LDL-Cholesterol – Calculated and Direct
LDL–cholesterol plays a causal role in the development of atherosclerosis. Its measurement is therefore important for the diagnosis and treatment of hyperlipidaemia.
Most laboratories estimate LDL Cholesterol by the Friedewald equation:
LDLC = TC – HDLC -Trig/2.2
where Trig/2.2 approximates to VLDL Cholesterol.
This approximation assumes that total cholesterol is normally distributed across the three major lipoprotein classes. It is not valid at high triglyceride concentrations (>4.5mmol/L) or for non-fasting samples containing chylomicrons, when it under-estimates LDLC; or in Type III Hyperlipoproteinaemia, when it over-estimates LDLC.
Direct LDL measurements are indicated when the triglyceride concentration exceeds 4mmol/L. Beta-quantitation, based on ultracentrifugation, is the reference method, but automated methods evaluated against this procedure are now available.
Clinical indications:
LDLC concentration is a primary criterion which, together with other risk factors, is used in the treatment recommendations for hyperlipidaemia.
Reference Range:
LDL cholesterol (Target value) Less than 2.0mmol/L
(derived from patients with a >20% risk of CVD over 10 years)
Patient preparation:
Patients should follow their normal diet for 3 weeks prior to sampling. A fasting sample is required. Standardise posture to reduce effect of change in plasma volume – seat the patient for 5 minutes before sampling. Avoid venous stasis – apply tourniquet briefly before inserting the needle and release before drawing the sample.
Sample details:
EDTA plasma or serum (min. vol. 2mL).
Stable 4 days at 4°C, 2months at -20°C
Transport – First Class Post (avoid weekends)
Information required:
Age, sex, NHS/hospital No.
Medication
Reference:
Friedewald WT, Levy RI, Fredrickson DS ;Estimation of the concentration of low-density lipoprotein cholesterol in plasma without the use of the preparative ultracentrifuge. Clin Chem 1972:18; 499-502