Diabet. Med. 29, 266–271 (2012)
Aims 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).
Conclusion 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.