Hypertriglyceridemia is a heterogeneous disorder of lipoprotein metabolism with a less definite association to atherosclerosis risk than hypercholesterolemia or increased low-density lipoprotein (LDL)-cholesterol . Patients with moderate hypertriglyceridemia such as familial combined hyperlipidemia, diabetic dyslipidemia, or metabolic syndrome more often develop premature atherosclerotic diseases, because smaller-sized triglyceride (TG)-rich lipoproteins such as chylomicron remnants and very-low-density lipoprotein (VLDL) remnants penetrate the arterial intima from plasma, than larger-sized chylomicrons [1–6]. Remnant lipoproteins are atherogenic, and elevated remnant lipoproteins are associated with the increased risk of cardiovascular disease [2–8].
Two clinically available methods to determine cholesterol levels of remnant lipoproteins have ever been developed, but these assay procedures are basically different. First, remnant-like particle-cholesterol (RLP-C), an immunoaffinity separation method (RLP-C assay; Otsuka, Japan) was developed, and this assay isolates remnant-like particles (RLPs) from human serum using an immunoaffinity gel containing two different immobilized monoclonal antibodies to human apolipoproteins A-1 and B-100 [9, 10]. Many clinical studies have demonstrated that RLP-C is a risk factor for cardiovascular disease, and serum RLP-C levels are higher in patients with coronary artery disease, diabetes, and metabolic syndrome than in healthy subjects [2, 4, 11]. Thus, RLP-C measurement can be performed without an ultracentrifugation, but it takes some time and is not able to be run on an autoanalyzer.
Next, Remnant Lipoprotein Cholesterol Homogenous assay (RemL-C assay; Kyowa Medex, Japan) was developed, and this assay utilizes special surfactant [polyoxyethylene-polyoxybutylene (POE-POB) block copolymer] and phospholipase D, which can selectively solubilize and degrade TG-rich remnant lipoproteins, VLDL remnants and chylomicron remnants [12, 13]. In contrast to RLP-C assay, RemL-C assay is able to be performable on a universal autoanalyzer, thereby allowing quick and high throughput measurements. Nakada et al  reported that remnant lipoproteins, measured by RemL-C, were increased in patients with coronary artery disease, indicating the clinical significance of coronary risk assessment by remnant lipoprotein levels measured by RemL-C. In samples from patients with diabetes, RemL-C correlated with RLP-C, but discrepant data between the 2 methods were found investigated by the gel filtration method, suggesting a murky difference in the affinity of respective assay reagents to various TG-rich lipoproteins .
We developed a novel high performance liquid chromatography (HPLC) method for measuring cholesterol levels in the major classes of serum lipoproteins within 25 min using an anion exchange column filled with diethylaminoethyl-ligand nonporous polymer-based gel by elution with a step gradient of sodium perchlorate concentration [14, 15]. This HPLC method is able to determine cholesterol levels of HDL, LDL, IDL (intermediate-density lipoprotein), VLDL, and chylomicron similarly to the ultracentrifugation method, a golden standard method to determine cholesterol levels of lipoprotein fractions despite its the technical complexity inappropriate for routine clinical laboratory use. Cholesterol values of HDL, LDL, IDL, VLDL and chylomicron measured by this HPLC method are correlated to those estimated by the ultracentrifugation method , and therefore this HPLC method may be employed as a substitute of the ultracentrifugation method. RLP-C correlated well with VLDL-cholesterol but poorly with IDL-cholesterol, measured by the HPLC method  similarly as RLP-C was correlated to VLDL-cholesterol but not to IDL-cholesterol, measured by the ultracentrifugation method . However, the associations of RemL-C with VLDL and IDL have never been examined quantitatively by the HPLC method, although the RemL-C values contain VLDL remnant (IDL) cholesterol concentrations qualitatively estimated by the gel filtration method and polyacrylamide gel electrophoresis analysis in the previous report .
In the context, we evaluated the correlations and data validation between the 2 assays (RLP-C and RemL-C) in subjects without diabetes, hypertension and medications for hyperlipidemia, diabetes, and hypertension, and investigated the characteristics of remnant lipoproteins obtained by the two assays and their relationships with IDL-cholesterol determined quantitatively by our HPLC method.