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G R Warnick

Publications and source records attributed to G R Warnick.

At least 19 recordsLinked to original sources
PubMed2001

Measurement of cholesterol in plasma and other body fluids.

Measurement of cholesterol has become increasingly important with recognition of its predictive association with cardiovascular diseases. Government and professional groups in the United States and other countries have developed consensus guidelines for using cholesterol and the lipoproteins in identifying patients at risk and in managing therapies. Accuracy in the measurements is essential not only for reliable classification of patients, but also in public health/wellness programs and in research. Laboratory experts have developed requisite analytical performance targets and guidelines for measurements. Manufacturers of diagnostic reagents and laboratories can assure accuracy by accessing the Cholesterol Reference Method Laboratory Network, which offers reference methods for total, high-density lipoprotein, and low-density lipoprotein cholesterol. Recommendations for controlling pre-analytical sources of variation and uniform interpretation of patient results facilitate reliable classification of patients. Driven by increasing workloads and the need for increased efficiency in laboratory testing, dramatic improvements have been made in recent years in automating laboratory analyzers, as well as the methods used for lipoprotein analysis. Newer technology allows multiple sequential measurements in the same cuvette, as well as point-of-care measurements, using strip tests and compact analyzers. Efforts continue to develop reliable, minimally invasive tests in body fluids other than serum.

Analysis of Variance↗
PubMed2001

Excretion of sweat and urine pyridinoline crosslinks in healthy controls and subjects with established metabolic bone disease.

Convenient techniques for measuring rates of bone turnover have been developed in recent years with the advent of biochemical markers of bone metabolism. One recent of these techniques is a collection method and quantitative enzyme immunoassay for free pyridinoline crosslinks in human sweat. The concentrations of pyridinoline crosslinks in 5-day sweat collections and first morning void and 24-hour urine collections from healthy subjects and subjects with established metabolic bone disorders were determined. T-scores were higher in the sweat system than in the urine system by up to 10-fold in postmenopausal subjects, women with hyperparathyroidism, and subjects with postmenopausal osteoporosis. For subjects with postmenopausal osteoporosis, receiver-operating characteristic curve analysis yielded areas under the curve of 0.699, 0.629, and 0.520 for sweat pyridinoline, first morning void urine pyridinoline, and 24 hour urine pyridinoline respectively. The areas under the curve of the sweat and first morning void urine measurements were significantly greater (p<0.05) than the 24-hour pyridinoline measurements. Healthy postmenopausal subjects and subjects with postmenopausal osteoporosis were monitored before and during estrogen replacement therapy or alendronate therapy. Sweat pyridinoline values declined by 49.0 +/- 12.4% and 19.4 +/- 19.9% for estrogen and alendronate subjects respectively. We conclude that this non-invasive technique is a sensitive and specific measure of bone resorption and is appropriate as an adjunct to techniques such as bone density and may also be useful in monitoring of response to anti-resorptive therapies.

Adult↗
PubMed2001

Evolution of methods for measurement of HDL-cholesterol: from ultracentrifugation to homogeneous assays.

BACKGROUND: Adoption of automated homogeneous assays for HDL-cholesterol (HDL-C) is increasing, driven by the need of clinical laboratories to cope with increasing workloads while containing costs. However, performance characteristics of homogeneous assays often differ in important aspects from those of the earlier precipitation methods. This review provides an overview of the new generation of homogeneous assays for HDL-C within the historical context of the evolution of methods and the efforts to standardize measurements of the lipoproteins. APPROACH: This is a narrative review based on method evaluations conducted in the laboratories of the authors as well as on relevant publications, especially comparative evaluation studies, from the literature. Publications considered here have been collected by the authors over the past 30 years of involvement as methods for HDL-C made the transition from their early use in lipid research laboratories to clinical laboratories and the recent emergence of homogeneous assays. CONTENT: The presentation includes descriptions of methodologies, including homogeneous, precipitation, electrophoresis, and ultracentrifugation assays. Reference methods and recommended approaches for assessing accuracy are described. Accuracy and imprecision are summarized in the context of the National Cholesterol Education Program (NCEP) standards for analytical performance. The effects of interfering substances and preanalytical sources of variation are presented. SUMMARY: Homogeneous assays have been shown to be reasonably well suited for use in routine clinical laboratories, generally meeting the NCEP criteria for precision, accuracy, and total error. However, discrepant results compared with the reference methods have been observed with some of the assays, and the sources of discrepancies are not well characterized. Some homogeneous reagents have not been thoroughly evaluated. At least three of the reagents have experienced successive adjustments in formulation; hence, the reagents may not yet be fully optimized. For these reasons, the homogeneous assays cannot be confidently recommended for use in long-term clinical trials and other research applications without thorough validation.

Chemical Precipitation↗
PubMed2000

Standardization of lipoprotein reporting.

We wanted to ascertain whether the current format of lipid laboratory reports seemed adequate to promote identification and treatment of patients with dyslipidemia. In a random survey of lipid laboratory reports from 25 laboratories, we found great inconsistencies among reporting formats and contents. Fewer than half the laboratories correctly reported the ranges for cholesterol, only 4 correctly reported ranges for high-density lipoprotein cholesterol, only 2 correctly reported ranges for triglycerides, and none presented low-density lipoprotein cholesterol ranges in terms of risk factors for coronary heart disease. Reports typically were disjointed and difficult to read. The current practice of reporting results for lipid panels is confusing and does not follow the National Cholesterol Education Program (NCEP) guidelines. We recommend that reporting of results be standardized, and a "model" standardized report is presented herein, based on consensus from a team of experts. The standardized report uses current recommendations for ranges, follows the flowcharts of the NCEP guidelines, and takes the patient's clinical condition (the number of risk factors and the presence of coronary heart disease) into consideration. Standardizing lipid reports should decrease confusion and perhaps increase application of the guidelines and patient compliance with treatment.

Cholesterol↗
PubMed2000

Measurement of cholesterol and other lipoprotein constituents in the clinical laboratory.

Measurements of lipids and lipoproteins in the clinical laboratory have become increasingly important because of their predictive association with cardiovascular diseases, especially coronary artery disease. The US National Institutes of Health-sponsored National Cholesterol Education Program and counterparts in other countries have developed national consensus guidelines for diagnosis and treatment of coronary artery disease which provide risk cut-points and define use of the lipid/lipoprotein analytes in case finding and therapy. Total and low density lipoprotein cholesterol and triglycerides are measured as positive risk factors and high density lipoprotein cholesterol as an inverse risk factor for coronary artery disease. A National Cholesterol Education Program-sponsored expert laboratory panel has developed guidelines for measurements with requisite analytical performance targets for total error and corresponding precision and bias. The US Centers for Disease Control and Prevention have established reference methods for total and high density lipoprotein cholesterol and for triglycerides, with a method for low density lipoprotein cholesterol in development. Standardization programs for research laboratories and a Cholesterol Reference Method Laboratory Network for diagnostic manufacturers and clinical laboratories provide reliable access and documentation of traceability to accepted reference methods. Methods for the lipid/lipoprotein analytes have improved dramatically in recent years and, coupled with improved chemistry analyzer systems and more attention to standardization by manufacturers, offer considerable improvement in analytical performance. Fully automated homogeneous assays for high density lipoprotein cholesterol and newer similar assays for low-density lipoprotein cholesterol have potential for better precision as well as more convenient and cost-effective measurements. Attention to pre-analytical sources of variation is also important in making reliable classification of patients.

Cholesterol↗
PubMed1999

A collection method and high-sensitivity enzyme immunoassay for sweat pyridinoline and deoxypyridinoline cross-links.

BACKGROUND: Collagen cross-link molecules such as pyridinoline (PYD), deoxypyridinoline (DPD), and N-terminal cross-linked peptides (NTX) have been measured in urine as indices of bone resorption. However, very little is known regarding the excretion of pyridinolines into other biological fluids. We report a collection device, normalizing analyte, and high-sensitivity immunoassay for quantitative analysis of free pyridinoline cross-links in sweat. METHODS: Flame atomic emission and ion-selective electrode techniques were used to measure potassium as a sweat volume marker. The Pyrilinks immunoassay for urine free pyridinolines was optimized to increase sensitivity for measurements in sweat. The precision, accuracy, and detection limit of this assay were characterized. To assess values and variability of sweat pyridinolines in human subjects, a nonocclusive skin patch was used to collect sweat samples from a reference group and from a mixed group experiencing accelerated bone resorption, postmenopausal women and men receiving gonadotropin-releasing hormone for prostate cancer. RESULTS: The immunoassay intra- and interassay variations were </=10% and <16%, respectively, with a detection limit of 309 pmol/L. Linearity upon dilution and analytical recovery ranged from 93% to 109% and 85% to 122%, respectively. Sweat PYD values normalized to potassium output yielded a weekly intraindividual biological variability of 14.7%. The mean increase in the population experiencing increased bone resorption vs the reference group was 36% (P <0.05) for sweat PYD/K vs 23-40% (P <0.05) for urinary PYD/Cr, DPD/Cr, and NTX/Cr. CONCLUSION: We conclude that this new platform sweat collection technology and PYD immunoassay show potential as an indicator of bone resorption.

Amino Acids↗
PubMed1999

Ratio of remnant-like particle-cholesterol to serum total triglycerides is an effective alternative to ultracentrifugal and electrophoretic methods in the diagnosis of familial type III hyperlipoproteinemia.

BACKGROUND: Familial type III hyperlipoproteinemia (HLP) is characterized by the presence of beta-migrating VLDL (beta-VLDL) and increased risk of cardiovascular disease. Assessment of plasma beta-VLDL is achieved by measuring the ratio of VLDL-cholesterol (VLDL-C) to total plasma triglycerides (TGs) or by detecting beta-VLDL in total VLDL. The objective of this study was to compare the clinical utility of the ratio of remnant-like particle-cholesterol (RLP-C) to total TGs with that of the current methods for diagnosing type III HLP. METHODS: Detection of beta-VLDL by electrophoresis of VLDL was used to define type III HLP. Twenty-eight patients with type III HLP and 43 subjects lacking beta-VLDL were investigated. Fasting TG concentrations were >2.26 mmol/L in all subjects. Subjects were separated into three groups: group 1, serum total cholesterol </=5.18 mmol/L (n = 11); group 2, total cholesterol >5.18 mmol/L and TGs between 2.26 and 9.04 mmol/L (n = 51); and group 3, TGs >9.04 mmol/L (n = 9). RESULTS: In group 2, a RLP-C-to-total TG molar ratio >/=0.23 (>/=0.10 when using mg/dL) and a VLDL-C-to-total TG molar ratio >/=0.69 (>/=0.30 when using mg/dL) correctly classified 94% and 90% of the subjects, respectively. The utility of the RLP-C-to-total TG ratio in diagnosing type III HLP decreased in patients in the other two groups. CONCLUSION: When used in an appropriate target population, the RLP-C-to-total TG ratio is a convenient and effective alternative to ultracentrifugal and electrophoretic methods for diagnosing type III HLP.

Apolipoproteins↗
PubMed1998

Evaluation of an immunoseparation method for quantitative measurement of remnant-like particle-cholesterol in serum and plasma.

Substantial evidence indicates that triglyceride-rich lipoprotein remnants are atherogenic. Additional research has, however, been limited by available methods for separation and quantification of remnants. We have evaluated an immunoseparation assay developed to measure cholesterol in remnant-like particles (RLP-C). This method uses monoclonal antibodies to human apolipoproteins B-100 and A-I to remove most of the apolipoprotein B-100-containing lipoproteins (namely LDL and nascent VLDL) and apolipoprotein A-I-containing lipoproteins (namely chylomicrons and HDL), leaving behind a fraction of triglyceride-rich lipoproteins, including chylomicron and VLDL remnants, both of which are enriched in apolipoprotein E. Cholesterol in the unbound fraction is measured with a sensitive enzymatic assay. The RLP-C concentration was highly correlated with total triglyceride-rich lipoproteins (sum of VLDL-cholesterol and IDL-cholesterol) separated by ultracentrifugation and by polyacrylamide gel electrophoresis (r = 0.86 and 0.76, respectively). The within-run and run-to-run imprecision (CV) of the assay was approximately 6% and 10%, respectively. The assay was not affected by hemoglobin up to 5000 mg/L (500 mg/dL), bilirubin up to 342 mmol/L (20 mg/dL), glucose up to 67 mmol/L (1200 mg/dL), or ascorbic acid up to 170 mmol/L (3.0 mg/dL). In 726 subjects (men, n = 364; women, n = 362) in the US, the 75th percentiles of RLP-C concentration were 0.17 mmol/L (6.6 mg/dL) and 0.23 mmol/L (8.8 mg/dL) in sera obtained after overnight fasting or randomly, respectively. A group of 151 patients from nine US centers and one Canadian center with coronary artery atherosclerosis established by angiography had higher median RLP-C concentrations than 302 gender- and age-matched controls (P <0.05). We conclude that the RLP-C assay compares favorably to ultracentrifugation and electrophoresis and provides a convenient and economical approach to measure triglyceride-rich lipoprotein remnants in routine clinical laboratories.

Adolescent↗
PubMed1995

Accurate direct determination of low-density lipoprotein cholesterol using an immunoseparation reagent and enzymatic cholesterol assay.

Clinical laboratories currently estimate low-density lipoprotein cholesterol using the Friedewald formula, which requires fasting specimens and is subject to error with increasing triglyceride levels. We describe a rapid method for isolating low-density lipoproteins using the Direct LDL Immunoseparation Reagent for subsequent measurement of cholesterol by conventional assay. This method meets current guidelines for precision with within-run and run-to-run coefficients of variation of less than 3%. Results are in good agreement with the beta quantification reference method (Direct LDL-C = 1.03 [beta quantification] -0.06 mmol/L, [2.4 mg/dL] r = 0.980), there is minimal bias associated with increasing triglycerides or high-density lipoprotein cholesterol, and patient fasting is not required for accurate analysis. The Direct LDL Immunoseparation Reagent overcomes drawbacks of the Friedewald formula and appears to be suitable for accurate quantitation of low-density lipoprotein cholesterol in the routine laboratory.

Bias↗
PubMed1994

Capabilities of compact analyzers for decentralized testing of lipids and lipoproteins.

Advances in compact analyzer technology have provided the capability to move laboratory testing to the patient, the bedside, the physician office, wellness sites and even into the home. The future has arrived in the sense that lipid testing can be performed in a simple manner similar to blood pressure testing with results immediately available to facilitate treatment decisions. The realities of access and utilization as well as the rate of future technology developments will be determined to a large degree by government regulatory and reimbursement decisions.

Blood Chemical Analysis↗
PubMed1994

Modification of the dextran-Mg2+ high-density lipoprotein cholesterol precipitation method for use with previously frozen plasma.

Although dextran-Mg2+ precipitation produces accurate and precise results for high-density lipoprotein (HDL) cholesterol in fresh plasma and serum, precipitation of frozen specimens with triglycerides > 2.26 mmol/L (> 200 mg/dL) is difficult. We developed a modification that dilutes thawed samples by 35% and increases dextran-Mg2+ reagent to 15% of sample volume. Standard precipitations were performed on 62 fresh EDTA-treated plasma specimens; supernatant solutions were analyzed fresh and after freezing. Standard and modified methods were also performed on thawed, paired plasmas. In specimens with triglycerides < or = 2.26 mmol/L, HDL cholesterol results for all methods were similar. For triglycerides > 2.26 mmol/L, however, bias and precision were significantly affected by freezing, and 38.5% of samples with standard precipitation required additional procedures to produce clear supernatant solutions. HDL cholesterol concentrations for thawed samples with standard precipitation were significantly greater than for fresh samples (P < 0.02), but those for the modified method were not different from fresh samples, and only one specimen required additional steps to produce a clear supernate.

Chemical Precipitation↗
PubMed1994

Cholesterol in fingerstick capillary specimens can be equivalent to conventional venous measurements.

Current interest in coronary heart disease and cholesterol has led to the development of a new generation of compact analysis systems designed for fingerstick whole blood measurement. Since reliable classification of patients based on national cut-points for serum cholesterol concentration requires accurate results, the question whether results from fingerstick capillary specimens are equivalent to those from conventional venous-derived serum specimens, the basis for the national cut-points, is germane. Earlier studies in the literature are contradictory, with fingerstick differences ranging from 9% low to 6% high. We developed guidelines for reliable fingerstick collection and, following these guidelines, achieved results that were comparable to results derived from concurrently collected venous serum specimens. Results measured either by an accurate, standardized enzymatic assay or by the AccuMeter, a new noninstrumented device, were in close agreement with serum results, ie, within 1% and 1.7%, respectively, suggesting that fingerstick measurements are appropriate for identifying individuals with elevated cholesterol levels and monitoring their treatment.

Blood Specimen Collection↗
PubMed1993

Multicenter evaluation of Reflotron direct dry-chemistry assay of high-density lipoprotein cholesterol in venous and fingerstick specimens.

The Reflotron HDL Cholesterol test (Boehringer Mannheim GmbH) directly separates and analyzes high-density lipoprotein (HDL) cholesterol in plasma collected with EDTA in an integrated dry-reagent system suitable for alternative site testing of lipoproteins. We describe a multicenter evaluation of this test by two US and six European laboratories experienced in lipid analysis. Each laboratory compared the Reflotron with the same conventional wet-chemistry method, Boehringer phosphotungstate-Mg2+ precipitation with enzymatic cholesterol assay. Imprecision was within accepted guidelines, with CVs of < or = 8% for fresh and frozen plasmas (median CV 1.7-3.9%) and for lyophilized sera (median CV 3.8-4.7%), similar to those of the conventional method. Results of linear-regression analysis were as follows: Reflotron HDL Cholesterol = 1.03 conventional - 3.9 mg/L, r = 0.987. The Reflotron results were somewhat low in the two US laboratories, demonstrating the need for general standardization of methods for measuring HDL cholesterol. Results from capillary fingerstick plasma agreed well with those from venous-derived plasma; capillary = 1.04 venous + 4.5 mg/L, r = 0.967. The system is relatively insensitive to interference from hemoglobin (< or = 0.75 g/L), ascorbic acid (< or = 0.3 g/L), bilirubin (< or = 50 mg/L), cholesterol (< or = 3.5 g/L), and triglycerides (< or = 4 g/L). The relative ease of operation and the rapid availability of results (within 90 s for plasma collected in EDTA) make the method appropriate for use by well-trained, but not necessarily technical, operators in the physician's office or other alternative sites.

Aminopyrine↗
PubMed1992

Measurement of low-density-lipoprotein cholesterol in serum: a status report.

Current recommendations of the Adult Treatment Panel and the Children and Adolescents Treatment Panel of the National Cholesterol Education Program make the concentration of low-density lipoproteins cholesterol (LDL-C) in serum the basis for the classification and treatment of hypercholesterolemia. Numerous methodologies for the determination of serum LDL-C concentrations, in research and clinical laboratories, have been described. Here, we review the principles, performance, and limitations of major current methodologies for determining LDL-C concentrations. These methods include sequential and density-gradient ultracentrifugation, chromatographic and electrophoretic techniques, and precipitation methods. In addition, the advantages and disadvantages of estimating LDL-C concentration by the Friedewald equation, the most commonly used approach in clinical laboratories, are addressed.

Chemical Precipitation↗
PubMed1990

Laboratory measurement of lipid and lipoprotein risk factors.

Accuracy in measurement of serum cholesterol and the other lipid risk factors is essential for reliable patient classification within the context of newly implemented national coronary heart disease intervention programs utilizing uniform cutpoints. This requires agreement or traceability of laboratory methods to national reference systems, which provided the accuracy base for the population studies from which the cutpoints are derived. Adhering to accepted guidelines for patient preparation, blood collection and processing will minimize preanalytical variability. Standardization of the lipid/lipoprotein assays is the next essential step. The assay technology for cholesterol measurement is reasonably mature, although still not perfected; recent activities have focused primarily on achieving traceability of field methods through improving access to the accuracy base, the National Reference System for Cholesterol and developing better (commutable) reference materials. A major hindrance to standardization has been matrix interactions; alterations in reference materials from their preparation which change the measurement characteristics, necessitating the use of fresh specimens in accuracy studies. Considering the importance of LDL cholesterol which is the primary decision parameter in the new clinical guidelines; methodology for routine laboratory quantification is substantially lacking, the usual routine approach still involving estimation. Efforts to develop a reference system for LDL are just beginning. Field methods for HDL cholesterol are reasonably reliable but with some inter-method differences. Agreement on and access to a Reference Method is needed. Accuracy in triglyceride measurement is less important compared to the other risk factors. Convenient methods allowing correction for the free glycerol blank and ready access to a Reference Method are needed.

Chemistry, Clinical↗
PubMed1990

Estimating low-density lipoprotein cholesterol by the Friedewald equation is adequate for classifying patients on the basis of nationally recommended cutpoints.

We compared low-density lipoprotein cholesterol (LDL) values obtained by the Friedewald formula--i.e., total cholesterol minus high-density lipoprotein (HDL) cholesterol minus very-low-density lipoprotein (VLDL) cholesterol (estimated as triglyceride divided by 5)--with those obtained by lipoprotein fractionation, using 4736 specimens. When triglycerides were less than 2.0 g/L, greater than 90% of estimated LDL cholesterol values were acceptable, within +/- 10% of measured values. At triglyceride concentrations of 2.0-4.0 g/L and 4.0-6.0 g/L, only 72% and 39%, respectively, of the estimates were acceptable. LDL values derived from an alternative formula, estimating VLDL as triglycerides divided by 6, were even less accurate. Nevertheless, the use of estimated LDL for risk classification based on the National Cholesterol Education Program Adult Treatment Panel cutpoints of 1.30 and 1.60 g/L was considered acceptable. At triglyceride concentrations less than or equal to 5.0 g/L, 88% of classifications based on estimated LDL (using triglycerides divided by 5) were concordant with those by measured LDL. Eleven percent of classifications were shifted across one cutpoint, evenly distributed between high and low. Fewer than 1% of classifications, all with Type III hyperlipoproteinemia, were misclassified two cutpoints high. Refinements in the estimation model did not substantially improve LDL estimation or concordance of risk classification.

Adult↗