Reference Measurement Procedure for Total Glycerides by Isotope Dilution GC-MS

Optimal Cut-off Points of Nonfasting and Fasting Triglycerides for Prediction of Ischemic Heart Disease in Japanese General Population: The Circulatory Risk in Communities Study (CIRCS)

AIMS

We investigated the optimal cut-off points of nonfasting and fasting triglycerides in Japanese individuals with lower average triglyceride levels than westerners.

METHODS

Residents aged 40-69 years without a history of ischemic heart disease or stroke were enrolled between 1980 and 1994 and followed. Serum triglyceride concentrations were measured from 10851 nonfasting (＜8 h after meal) and 4057 fasting (≥ 8 h) samples. As a prerequisite, we confirmed the shape of a receiver operating characteristic (ROC) curves, the area under ROC curves (AUC), and the integrated time-dependent AUC. We identified optimal cut-off points for incident ischemic heart disease based on C-statistic, Youden index, and Harrell's concordance statistic. We used dichotomized concentrations of triglycerides via the univariate logistic regression and Cox proportional hazards regression models. We also calculated multivariable hazard ratios and population attributable fractions to evaluate the optimal cut-off points.

RESULTS

Nonfasting and fasting optimal cut-off points were 145 mg/dL and 110 mg/dL, with C-statistic of 0.594 and 0.626, Youden index of 0.187 and 0.252, and Harrell's concordance statistic of 0.590 and 0.630, respectively. The corresponding multivariable hazard ratios of ischemic heart disease were 1.43 (95%CI 1.09-1.88) and 1.69 (1.03-2.77), and the corresponding population attributable fractions were 16.1% (95%CI 3.3-27.2%) and 24.6 (-0.3-43.3).

CONCLUSION

The optimal cut-off points of nonfasting and fasting triglycerides in the Japanese general population were 145 mg/dL and 110 mg/dL, respectively, lower than the current cut-off points recommended in the US and Europe.

Hypertriglyceridemia, a causal risk factor for atherosclerosis, and its laboratory assessment

Epidemiological and clinical studies show a causal association between serum triglyceride (TG) level, the number of triglyceride-rich lipoproteins (TRLs) and their remnants, and the increased risk of atherosclerosis and cardiovascular disease (CVD) development. In light of current guidelines for dyslipidemia management, the laboratory parameters reflecting TRL content are recommended as part of the routine lipid analysis process and used for CVD risk assessment, especially in people with hypertriglyceridemia (HTG), diabetes mellitus, obesity and low levels of low-density lipoprotein cholesterol (LDL-C), in which high residual CVD risk is observed. The basic routinely available laboratory parameters related with TRL are serum TG and non-high-density lipoprotein cholesterol (non-HDL-C) levels, but there are also other biomarkers related to TRL metabolism, the determination of which can be helpful in identifying the basis of HTG development or assessing CVD risk or can be the target of pharmacological intervention. In this review, we present the currently available laboratory parameters related to HTG. We summarise their link with TRL metabolism and HTG development, the determination methods as well as their clinical significance, the target values and interpretation of the results in relation to the current dyslipidemia guidelines.

A method for measuring glycerol-blanked triglyceride concentrations by using gas chromatography-isotope dilution mass spectrometry

BACKGROUND

Serum triglyceride concentrations are measured as total glycerides content in many Western countries. In Japan, glycerol-blanked triglycerides (TG-GB) are measured to identify postprandial hypertriglyceridaemia and to minimize the influence of glycerol formulation on serum triglyceride values. However, TG-GB measurements have not been standardized. Therefore, we developed an efficient quantification system for total glycerides and free glycerol that allows the calculation of TG-GB concentrations.

METHODS

We measured total glycerides and free glycerol in human serum by using gas chromatography-isotope dilution mass spectrometry and compared its performance to the reference method of the US Centers for Disease Control and Prevention (CDC).

RESULTS

Our practical method of total glycerides and free glycerol quantification achieved excellent precision for both within-run and among-run coefficients of variation (<1.5% and <2.7%, respectively), with an average recovery of 99.8% for free glycerol. However, we noted an average %bias of -0.26% for total glycerides and -3.15% for free glycerol between our TG-GB method and the CDC reference method.

CONCLUSIONS

This practical method of total glycerides and free glycerol quantification enables traceability assessment of TG-GB measurements. Differences between the output values of TG-GB and the CDC reference method might result from the differences in free glycerol values.

Simultaneous Quantification of Free Cholesterol, Cholesteryl Esters, and Triglycerides without Ester Hydrolysis by UHPLC Separation and In-Source Collision Induced Dissociation Coupled MS/MS

We demonstrate the application of in-source nitrogen collision-induced dissociation (CID) that eliminates the need for ester hydrolysis before simultaneous analysis of esterified cholesterol (EC) and triglycerides (TG) along with free cholesterol (FC) from human serum, using normal phase liquid chromatography (LC) coupled to atmospheric pressure chemical ionization (APCI) tandem mass spectrometry (MS/MS). The analysis requires only 50 μL of 1:100 dilute serum with a high-throughput, precipitation/evaporation/extraction protocol in one pot. Known representative mixtures of EC and TG species were used as calibrators with stable isotope labeled analogs as internal standards. The APCI MS source was operated with nitrogen source gas. Reproducible in-source CID was achieved with the use of optimal cone voltage (declustering potential), generating FC, EC, and TG lipid class-specific precursor fragment ions for multiple reaction monitoring (MRM). Using a representative mixture of purified FC, CE, and TG species as calibrators, the method accuracy was assessed with analysis of five inter-laboratory standardization materials, showing -10% bias for Total-C and -3% for Total-TG. Repeated duplicate analysis of a quality control pool showed intra-day and inter-day variation of 5% and 5.8% for FC, 5.2% and 8.5% for Total-C, and 4.1% and 7.7% for Total-TG. The applicability of the method was demonstrated on 32 serum samples and corresponding lipoprotein sub-fractions collected from normolipidemic, hypercholesterolemic, hypertriglyceridemic, and hyperlipidemic donors. The results show that in-source CID coupled with isotope dilution UHPLC-MS/MS is a viable high precision approach for translational research studies where samples are substantially diluted or the amounts of archived samples are limited. Graphical Abstract ᅟ.

Ten-year standardization of lipids and high-sensitivity C-reactive protein in a randomized controlled trial to assess the effects of statins on secondary stroke prevention: Japan Statin Treatment Against Recurrent Stroke

Background The Japan Statin Treatment Against Recurrent Stroke (J-STARS) is a clinical trial that administered pravastatin for secondary stroke prevention. Lipid concentrations are a well-established risk factor for cerebrovascular diseases. Elevated high-sensitivity C-reactive protein (hs-CRP) indicates a high risk of inflammatory reactions. In clinical trials, internationally approved standardization is essential for obtaining study results that are comparable with those from overseas. Therefore, total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), triglycerides (TG) and hs-CRP were standardized throughout a 10-year study period. Methods J-STARS specified a single clinical laboratory for blood analyses. Four lipids were evaluated by calculating the total error: accuracy (absolute mean %bias vs. reference value) + precision (1.96 among-run coefficient of variation [CV, %]). Accuracy for hs-CRP was ensured using a calibrator traceable to the international plasma protein reference material and precision was evaluated by CV. Results Average total errors (standard deviation, %) throughout the study period were as follows: TC 1.35% (0.290%), HDL-C 2.45% (1.087%), LDL-C 2.65% (0.956%) and TG 3.70% (0.559%). Four lipids met the performance criteria of the US Centers for Disease Control and Prevention (CDC). The precision of hs-CRP was 3.28% (0.627%), which met the performance criterion established by the American Heart Association/CDC. Conclusions Based on standardization, the results of J-STARS appear to be comparable with those of similar intervention-based clinical studies on statins overseas. These study results will contribute to the establishment of preventive measures against recurrent stroke in Japanese patients. J-STARS is registered in ClinicalTrials.gov under NCT00221104.

Quantification of prospective type 2 diabetes mellitus biomarkers by stable isotope dilution with bi-labeled standard glycated peptides

A new analytical approach is proposed for quantification of multiple glycated peptides in human plasma tryptic digests.

Quantitative analysis of glycerol in dicarboxylic acid-rich cutins provides insights into Arabidopsis cutin structure

Cutin is an extracellular lipid polymer that contributes to protective cuticle barrier functions against biotic and abiotic stresses in land plants. Glycerol has been reported as a component of cutin, contributing up to 14% by weight of total released monomers. Previous studies using partial hydrolysis of cuticle-enriched preparations established the presence of oligomers with glycerol-aliphatic ester links. Furthermore, glycerol-3-phosphate 2-O-acyltransferases (sn-2-GPATs) are essential for cutin biosynthesis. However, precise roles of glycerol in cutin assembly and structure remain uncertain. Here, a stable isotope-dilution assay was developed for the quantitative analysis of glycerol by GC/MS of triacetin with simultaneous determination of aliphatic monomers. To provide clues about the role of glycerol in dicarboxylic acid (DCA)-rich cutins, this methodology was applied to compare wild-type (WT) Arabidopsis cutin with a series of mutants that are defective in cutin synthesis. The molar ratio of glycerol to total DCAs in WT cutins was 2:1. Even when allowing for a small additional contribution from hydroxy fatty acids, this is a substantially higher glycerol to aliphatic monomer ratio than previously reported for any cutin. Glycerol content was strongly reduced in both stem and leaf cutin from all Arabidopsis mutants analyzed (gpat4/gpat8, att1-2 and lacs2-3). In addition, the molar reduction of glycerol was proportional to the molar reduction of total DCAs. These results suggest "glycerol-DCA-glycerol" may be the dominant motif in DCA-rich cutins. The ramifications and caveats for this hypothesis are presented.

Gas chromatography electrospray ionization mass spectrometry analysis of trimethylsilyl derivatives

A method based on the analysis of trimethylsilyl (TMS) derivatives by capillary gas chromatography electrospray ionization mass spectrometry (GC-ESI/MS) was proposed. To improve separation, analytes were derivatized to their TMS derivative. During ESI analysis, TMS derivatives may hydrolyze back to their polar native form and are thus suitable for ESI analysis. Several types of analytes were studied to investigate the potential of the approach. Not all TMS derivatives hydrolyzed back to their native form as anticipated. Incomplete hydrolysis was observed for TMS-organic acids and TMS-nonchlorinated phenols. For TMS-chlorophenols, the observation of only the [M - H]^- ion suggested that these phenols were hydrolyzed back to their native form. For TMS-beta agonists, the hydrolysis rate was low; therefore, the hydrolysis product was not detected. Both TMS-chlorophenols and TMS-beta agonists provide a sensitivity in the range of low parts per billion (0.25-5 ng/ml and 0.5-10 ng/ml respectively). Copyright © 2016 John Wiley & Sons, Ltd.

Comparison between the triglycerides standardization of routine methods used in Japan and the chromotropic acid reference measurement procedure used by the CDC Lipid Standardization Programme

Background The US Centers for Disease Control and Prevention ensured adequate performance of the routine triglycerides methods used in Japan by a chromotropic acid reference measurement procedure used by the Centers for Disease Control and Prevention lipid standardization programme as a reference point. We examined standardized data to clarify the performance of routine triglycerides methods. Methods The two routine triglycerides methods were the fluorometric method of Kessler and Lederer and the enzymatic method. The methods were standardized using 495 Centers for Disease Control and Prevention reference pools with 98 different concentrations ranging between 0.37 and 5.15 mmol/L in 141 survey runs. The triglycerides criteria for laboratories which perform triglycerides analyses are used: accuracy, as bias ≤5% from the Centers for Disease Control and Prevention reference value and precision, as measured by CV, ≤5%. Results The correlation of the bias of both methods to the Centers for Disease Control and Prevention reference method was: y (%bias) = 0.516 × (Centers for Disease Control and Prevention reference value) -1.292 ( n = 495, R²= 0.018). Triglycerides bias at medical decision points of 1.13, 1.69 and 2.26 mmol/L was -0.71%, -0.42% and -0.13%, respectively. For the combined precision, the equation y (CV) = -0.398 × (triglycerides value) + 1.797 ( n = 495, R²= 0.081) was used. Precision was 1.35%, 1.12% and 0.90%, respectively. It was shown that triglycerides measurements at Osaka were stable for 36 years. Conclusions The epidemiologic laboratory in Japan met acceptable accuracy goals for 88.7% of all samples, and met acceptable precision goals for 97.8% of all samples measured through the Centers for Disease Control and Prevention lipid standardization programme and demonstrated stable results for an extended period of time.

Total cholesterol performance of Abell-Levy-Brodie-Kendall reference measurement procedure: Certification of Japanese in-vitro diagnostic assay manufacturers through CDC's Cholesterol Reference Method Laboratory Network

BACKGROUND

Accurate measurement of total cholesterol (TC) is important for cardiovascular disease risk management. The US Centers for Disease Control and Prevention (CDC) and Cholesterol Reference Method Laboratory Network (CRMLN) perform Abell-Levy-Brodie-Kendall (AK) reference measurement procedure (RMP) for TC as a secondary reference method, and implement Certification Protocol for Manufacturers. Japanese CRMLN laboratory at Osaka performed the AK RMP for 22 years, and conducted TC certification for reagent/calibrator/instrument systems of six Japanese manufacturers every 2 years for 16 years. Osaka TC performance was examined and compared to CDC's reference values.

METHODS

AK RMP involved sample hydrolysis, cholesterol extraction, and determination of cholesterol levels by spectrophotometry. The Certification Protocol for Manufacturers includes comparison with AK RMP using at least 40 fresh specimens. Demonstration of average bias ≤3% and total coefficient of variation ≤3% qualified an analytical system for certification.

RESULTS

In the AK RMP used in the Osaka CRMLN laboratory, the regression equation for measuring TC was y (Osaka)=1.000x (CDC)+0.032 (n=619, R(2)=1.000). Six Japanese manufacturers had allowable performance for certification.

CONCLUSIONS

The AK RMP for TC measurement was accurate, precise, and stable for 22 years. Six Japanese manufacturers were certified for 16 years.