you undergo a cholesterol test to determine whether you are at risk of heart disease. The test measures the amount of cholesterol and triglycerides in the serum (the non-cellular fluid part) of your blood.
A total-cholesterol test will examine all the blood fats, including the overall cholesterol level, the LDL (low-density lipoproteins, or 'bad' cholesterol), HDL (high-density lipoproteins, or 'good' cholesterol), triglycerides (the form of fat that transports and stores energy derived from food), plus several other varieties of fats. However, these days, most cholesterol tests concentrate only on LDL cholesterol.
What does the test consist of?
It requires a relatively straightforward blood test. You must fast for 9-12 hours before the test, and only water should be drunk during the fast, as even tea or coffee can affect results. A tourniquet is applied to your arm, so that the lower veins pool with blood, and the blood sample is drawn from a vein either on the inside of the elbow or the back of the hand.
All tests for blood fats (or lipids, as they are known in medicalspeak) are measured in terms of milligrammes per deciliter of blood (mg/dL). A total cholesterol count of less than 200 mg/dL is considered acceptable. The current medical wisdom is that the higher the cholesterol count, the greater the risk of heart disease or atherosclerosis (fat-clogged arteries). If your levels are over 240 mg/dL, you are believed to have nearly twice the risk of developing heart disease compared with someone within the normal range. In Europe, the patients' test results are given in SI (Syst`eme International) units. For cholesterol, they are given as mmol/L (1 mmol/L = 38.5 mg/dL). Basically, a healthy person should have a cholesterol level of less than 5.2 mmol/L and a triglyceride level of 15 g/L (1 g/L = 10 mg/dL).
What is considered a high count?
The words ' high cholesterol' inspire a feeling of dread in all of us as something akin to a death sentence. The (largely unsubstantiated) view is that high LDL cholesterol may be the best predictor of heart disease. The current medical opinion is that if you have no other risk factors, your LDL count should be below 160 mg/dL. People with one or more risk factors should aim for even lower levels - at below 100 mg/dL for those with heart or vascular disease or diabetes, or below 130 mg/dL for those with two or more heart-disease risk factors (such as high blood pressure, smoking, a family history of heart disease, or being over 45 (men) or over 55 (women).
A high LDL is thought to be countered by high HDL, and vice versa. HDL levels of 60 mg/dL or higher are thought to counteract other risk factors whereas HDL levels below 40 mg/dL themselves become a risk factor.
Even if you have low LDL- and high HDL-cholesterol levels, high triglycerides may still put you at risk. An acceptable triglyceride level is thought to be less than 150 mg/dL.
Is the test accurate?
Not particularly. A vast array of conditions can result in an inaccurate test.
One problem is the inherent inaccuracy of the test itself. According to one study, 70 per cent of the samples analysed showed evidence of bias in computing the results (Arch Pathol Lab Med, 1993; 117: 393-400). The equipment used in the analysis may also be inaccurate. In a study of eight such devices, one was judged completely unacceptable, and every one of the remaining seven had drawbacks (Health Devices, 1990; 19: 343-71).
Inaccurate test results can have alarming consequences. In one study of 142 patients undergoing cholesterol blood tests, 22 per cent were wrongly classified. More alarmingly, 19 per cent were a false positive (a high cholesterol level when it wasn't) and, among those at 'high-risk', 50 per cent had a false-positive result. Even more worrisome, these misclassifications appeared to have nothing to do with whether a patient had one or another risk factor (such as diabetes, overweight or smoking) (Can Fam Physician, 1995; 41: 240-5).
Fat levels may be artificially raised if you haven't carried out a complete fast for at least nine hours.
Thus, looking at LDL cholesterol alone is not an accurate predictor of heart disease. A large health survey in England found that predicting heart disease was far more accurate when HDL was also considered (Arch Intern Med, 1995; 155: 2146-7).
Cirrhosis or other liver disease, heredity, thyroid or kidney dysfunction, malabsorption of food (such as from a 'leaky' gut), malnutrition, pernicious anaemia, infection, vitamin D supplements and uncontrolled diabetes all can cause a false-positive test. Pregnant women and those whose ovaries have been removed may also test wrong.
* Watch out for prescription drugs, which can throw off your test results. These include adrenocorticotropic hormone (ACTH), anabolic steroids like androgens, beta-blockers, corticosteroids, epinephrine (adrenaline), oral contraceptives, phenytoin, sulphonamides, thiazide diuretics and even niacin or vitamin D.
Specific drugs include allopurinol, captopril, chlorpropamide, clofibrate, colchicine, colestipol, erythromycin, isoniazid, lovastatin, MAO inhibitors and neomycin.
Nitrates, used in bacon or ham as a preservative, can also affect your test.
Is the test even necessary?
Cholesterol has been demonised in medicine but, in fact, it is an important and essential component of the body. Cholesterol is used to make cell membranes, steroidal hormones and bile acids. Without it, we would die.
Is high cholesterol an indication of anything? At best, it's a crude marker than something is awry. However, half of all patients who suffer heart attack have normal cholesterol levels, while many populations with very-high-fat diets have normal cholesterol. Studies show that people on low-fat diets or with the lowest cholesterol levels are up to 40 per cent more likely to die earlier than those with higher cholesterol (Lancet, 2001; 358: 351-5).
Has general cholesterol screening ever saved lives? No. General cholesterol screening has not been shown to be beneficial, particularly in those over 60 (Arch Intern Med, 1995; 155: 2146-7).