title: Establishing an evidence base for frequency of monitoring glycated haemoglobin levels in patients with Type 2 diabetes: projections of effectiveness from a regression model.
abstract: OBJECTIVE
  Glycated haemoglobin (HbA1c) is monitored to guide treatment decisions in relation to glycaemic goals over time. Changes between two consecutive HbA1c tests result not only from deterioration or improvement in glycaemic control, but also from biovariability and measurement error. We model how this short-term variability impacts on HbA1c monitoring.
  
  
  METHODS
  Using data from a randomized trial of non-insulin treated patients with Type 2 diabetes we fitted a random-effects model for progression and variability of HbA1c. We estimated how many tests where HbA1c ≥ 7.5% (58.5 mmol/mol) would be false-positive (underlying HbA1c < 7.5% but test ≥ 7.5% owing to variability) vs. true-positive, in people with initial HbA1c between 6.5% and 7.3% (48 mmol/mol and 56 mmol/mol).
  
  
  RESULTS
  Participants (n = 320) had mean (SD) age 66 (10) years, BMI 31.3 (6.0) kg/m2 and median HbA1c was 7.1% (54 mmol/mol) with interquartile range 6.6% (49 mmol/mol) to 7.7% (61 mmol/mol). Mean (95% CI) change in HbA1c was 0.1% (1 mmol/mol) with 95% confidence interval 0.05% (0.5 mmol/mol) to 0.15% (2 mmol/mol) per 6 months. The minimum interval at which a true-positive test is more likely than a false positive test is 270 days for a starting HbA1c of 6.9% (52 mmol/mol) and 360 days at a starting value of 6.5% (48 mmol/mol).
  
  
  CONCLUSIONS
  In patients with initial HbA1c close to treatment goal, retesting at 6 months would yield more true-positive than false-positive tests. For patients with lower initial HbA1c, retesting at 6 months would yield more false than true-positive tests. In all patients, retesting at 12 months yields more true than false-positive tests. In very few patients would retesting at 3 months be justified.
authors.affiliation: Department of Primary Health Care, University of Oxford and School of Primary Care Research, National Institute for Healthcare Research Somerville College, University of Oxford, UK. jason.oke@phc.ox.ac.uk.
authors.forename: J L
authors.initials: JL
authors.lastname: Oke
authors.forename: R J
authors.initials: RJ
authors.lastname: Stevens
authors.forename: K
authors.initials: K
authors.lastname: Gaitskell
authors.forename: A J
authors.initials: AJ
authors.lastname: Farmer
mesh_terms: D000368:Aged; D001786:Blood Glucose / Q000378:metabolism*; D015190:Blood Glucose Self-Monitoring; D003924:Diabetes Mellitus, Type 2 / Q000097:blood*; D019317:Evidence-Based Medicine; D005260:Female; D006442:Glycated Hemoglobin / Q000378:metabolism*; D006801:Humans; D008297:Male; D010349:Patient Compliance / Q000706:statistics & numerical data; D012044:Regression Analysis; D016896:Treatment Outcome
chemical_list: D001786:Blood Glucose; D006442:Glycated Hemoglobin