title: Repeatability of measurements of effectiveness of glaucoma medication.
abstract: OBJECTIVE
  To determine short-term repeatability of the effect of intraocular pressure (IOP) reducing medication and the number of repeated measurements necessary to estimate therapeutic effect with a given degree of precision.
  
  
  METHODS
  IOP was measured at 8:00, 11:00 and 16:00 h at each of three weekly visits in untreated patients with primary open-angle glaucoma or ocular hypertension. After starting travaprost (0.004%) to both eyes, the measurements were repeated for a further three weekly visits. Repeatability of the change in IOP 1, 2 and 3 weeks after starting treatment was reported as the coefficient of repeatability (CR) and coefficient of variability (CV).
  
  
  RESULTS
  Mean reduction in IOP was 7.5 mm Hg (29.9%). CR and CV between visits were 7.8 mm Hg and 37.2%, respectively. Repeated estimates of the effectiveness of treatment in the same subject at the same time of day would, therefore, be expected to lie within a range of 7.8 mm Hg and within ±73.2% of the mean effect with 95% confidence. A reduction in IOP less than 7.8 mm Hg over a single pair of measurements would be indistinguishable from measurement error. Precision improves from ±73.2% for a single pair of readings to ±25.9% for eight pairs.
  
  
  CONCLUSIONS
  Day-to-day repeatability in the estimation of medication effectiveness is poor even for time-of-day standardised measurements and the number of IOP measurements required to achieve precision adequate for use in clinical practice is large.
authors.affiliation: Department of Ophthalmology, Tennent Institute of Ophthalmology, Gartnavel Hospital, Great Western Road, Glasgow G12 0YN, UK. rotchford@doctors.org.uk
authors.forename: Alan P
authors.initials: AP
authors.lastname: Rotchford
authors.forename: Anthony J
authors.initials: AJ
authors.lastname: King
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000959:Antihypertensive Agents / Q000627:therapeutic use*; D005260:Female; D005500:Follow-Up Studies; D005902:Glaucoma, Open-Angle / Q000188:drug therapy / Q000503:physiopathology*; D006801:Humans; D007429:Intraocular Pressure / Q000187:drug effects*; D008297:Male; D008875:Middle Aged; D009798:Ocular Hypertension / Q000188:drug therapy / Q000503:physiopathology*; D015203:Reproducibility of Results; D014065:Tonometry, Ocular / Q000706:statistics & numerical data*; D016896:Treatment Outcome
chemical_list: D000959:Antihypertensive Agents