Time-dependent variability in repeated measurements of cholesterol levels: clinical implications for risk misclassification and intervention monitoring

Intraindividual variability (IIV) in total cholesterol levels based on measurements taken 1 week apart is compared with an estimate based on measurements taken 2 years apart. Single-subject 95% confidence intervals around the mean of two repeated measurements were Xi +/- 21 and +/- 28 mg/dl, respectively, and Xi +/- 30 and +/- 40 mg/dl for a single measurement. Comparing these results with published estimates over varying time intervals shows a trend of decreasing IIV with shorter intervals, suggesting that confidence interval widths based on short-term repeated measurements and those based on longer-term repeated measurements may differ more than previously assumed. The practical consequences are that: (1) the level of misclassification inherent in the National Cholesterol Education Program (NCEP) guidelines may be less than had been estimated; and (2) reliable cholesterol reductions resulting from dietary or other interventions may be somewhat easier to detect. These findings have implications for the cost-effectiveness of cholesterol screening strategies and interventions to reduce cholesterol.

Methods for assessing difference between groups in change when initial measurements is subject to intra-individual variation

Statistical analysis of difference between groups in change for some variable, adjusting for initial value, is complicated by the presence of intra-individual variation in that variable. We estimate here the asymptotic bias that results from calculating the adjusted between-group difference by ordinary least squares (OLS) from observed data. We also present explicit formulae that use the OLS estimates, the difference between treatment groups in mean initial values, and a measure of the intra-individual variation to compute a corrected estimator and its variance. Alternatively, we can use OLS on transformed data to obtain unbiased estimates, in which we replace initial observed values by conditional Stein estimates of true values. We illustrate the results with data from an observational study and a clinical trial.

Effects of chromium supplementation on serum high-density lipoprotein cholesterol levels in men taking beta-blockers. A randomized, controlled trial

OBJECTIVE

To determine the efficacy of glucose tolerance factor (GTF)-chromium for increasing serum levels of high-density lipoprotein (HDL) cholesterol in patients taking beta-blockers.

DESIGN

Randomized, double-blind, placebo-controlled trial.

SETTING

Mixed primary and referral-based outpatient clinic at a university-affiliated VA Medical Center.

PATIENTS

Referred sample of 72 men receiving beta-blockers, mainly for hypertension. Sixty-three patients (88%) completed the study.

INTERVENTIONS

Current medications, including beta-blockers, were continued. During the 8-week treatment phase, patients in the chromium group received a total daily dose of 600 micrograms of biologically active chromium divided into three equal doses; control patients received a placebo of identical appearance and taste.

MEASUREMENTS

Serum levels of total cholesterol and HDL cholesterol were measured.

MAIN RESULTS

Mean baseline levels of HDL and total cholesterol (+/- SD) were 0.93 +/- 0.28 mmol/L and 6.0 +/- 1.0 mmol/L (36 +/- 11.1 mg/dL and 232 +/- 38.5 mg/dL), respectively. The difference between groups in adjusted mean change in HDL cholesterol levels, accounting for baseline HDL cholesterol levels, age, weight change, and baseline total cholesterol levels, was 0.15 mmol/L (5.8 mg/dL) (P = 0.01) with a 95% Cl showing that the treatment effect was greater than +0.04 mmol/L (+1.4 mg/dL). Mean total cholesterol, triglycerides and body weight did not change significantly during treatment for either group. Compliance as measured by pill count was 85%, and few side effects were reported. Two months after the end of treatment, the between-group difference in adjusted mean change from baseline to end of post-treatment follow-up was -0.003 mmol/L (-0.1 mg/dL).

CONCLUSION

Two months of chromium supplementation resulted in a clinically useful increase in HDL cholesterol levels in men taking beta-blockers.

Improving the reliability of total and high-density lipoprotein cholesterol measurements. Four testing strategies compared in a high-risk population

Four testing strategies for assessing total and high-density lipoprotein cholesterol are compared in a homogeneous group of male outpatients at increased risk for cardiovascular disease: (1) a single measurement at one occasion; (2) the mean of duplicate measurements at one occasion; (3) the mean of single measurements in specimens collected 1 week apart; and (4) the overall mean of duplicate measurements at two occasions 1 week apart. Results of strategy 1 were comparatively less reliable as demonstrated by lower intraclass correlation coefficients and higher within-subject variance components. Use of strategy 3 decreased within-subject variance by 50% and improved the 95% confidence interval by 30% for both total and high-density lipoprotein cholesterol, compared with strategy 1. Duplicate testing on either one or two occasions resulted in a nominal improvement in reliability and confidence. Calculating the mean of single measurements in specimens collected 1 week apart is clinically useful because: (1) it reduces the risk of misclassification, (2) it improves intervention monitoring, (3) it supports the National Cholesterol Education Program guidelines for total cholesterol, and (4) it improves the use of high-density lipoprotein as an independent risk factor.