John H. Eckfeldt

Estimating Glomerular Filtration Rate from Serum Creatinine and Cystatin C

BACKGROUND Estimates of glomerular filtration rate (GFR) that are based on serum creatinine are routinely used; however, they are imprecise, potentially leading to the overdiagnosis of chronic kidney disease. Cystatin C is an alternative filtration marker for estimating GFR. METHODS Using cross-sectional analyses, we developed estimating equations based on cystatin C alone and in combination with creatinine in diverse populations totaling 5352 participants from 13 studies. These equations were then validated in 1119 participants from 5 different studies in which GFR had been measured. Cystatin and creatinine assays were traceable to primary reference materials. RESULTS Mean measured GFRs were 68 and 70 ml per minute per 1.73 m(2) of body-surface area in the development and validation data sets, respectively. In the validation data set, the creatinine-cystatin C equation performed better than equations that used creatinine or cystatin C alone. Bias was similar among the three equations, with a median difference between measured and estimated GFR of 3.9 ml per minute per 1.73 m(2) with the combined equation, as compared with 3.7 and 3.4 ml per minute per 1.73 m(2) with the creatinine equation and the cystatin C equation (P=0.07 and P=0.05), respectively. Precision was improved with the combined equation (interquartile range of the difference, 13.4 vs. 15.4 and 16.4 ml per minute per 1.73 m(2), respectively [P=0.001 and P<0.001]), and the results were more accurate (percentage of estimates that were >30% of measured GFR, 8.5 vs. 12.8 and 14.1, respectively [P<0.001 for both comparisons]). In participants whose estimated GFR based on creatinine was 45 to 74 ml per minute per 1.73 m(2), the combined equation improved the classification of measured GFR as either less than 60 ml per minute per 1.73 m(2) or greater than or equal to 60 ml per minute per 1.73 m(2) (net reclassification index, 19.4% [P<0.001]) and correctly reclassified 16.9% of those with an estimated GFR of 45 to 59 ml per minute per 1.73 m(2) as having a GFR of 60 ml or higher per minute per 1.73 m(2). CONCLUSIONS The combined creatinine-cystatin C equation performed better than equations based on either of these markers alone and may be useful as a confirmatory test for chronic kidney disease. (Funded by the National Institute of Diabetes and Digestive and Kidney Diseases.).

Relation Between Folate Status, a Common Mutation in Methylenetetrahydrofolate Reductase, and Plasma Homocysteine Concentrations

BACKGROUND Methylenetetrahydrofolate reductase (MTHFR) synthesizes 5-methyltetrahydrofolate, the major carbon donor in remethylation of homocysteine to methionine. A common MTHFR mutation, an alanine-to-valine substitution, renders the enzyme thermolabile and may cause elevated plasma levels of the amino acid homocysteine. METHODS AND RESULTS To assess the potential interaction between this mutation and vitamin coenzymes in homocysteine metabolism, we screened 365 individuals from the NHLBI Family Heart Study. Among individuals with lower plasma folate concentrations ( < 15.4 nmol/L), those with the homozygous mutant genotype had total fasting homocysteine levels that were 24% greater (P<.05) than individuals with the normal genotype. A difference between genotypes was not seen among individuals with folate levels > or = 15.4 nmol/L. CONCLUSIONS Individuals with thermolabile MTHFR may have a higher folate requirement for regulation of plasma homocysteine concentrations; folate supplementation may be necessary to prevent fasting hyperhomocysteinemia in such persons.

Relation of carotid artery wall thickness to diabetes mellitus, fasting glucose and insulin, body size, and physical activity. Atherosclerosis Risk in Communities (ARIC) Study Investigators.

Background and Purpose We tested the hypothesis that body mass, waist-to-hip circumference ratio, physical inactivity, diabetes, hyperglycemia, and fasting insulin are each positively associated with asymptomatic carotid artery wall thickness. Methods Average intimal-medial carotid wall thickness (an indicator of atherosclerosis) was measured noninvasively by B-mode ultrasonography in cross-sectional samples of 45- to 64-year-old adults, both blacks and whites, free of symptomatic cardiovascular disease, in four US communities. Results Sample mean carotid wall thickness was approximately 0.7 mm in women (n=7956) and 0.8 mm in men (n=6474). Body mass, waist-to-hip ratio, work physical activity, diabetes, and fasting insulin were associated (P < .05) with carotid wall thickness in the hypothesized direction. Adjusted for age, race, smoking, body mass index, artery depth, and Atherosclerosis Risk in Communities field center, mean wall thickness was greater by 0.02 mm in women and 0.03 mm in men for a 0.07-unit (one SD) larger waist-to-hip ratio. Adjusted mean wall thickness was about 0.07 mm thicker in participants with diabetes mellitus and 0.02 mm thicker in participants with hyperglycemia (fasting glucose 6.4 to 7.7 mmol/L) than in subjects with fasting glucose <6.4 mmol/L. Adjusted mean wall thickness increased by about 0.02 mm with an increase of 100 mmol/L in fasting serum insulin. Conclusions Abdominal adiposity, physical inactivity, and abnormal glucose metabolism are associated positively with carotid intimal-medial wall thickness, suggesting these factors contribute to atherogenesis.

Associations of serum and dietary magnesium with cardiovascular disease, hypertension, diabetes, insulin, and carotid arterial wall thickness: The aric study

The objective of this study was to examine the relationships of serum and dietary magnesium (Mg) with prevalent cardiovascular disease (CVD), hypertension, diabetes mellitus, fasting insulin, and average carotid intimal-medial wall thickness measured by B-mode ultrasound. A cross-sectional design was used. The setting was the Atherosclerosis Risk in Communities (ARIC) Study in four US communities. A total of 15,248 participants took part, male and female, black and white, aged 45-64 years. Fasting serum Mg, lipids, fasting glucose and insulin were measured; as was usual dietary intake by food frequency questionnaire and carotid intima-media thickness by standardized B-mode ultrasound methods. The results showed that serum Mg levels and dietary Mg intake were both lower in blacks than whites. Mean serum Mg levels were significantly lower in participants with prevalent CVD, hypertension, and diabetes than in those free of these diseases. In participants without CVD, serum Mg levels were also inversely associated with fasting serum insulin, glucose, systolic blood pressure and smoking. Dietary Mg intake was inversely associated with fasting serum insulin, plasma high density lipoprotein-cholesterol, systolic and diastolic blood pressure. Adjusted for age, race, body mass index, smoking, hypertension, Low density lipoprotein-cholesterol, and field center, mean carotid wall thickness increased in women by 0.0118 mm (p = 0.006) in diuretic users and 0.0048 mm (p = 0.017) in nonusers for each 0.1 mmol/l decrease in serum Mg level; the multivariate association in men was not significant. In conclusion, low serum and dietary Mg may be related to the etiologies of CVD, hypertension, diabetes, and atherosclerosis.

Familial and genetic determinants of systemic markers of inflammation: the NHLBI family heart study.

Inflammation is thought to play a central role in the etiology and outcome of atherosclerosis. Animal studies as well as in vitro and in vivo human studies suggest that host factors modulate the magnitude and extent of inflammatory responses. We investigated familial aggregation of three systemic markers of inflammation (C-reactive protein (CRP), white blood cell count (WBC), and albumin) in a large, cross-sectional study conducted in four US communities. We found evidence of substantial heritability (35-40%) for CRP levels as well as for WBC and albumin levels. Negligible spouse correlations suggested little influence of shared household environment on these traits. The combination of sociodemographic factors (age, center, education), behavioral and lifestyle factors (cigarette smoking, alcohol intake, hormone replacement therapy), obesity and fat patterning, and prevalent diabetes explained 13-30% the interindividual variability of these traits. There was no evidence that these inflammation phenotypes were linked to a microsatellite marker in the interleukin-1 gene cluster on chromosome 2q, a region that includes several candidate genes for chronic inflammatory diseases. Our findings suggest that CRP levels, albumin levels, and WBC are determined at least partially by genetic factors. Further efforts to identify gene loci affecting these traits are warranted.

The 1298A C polymorphism in methylenetetrahydrofolate reductase (MTHFR): in vitro expression and association with homocysteine

A common mutation in methylenetetrahydrofolate reductase (MTHFR), 677C-->T, is associated with reduced enzyme activity, a thermolabile enzyme and mild hyperhomocysteinemia, a risk factor for vascular disease. Recently, a second common mutation (1298A-->C; glutamate to alanine) was reported, but this mutation was suggested to increase homocysteine only in individuals who carried the bp677 variant. To evaluate the functional consequences of this mutation, we performed site-directed mutagenesis and in vitro expression. For in vivo assessment of clinical impact, we examined the 1298A-->C genotypes and plasma homocysteine in 198 individuals from the NHLBI Family Heart Study that had previously been assessed for the 677 substitution. Site-directed mutagenesis of the human cDNA was performed to generate enzymes containing each of the two mutations, as well as an enzyme containing both substitutions. Enzyme activity and thermolability were assessed in bacterial extracts. The activity of the wild-type cDNA was designated as 100%; mutant enzymes containing the 1298 and 677 mutations separately had 68% (+/-5.0) and 45% (+/-10.8), respectively, of control activity while the enzyme containing both mutations had 41% (+/-12.8) of control activity. The 1298 mutation was not associated with a thermolabile enzyme. In the Family Heart Study, fasting homocysteine was significantly higher (P<0.05) in individuals heterozygous for both substitutions, compared to individuals who carried only the 677C-->T variant. This study suggests that two variants in MTHFR should be assessed as genetic risk factors for hyperhomocysteinemia.

A Prospective Study of Coronary Heart Disease in Relation to Fasting Insulin, Glucose, and Diabetes: The Atherosclerosis Risk in Communities (ARIC) Study

OBJECTIVE To determine the association of coronary heart disease (CHD) incidence with diabetes, fasting serum glucose, and insulin in a biracial cohort of middle-aged men and women. RESEARCH DESIGN AND METHODS We examined a population-based sample (n = 13,446 free of baseline CHD) from four U.S. communities in 1987–1989. We defined diabetes on the basis of baseline fasting glucose concentration (≥ 7.8 mmol/l), medical history, and current medications. A central laboratory measured fasting insulin with a nonspecific radioimmunoassay. After 4–7 years, 209 men and 96 women developed CHD. RESULTS After adjustment for sociodemographic characteristics, smoking status, ethanol intake, sports participation, and hormone replacement therapy, the relative risk of CHD for people with diabetes versus those without diabetes was 3.45 (95% CI 2.16–5.50) among women and 2.52 (1.78–3.56) among men. Relative risks of CHD with diabetes were somewhat lower in blacks than non-blacks, but because diabetes was more than twice as prevalent in blacks, the percentage of CHD cases attributable to diabetes (population attributable risk) was 27% for black women, 15% for non-black women, 8% for black men, and 12% for non-black men. Among people without diabetes, fasting glucose was not independently associated with CHD incidence. Among women without diabetes, there was a positive association between fasting insulin and CHD; multivariable adjusted relative risks of CHD across quintiles of fasting insulin were 1.00, 0.76, 2.08, 2.08 and 2.82 (P for linear trend = 0.02). However, among men without diabetes, fasting insulin and CHD were not associated. CONCLUSIONS Diabetes conveys a high risk of CHD in black and non-black middle-aged men and women. Fasting insulin, however, is a CHD risk factor only among women in this cohort.

Association of C-reactive protein with markers of prevalent atherosclerotic disease

Recent prospective studies have demonstrated that elevated C-reactive protein (CRP) is a marker of increased risk of atherothrombotic clinical events. We examined in a large, cross-sectional family-based study (n = 875 men, 948 women) whether serum CRP was associated with prevalent coronary heart disease (CHD), the ankle/brachial blood pressure index, or carotid intima-media thickness, an indicator of subclinical atherosclerosis as assessed by B-mode ultrasound. CRP was associated with many other cardiovascular risk factors, particularly markers of obesity and insulin resistance, markers of inflammation and acute phase reaction, and hormone replacement therapy. Adjusted for age and family type, there was a weak positive association of CRP with carotid intima-media thickness in both genders and with prevalent CHD in women. However, adjustment for other risk factors completely eliminated the associations. For example, among women, the risk factor-adjusted mean values of intima-media thickness across quartiles of CRP were 0.76, 0.74, 0.75, and 0.76 mm (p >0.5). In men there was a weak inverse association between CRP and ankle/brachial blood pressure index, independent of other risk factors, but no such association in women. Our findings indicate that CRP is not strongly and independently associated with prevalent atherosclerosis. Because CRP has been associated with clinical events, it could be that elevated CRP may be a stronger marker of thrombotic risk than of the degree of atherosclerosis.

Circulating TNF Receptors 1 and 2 Predict ESRD in Type 2 Diabetes

Levels of proinflammatory cytokines associate with risk for developing type 2 diabetes but whether chronic inflammation contributes to the development of diabetic complications, such as ESRD, is unknown. In the 1990s, we recruited 410 patients with type 2 diabetes for studies of diabetic nephropathy and recorded their characteristics at enrollment. During 12 years of follow-up, 59 patients developed ESRD (17 per 1000 patient-years) and 84 patients died without ESRD (24 per 1000 patient-years). Plasma markers of systemic inflammation, endothelial dysfunction, and the TNF pathway were measured in the study entry samples. Of the examined markers, only TNF receptors 1 and 2 (TNFR1 and TNFR2) associated with risk for ESRD. These two markers were highly correlated, but ESRD associated more strongly with TNFR1. The cumulative incidence of ESRD for patients in the highest TNFR1 quartile was 54% after 12 years but only 3% for the other quartiles (P<0.001). In Cox proportional hazard analyses, TNFR1 predicted risk for ESRD even after adjustment for clinical covariates such as urinary albumin excretion. Plasma concentration of TNFR1 outperformed all tested clinical variables with regard to predicting ESRD. Concentrations of TNFRs moderately associated with death unrelated to ESRD. In conclusion, elevated concentrations of circulating TNFRs in patients with type 2 diabetes at baseline are very strong predictors of the subsequent progression to ESRD in subjects with and without proteinuria.

Ethical and Practical Guidelines for Reporting Genetic Research Results to Study Participants: Updated Guidelines From a National Heart, Lung, and Blood Institute Working Group

In January 2009, the National Heart, Lung, and Blood Institute convened a 28-member multidisciplinary Working Group to update the recommendations of a 2004 National Heart, Lung, and Blood Institute Working Group focused on Guidelines to the Return of Genetic Research Results. Changes in the genetic and societal landscape over the intervening 5 years raise multiple questions and challenges. The group noted the complex issues arising from the fact that technological and bioinformatic progress has made it possible to obtain considerable information on individuals that would not have been possible a decade ago. Although unable to reach consensus on a number of issues, the working group produced 5 recommendations. The working group offers 2 recommendations addressing the criteria necessary to determine when genetic results should and may be returned to study participants, respectively. In addition, it suggests that a time limit be established to limit the duration of obligation of investigators to return genetic research results. The group recommends the creation of a central body, or bodies, to provide guidance on when genetic research results are associated with sufficient risk and have established clinical utility to justify their return to study participants. The final recommendation urges investigators to engage the broader community when dealing with identifiable communities to advise them on the return of aggregate and individual research results. Creation of an entity charged to provide guidance to institutional review boards, investigators, research institutions, and research sponsors would provide rigorous review of available data, promote standardization of study policies regarding return of genetic research results, and enable investigators and study participants to clarify and share expectations for the handling of this increasingly valuable information with appropriate respect for the rights and needs of participants.