Categorisation of continuous risk factors in epidemiological publications: a survey of current practice.

BACKGROUND: Reports of observational epidemiological studies often categorise (group) continuous risk factor (exposure) variables. However, there has been little systematic assessment of how categorisation is practiced or reported in the literature and no extended guidelines for the practice have been identified. Thus, we assessed the nature of such practice in the epidemiological literature. Two months (December 2007 and January 2008) of five epidemiological and five general medical journals were reviewed. All articles that examined the relationship between continuous risk factors and health outcomes were surveyed using a standard proforma, with the focus on the primary risk factor. Using the survey results we provide illustrative examples and, combined with ideas from the broader literature and from experience, we offer guidelines for good practice. RESULTS: Of the 254 articles reviewed, 58 were included in our survey. Categorisation occurred in 50 (86%) of them. Of those, 42% also analysed the variable continuously and 24% considered alternative groupings. Most (78%) used 3 to 5 groups. No articles relied solely on dichotomisation, although it did feature prominently in 3 articles. The choice of group boundaries varied: 34% used quantiles, 18% equally spaced categories, 12% external criteria, 34% other approaches and 2% did not describe the approach used. Categorical risk estimates were most commonly (66%) presented as pairwise comparisons to a reference group, usually the highest or lowest (79%). Reporting of categorical analysis was mostly in tables; only 20% in figures. CONCLUSIONS: Categorical analyses of continuous risk factors are common. Accordingly, we provide recommendations for good practice. Key issues include pre-defining appropriate choice of groupings and analysis strategies, clear presentation of grouped findings in tables and figures, and drawing valid conclusions from categorical analyses, avoiding injudicious use of multiple alternative analyses.

Prognostic significance of visit-to-visit variability, maximum systolic blood pressure, and episodic hypertension.

BACKGROUND: The mechanisms by which hypertension causes vascular events are unclear. Guidelines for diagnosis and treatment focus only on underlying mean blood pressure. We aimed to reliably establish the prognostic significance of visit-to-visit variability in blood pressure, maximum blood pressure reached, untreated episodic hypertension, and residual variability in treated patients. METHODS: We determined the risk of stroke in relation to visit-to-visit variability in blood pressure (expressed as standard deviation [SD] and parameters independent of mean blood pressure) and maximum blood pressure in patients with previous transient ischaemic attack (TIA; UK-TIA trial and three validation cohorts) and in patients with treated hypertension (Anglo-Scandinavian Cardiac Outcomes Trial Blood Pressure Lowering Arm [ASCOT-BPLA]). In ASCOT-BPLA, 24-h ambulatory blood-pressure monitoring (ABPM) was also studied. FINDINGS: In each TIA cohort, visit-to-visit variability in systolic blood pressure (SBP) was a strong predictor of subsequent stroke (eg, top-decile hazard ratio [HR] for SD SBP over seven visits in UK-TIA trial: 6.22, 95% CI 4.16-9.29, p<0.0001), independent of mean SBP, but dependent on precision of measurement (top-decile HR over ten visits: 12.08, 7.40-19.72, p<0.0001). Maximum SBP reached was also a strong predictor of stroke (HR for top-decile over seven visits: 15.01, 6.56-34.38, p<0.0001, after adjustment for mean SBP). In ASCOT-BPLA, residual visit-to-visit variability in SBP on treatment was also a strong predictor of stroke and coronary events (eg, top-decile HR for stroke: 3.25, 2.32-4.54, p<0.0001), independent of mean SBP in clinic or on ABPM. Variability on ABPM was a weaker predictor, but all measures of variability were most predictive in younger patients and at lower (

Effects of beta blockers and calcium-channel blockers on within-individual variability in blood pressure and risk of stroke.

BACKGROUND: Analyses of some randomised trials show that calcium-channel blockers reduce the risk of stroke more than expected on the basis of mean blood pressure alone and that beta blockers are less effective than expected. We aimed to investigate whether the effects of these drugs on variability in blood pressure might explain these disparities in effect on stroke risk. METHODS: The Anglo-Scandinavian Cardiac Outcomes Trial Blood Pressure Lowering Arm (ASCOT-BPLA) compared amlodipine-based regimens with atenolol-based regimens in 19 257 patients with hypertension and other vascular risk factors and the Medical Research Council (MRC) trial compared atenolol-based and diuretic-based regimens versus placebo in 4396 hypertensive patients aged 65-74 years. We expressed visit-to-visit variability of blood pressure during follow-up in the two trials as standard deviation (SD) and as transformations uncorrelated with mean blood pressure. For ASCOT-BPLA, we also studied within-visit variability and variability on 24 h ambulatory blood-pressure monitoring (ABPM). RESULTS: In ASCOT-BPLA, group systolic blood pressure (SBP) SD was lower in the amlodipine group than in the atenolol group at all follow-up visits (p<0.0001), mainly because of lower within-individual visit-to-visit variability. Within-visit and ABPM variability in SBP were also lower in the amlodipine group than in the atenolol group (all p<0.0001). Analysis of changes from baseline showed that variability decreased over time in the amlodipine group and increased in the atenolol group. The lower risk of stroke in the amlodipine group (hazard ratio 0.78, 95% CI 0.67-0.90) was partly attenuated by adjusting for mean SBP during follow-up (0.84, 0.72-0.98), but was abolished by also adjusting for within-individual SD of clinic SBP (0.99, 0.85-1.16). Findings were similar for coronary events. In the ABPM substudy, reduced variability in daytime SBP in the amlodipine group (p<0.0001) partly accounted for the reduced risk of vascular events, but reduced visit-to-visit variability in clinic SBP had a greater effect. In the MRC trial, group SD SBP and all measures of within-individual visit-to-visit variability in SBP were increased in the atenolol group compared with both the placebo group and the diuretic group during initial follow-up (all p<0.0001). Subsequent temporal trends in variability in blood pressure during follow-up in the atenolol group correlated with trends in stroke risk. INTERPRETATION: The opposite effects of calcium-channel blockers and beta blockers on variability of blood pressure account for the disparity in observed effects on risk of stroke and expected effects based on mean blood pressure. To prevent stroke most effectively, blood-pressure-lowering drugs should reduce mean blood pressure without increasing variability; ideally they should reduce both.

Baseline heart rate, antihypertensive treatment, and prevention of cardiovascular outcomes in ASCOT (Anglo-Scandinavian Cardiac Outcomes Trial).

OBJECTIVES: The aim of this study was to evaluate the effect of baseline heart rate on the efficacy of atenolol-based compared with amlodipine-based therapy in patients with hypertension uncomplicated by coronary heart disease in the ASCOT-BPLA (Anglo-Scandinavian Cardiac Outcomes Trial-Blood Pressure Lowering Arm). BACKGROUND: Heart rate is an established risk factor for cardiovascular events. Consequently, it is a widely held belief that beta-blockers should be prescribed for management of hypertension in patients with higher heart rates. METHODS: Patients with atrial fibrillation or taking rate-limiting antihypertensive drugs at baseline were excluded. Primary analyses used Cox models to investigate the potential attenuation of the treatment effect with higher baseline heart rate on total cardiovascular events and procedures (TCVP) via introduction of an interaction term. Secondary analyses assessed coronary and total stroke outcomes. RESULTS: Primary unadjusted analyses included 12,759 patients and 1,966 TCVP. At the final visit, mean heart rate reduction from baseline was 12.0 (SD 13.7) and 1.3 (SD 12.1) beats/min in atenolol- and amlodipine-based groups, respectively. There was a reduction in TCVP in those allocated amlodipine-based therapy compared with atenolol-based therapy (unadjusted hazard ratio: 0.81, p < 0.001). This benefit was unattenuated at higher heart rates (interaction p value = 0.81). Similar results were obtained for coronary and total stroke outcomes. CONCLUSIONS: There was no evidence that the superiority of amlodipine-based over atenolol-based therapy for patients with hypertension uncomplicated by coronary heart disease was attenuated with higher baseline heart rate. These data suggest that, in similar hypertensive populations without previous or current coronary artery disease, higher baseline heart rate is not an indication for preferential use of beta-blocker-based therapy.