Medical devices, digital health and remote monitoring — formative SAQs

SAQ 1 (10)

A four-year-old with type 1 diabetes for eighteen months is brought to clinic. The family uses a continuous glucose monitor (CGM) and report recurrent night-time alarms and a recent reading of 48 mg/dL (2.7 mmol/L) at three o'clock in the morning, treated with juice. The family ask whether they should stop the device. ^[4]

1. Explain how a CGM works at sensor level, including the plasma-interstitial lag and its clinical consequence. (3) ^[4]
2. State the ISPAD 2024 ambulatory glucose profile targets for time in range, time below 70 mg/dL and time below 54 mg/dL, and outline how you would read the AGP at this visit. (4) ^[4]
3. Outline your immediate management of a CGM reading below 54 mg/dL (3.0 mmol/L), and your shared decision with the family about whether to continue, modify or stop the device. (3) ^[8]

Model answer

Sensor physiology and the lag. A CGM measures interstitial fluid glucose, not plasma glucose. A flexible sensor is placed subcutaneously — usually the upper arm in a child — and a glucose-oxidase enzyme electrode generates a current proportional to the interstitial glucose. Because glucose must move from plasma to interstitial fluid before the sensor reads it, the interstitial value lags the plasma value by five to fifteen minutes, longer during rapid change. The clinical consequence is the lag: a falling glucose can be missed at the trough, and a post-meal rise can lag the capillary value. For this reason any critical CGM reading — particularly a low — must be confirmed with a fingerstick before action. ^[4]

AGP targets and how to read the report. ISPAD 2024 and the Battelino international consensus set the operational targets: time in range — glucose between 70 and 180 mg/dL (3.9 and 10.0 mmol/L) — should exceed 70 per cent of the day; time below 70 mg/dL (3.9 mmol/L) should be under 4 per cent; and time below 54 mg/dL (3.0 mmol/L) should be under 1 per cent. At this visit I would open the AGP report, look first at the time in range percentage against the 70 per cent target, then at the time below range against the 4 per cent and 1 per cent targets, then at the median trace to see whether the hypoglycaemias cluster at night (suggesting basal insulin or alarm-threshold tuning), and finally at the sensor wear to confirm the data are complete enough to interpret. ^[4]

Immediate management and the shared decision. For a CGM reading below 54 mg/dL (3.0 mmol/L) I confirm with a fingerstick, give 10 to 15 grams of fast-acting oral carbohydrate if the child is conscious and able to swallow (glucose tablets, juice, glucose gel), give glucagon 0.5 milligrams intramuscularly for a child under twelve years or 1 milligram at twelve years and over if the child is unconscious or unable to swallow safely, and recheck the glucose in fifteen minutes. I counsel the family to keep glucagon available at home and to use it. On the shared decision, I would not stop the device — the recurrent night-time alarms are a threshold-tuning problem, not a device failure. I would raise the low alert threshold to limit non-critical alarms, schedule a period of protected sleep with only the critical alarm audible, review the basal insulin pattern that is driving the night-time lows, and arrange a follow-up review of the AGP in two to four weeks. Abandoning the device without a safety-net would expose the child to unrecognised hypoglycaemia. ^[8]