Table 6.

Comparison of LDL-C and apo B.

ParameterLDL-Capo B
Nature of target analyteLDL is not a unique molecular species but a heterogeneous and polydisperse population of particles with varying chemical composition and physicochemical properties. Therefore, LDL is defined functionally in terms of the method used to separate it from other lipoproteins.apo B is well defined as a molecular species (apo B-100 and apo B-48). Although methods for measuring apo B-48 are available, routine “apo B” methods measure either apo B-100 or total apo B.
Reference materialStandard Reference Material (SRM) 1951b (frozen human serum preparations) certified by the National Institute of Standards and Technology (NIST), Gaithersburg, MD. LDL-C determined by β-quantification (see below) at CDC, USA. Level I, 113.2 (3.1) mg/dL or 2.93 (0.08) mmol/L; level II, 152.6 (3.0) mg/dL. Note: Direct comparison with the “reference method” β-quantification (see below) is considered the only reliable accuracy test for an LDL-C method at present.1International Reference Material SP3–07 (a human serum preparation in liquid-stabilized form) developed by IFCC Standardization Project and endorsed by WHO.2 Accuracy-based mass value of 1.22 g/L [3.95 (0.08) mmol/L] assigned to apo B.2
Comparison methodsVarious ultracentrifugation methods sometimes combined with chemical precipitation agents [e.g., dextran sulfate or phosphotungstate with MgCl2, heparin with MnCl2, and polyethylene glycol (PEG) 6000].13Behring (now Siemens) Nephelometer at the Northwest Lipid Research Laboratories (NWLRL), University of Washington, Seattle, WA.4
Reference methodβ-Quantification.13 Widely accepted (including CDC in USA) but not formally credentialed.13 Defines LDL as a population of particles with hydrated density ≥1.006 kg/L and precipitation by polyanion-metal ions.Not defined.
Definitive methodNot defined.Not defined.
Principle of analytical methods for quantitationDifferent methodologies are based on different physicochemical properties of LDL particlesAll methodologies are based on the antigenicity of apo B and involve the use of specific anti-apo B antibodies.