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Lipid Variability

Lipid and lipoprotein concentrations vary during the normal course of daily activity. Studies have demonstrated that within person variability is sufficient to make an individual move in and out of the predefined risk categories defined by the National Cholesterol Education Program (NCEP). As many as 11% of patients would be mistreated if risk assessment were made on the basis of a single lipid panel. NCEP guidelines acknowledge this variation by stressing that patient management decisions should be based on an average of at least two results. The extent of within person biological variability (expressed as the coefficient of variation) for each lipid fraction is summarized in the following table. 

 

Interval

 Total-C

 HDL-C

 LDL-C

 TG

Day

 2.5%

 4.5%

 7.8%

 36%

Month

 4.8%

 7.7%

 9.6%

 24%

Year

 6.1%

 8.4%

 13.6%

 26%



Triglycerides fluctuate widely, even during a single day, because they exhibit diurnal variation. Triglyceride concentration is lowest at 3 a.m., rises until mid-afternoon and decreases thereafter. Cholesterol and LDL cholesterol are not subject to diurnal variation, but are affected by seasonal variation. Levels increase as much as 5% in winter. Other sources of intra-individual variation are listed below.

 Variable

 Effect

Menstruation 6-9% TC increase in follicular phase
Pregnancy 30% TC & 200% TG increase at term
Acute illness 15% TC & 25% HDL decrease
Weight loss 10% TC, 10% HDL, 40% TG decrease
Exercise 3-7% TC decrease & 3-10% HDL increase
Smoking 3% TC & 9% TG increase, 6-11% HDL decrease
Posture 10-20% higher standing than lying
Specimen handling Stable for 3 days



Besides biological variation, lipid values are also dependent on the laboratory's analytical performance. Today's chemistry analyzers are very precise; analytical variation in reputable laboratories is less than one half of biological variation. The coefficient of variation at Saint Luke's Hospital Laboratory is 2.2% for cholesterol, 3.8% for HDL cholesterol and 4.2% for triglycerides. Combining biological and analytical variability, it is possible to calculate the magnitude of change that must occur to have 95% confidence that a change in lipid value is medically significant.

 Lipid

 Result (mg/dL)

 Significant Change

Cholesterol

 180

 32

 200

 35

 220

 39

 240

 43

 260

 46

 280

 50

 300

 53

HDL

 25

 7

 30

 8

 35

 10

 40

 11

 45

 12

 50

 14

 55

 15

 60

 16

 65

 18

Triglycerides

 100

 84

 150

 126

 200

 168

 250

 210

 300

 252

 350

 293

 400

 336

 450

 378

 500

 420


Cholesterol concentration must change at least 18%, HDL cholesterol 27% and triglycerides 84% before one can be assured that the difference is not simply due to intra-individual and analytical changes.

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