Glycated haemoglobin (HbA 1c ) monitoring

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Abstract
Case 1 A 60 year old woman was seen for review of her diet controlled type 2 diabetes. She was also taking long term ferrous sulphate treatment because of caecal angiodysplasia. Otherwise, she seemed healthy with no specific problems except for being slightly overweight (body mass index 28). Her blood pressure was 144/94 mm Hg, and her haemoglobin was 12.3 g/dl (reference interval 11.5-16.5 g/dl). Random plasma glucose and HbA1c were measured and found to be 12.0 mmol/l and 7.4% respectively. Urine albumin excretion was also assessed—urinary albumin:creatinine ratio 1.5 mg/µmol (threshold < 3.5 mg/µmol to exclude micro-albuminuria). This was considered acceptable, and no change in treatment of her diabetes was considered necessary. Over the next year her haemoglobin remained stable; an HbA1c of 6.8% suggested good glycaemic control, and her diabetes treatment was not changed. The next year two consecutive urine albumin excretion ratios were 5.6 and 7.2 mg/µmol, and fundal examination showed early retinopathic changes. Her blood pressure was 152/98 mm Hg. HbA1c was measured and found to be 6.4% despite a random plasma glucose of 18.9 mmol/l. The patient was asked to record regular daily random home blood glucose measurements for a period of three weeks, the results of which were consistently between 12.4 mmol/l and 21.0 mmol/l. She was started on oral hypoglycaemic agents and advised to monitor her blood glucose and symptoms regularly to determine future treatment. She was also prescribed an angiotensin converting enzyme inhibitor in view of her hypertension and micro-albuminuria. Summary points Glycated haemoglobin (HbA1c) is a reliable indicator of diabetic control in most but not all situations Glycation of haemoglobin is non-linear over time and occurs over the whole lifespan of the red blood cell (normally 120 days) Situations in which red cell lifespan is reduced may give rise to low HbA1c results Testing intervals should not normally be less than two months to take account of the physiological half life Practitioners should be aware of situations in which results do not correctly reflect control, when home glucose records may add valuable information HbA1c is not a diagnostic test for diabetes and should not be used as a screening test

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