Clinical Trials

❘❙❚■ Chapter 28 | Repeated MeasurementsFigure 1. Repeated measurements of systolic blood pressure (SBP) for 10 patients in a clinical trial.1506SBP (mm Hg)14013012011010090101739458261017394582610173945826101739548261379548Baseline24Time (weeks)68The numbering on the lines refers to the subject’s number in the trial.What are repeated measurements?A repeated measurement in a clinical trial is an outcome variable that is measuredtwo or more times for each subject, usually over a period of time, eg, before,during, and/or after an intervention or treatment. Figure 1 illustrates repeatedmeasurements of systolic blood pressure (SBP) at baseline, 2, 4, 6, and 8 weeksfor 10 patients in a clinical trial.Repeated measurements are commonly encountered in pharmacokinetic andpharmacodynamic studies, eg, collection of blood samples at preselected timepoints in order to measure the blood plasma concentration of a drug. Manyfactors (eg, gender, race, and weight) can influence the absorption andelimination of a study drug. As a result, some people might exhibit consistentlyhigher blood plasma concentrations of a drug than others during the study period.This phenomenon is generally referred to as tracking. In Figure 1, the patients whohave a higher SBP (such as subjects 1, 6, and 10) or a lower SBP (such as subjects2, 4, 5, and 8) tend to have consistently higher or lower SBP throughout follow-up.318