Age and gender contribute to variations in total cholesterol measurements. Before age 50, women have lower lipid values than men, after which age the level in women is higher than in men. This finding may be modified by exogenous oral conjugated estrogens.
Seasonal variation also occurs, with lipid samples in December or January being found to be approximately 2.5% higher than those measured in June or July.
The effect of diet and obesity is well established. Weight reduction in an obese individual affects the triglyceride level, which may decrease as much as 40%. Total cholesterol and LDL decrease less than 10% with diet; however, HDL increases approximately 10%. Weight gain negates any benefit from prior weight loss. Accuracy of lipid measurements depends on the stability of the patient’s weight.
Alcohol and cigarette smoking are well-known modifiers of cholesterol. Moderate sustained alcohol intake increases HDL and decreases LDL; there is a complementary increase in triglycerides. Alcohol has a protective effect when taken in moderation (defined as approximately 2 alcoholic drinks [2 ounces of absolute alcohol] per day), but this effect is negated in higher quantities. The increase in HDL is in the HDL3 fraction, which is important in the scavenger mechanism of removing LDL. Smoking has the opposite effect, increasing LDL and triglyceride levels and decreasing HDL. HDL3 decreases with cigarette smoking. The critical number of cigarettes smoked is 15 to 20 per day, regardless of gender, and a variation in the number smoked will affect results. Caffeine has a mixed effect on lipoprotein measurements and should be avoided for 12 hours before blood collection.
Exercise is an important variable in the overall risk management of heart disease.
Moderate levels of exercise are as important in overall cardiovascular health as control of hypertension and cessation of cigarette smoking. Strenuous exercise lowers the concentrations of triglycerides and LDL and increases HDL in the serum. Because of these acute blood changes, vigorous exercise should be avoided within 12 hours of drawing the blood for testing.
Certain disease states and medications affect cholesterol measurements. Use of diuretics and propranolol increases triglycerides and decreases HDL. Diuretics may increase total cholesterol levels. Diabetes, especially in the presence of poor control, may be associated with very high levels of triglycerides and LDL and decreased levels of HDL. This may explain why these individuals are prone to cardiovascular diseases. Patients with diabetes under tight control have improved lipoprotein profiles.
Pregnancy is associated with decreased total cholesterol in the first trimester and continuous increases of all fractions over the second and third trimesters (32). The LDL and triglyceride concentrations are the lipoproteins most affected by pregnancy.
Patients with hypothyroidism have increased levels of total cholesterol and LDL.
Because of the diurnal variation of blood triglycerides, blood samples should be collected in the morning after a 12-hour fast. Excessive quantities of water should not be consumed before blood is drawn. After 2 minutes of venous occlusion, serum cholesterol levels can increase 2% to 5%. Therefore, the sample used for cholesterol testing should be collected first if multiple blood samples are required. One of the most important aspects of overall standardization of lipoprotein measurements is the laboratory used. It may be worthwhile to consult a clinical pathologist to determine if the laboratory complies with Centers for Disease Control and Prevention (CDC) standards for cholesterol and lipoprotein measurements.