Variability over time and correlates of cholesterol and blood pressure in systemic lupus erythematosus: a longitudinal cohort study
1 University of Toronto Lupus Clinic and the Centre for Prognosis Studies in the Rheumatic Diseases, Toronto Western Hospital, 399 Bathurst Street, Toronto, ON, M5T 2S8, Canada
2 University of Melbourne Department of Medicine, St. Vincent's Hospital, 41 Victoria Parade, Fitzroy, Melbourne, Victoria, 3065, Australia
3 Division of Cardiology and Clinical Pharmacology, Toronto Western Hospital, 399 Bathurst Street, Toronto, ON, M5T 2S8, Canada
Arthritis Research & Therapy 2010, 12:R125 doi:10.1186/ar3063Published: 30 June 2010
Total cholesterol (TC) and blood pressure (BP) are likely to take a dynamic course over time in patients with systemic lupus erythematosus (SLE). This would have important implications in terms of using single-point-in-time measurements of these variables to assess coronary artery disease (CAD) risk. The objective of this study was to describe and quantify variability over time of TC and BP among patients with SLE and to determine their correlates.
Patients in the Toronto lupus cohort who had two or more serial measurements of TC and systolic and diastolic BP (SBP and DBP) were included in the analysis. Variability over time was described in terms of the proportion of patients whose TC and BP profile fluctuated between normal and elevated (TC > 5.2 mmol/L; SBP ≥ 140 mm Hg or DBP ≥ 90 mm Hg), and also in terms of within- and between-patient variance quantified by using analysis of variance modeling. Generalized estimating equations (GEEs) were used to determine independent correlates of each of TC, SBP, and DBP, treated as continuous outcome variables.
In total, 1,260 patients, comprising 26,267 measurements of each of TC, SBP, and DBP, were included. Mean ± SD number of measurements per patient was 20.8 ± 20. Mean ± SD time interval between measurements was 5.4 ± 9.7 months. Mean ± SD time interval from the start to the end of the study was 9.3 ± 8.5 years. Over time, 64.7% of patients varied between having normal and elevated cholesterol levels, whereas the status of 46.4% of patients varied between normotensive and hypertensive. By using analysis of variance (ANOVA), the within-patient percentage of total variance for each of TC, SBP, and DBP was 48.2%, 51.2%, and 63.9%, respectively. By using GEE, independent correlates of TC and BP included age, disease activity, and corticosteroids; antimalarial use was negatively correlated with TC (all P values < 0.0001).
TC and BP vary markedly over time in patients with SLE. This variability is due not only to lipid-lowering and antihypertensive medications, but also to disease- and treatment-related factors such as disease activity, corticosteroids, and antimalarials. The dynamic nature of TC and BP in SLE makes a compelling case for deriving summary measures that better capture cumulative exposure to these risk factors.