Your current glucose number tells you where you are. Your trend arrow tells you where you are going. A flat arrow at 140 mg/dL and a double up arrow at 140 mg/dL require completely different responses.
A CGM reading is a snapshot. It tells you what your interstitial glucose concentration is at this moment. But your glucose is always moving. At any given reading, it could be rising fast, rising slowly, stable, falling slowly, or falling fast. The clinical response to each of these situations is different.
Before CGM technology, all dosing decisions were made from single-point glucose measurements. You knew where you were but not where you were going. The trend arrow changes that. It gives you velocity information that fundamentally changes how you should interpret any given glucose reading.
Different CGM systems use slightly different symbols, but the underlying classification is consistent:
| Arrow | Rate of change | What it means |
|---|---|---|
| Double up | More than 2 mg/dL per minute rising | Glucose rising very fast. In 15 minutes it will be 30 mg/dL higher. |
| Single up | 1 to 2 mg/dL per minute rising | Glucose rising steadily. In 15 minutes it will be 15 to 30 mg/dL higher. |
| Diagonal up | 0.5 to 1 mg/dL per minute rising | Slow rise. Less urgent but should be factored into dosing. |
| Flat | Less than 0.5 mg/dL per minute | Glucose stable. Standard dosing applies. |
| Diagonal down | 0.5 to 1 mg/dL per minute falling | Slow fall. Reduce correction dose slightly. |
| Single down | 1 to 2 mg/dL per minute falling | Falling meaningfully. Reduce or hold correction. Eat before bolusing if low-normal. |
| Double down | More than 2 mg/dL per minute falling | Falling fast. Treat immediately. Do not bolus. |
A randomised clinical trial using the Omnipod 5 system compared standard bolus calculation against CGM-informed bolus calculation with automatic trend adjustment. The CGM-informed approach produced significantly fewer glucose readings below 70 mg/dL in the 4 hours after a bolus — 2.1% versus 2.8% — without increasing hyperglycemia or total insulin use.
This sounds like a small difference but it represents a meaningful reduction in post-bolus hypoglycemia risk across thousands of dosing decisions over time. The trend adjustment did not make dosing more complicated — it made it safer.
The adjustment is applied as a percentage modifier on your correction dose. For a rising trend, you add insulin. For a falling trend, you reduce insulin. The meal bolus component stays the same regardless of trend — only the correction is modified.
Rising fast: add 15 to 20% to your correction dose. Rising: add 5 to 10%. Flat: no adjustment. Falling: reduce correction by 10 to 15% or hold entirely if near target. Falling fast: hold all correction insulin, treat the low.
These are starting percentages. Your actual adjustment will be calibrated by your ISF. If your ISF is 40 mg/dL per unit and you are rising fast, adding 15% to a 2-unit correction adds about 0.3 units. If your ISF is 20, the same percentage adds more absolute insulin.
CGM measures interstitial glucose, not blood glucose. There is a physiological lag of approximately 5 to 15 minutes between your actual blood glucose and what your CGM displays. This lag matters most when glucose is changing rapidly.
If your CGM shows 130 mg/dL with a double up arrow, your actual blood glucose may already be 145 to 160 mg/dL. This is one reason why trend-adjusted dosing can produce better outcomes than fixed-point dosing — it accounts for where you actually are, not just where the sensor says you were 10 minutes ago.
The practical implication: when you see a rapidly rising arrow, be willing to act more decisively because the sensor is behind reality.
The most important trend-based decision is when to hold insulin entirely. If your glucose is below 90 mg/dL (5 mmol/L) with any falling trend, do not bolus. Treat with fast carbs first, wait for the trend to stabilise, then reassess whether a meal bolus is still needed.
If your glucose is in a reasonable range but falling fast before a meal, eat first and delay your bolus by 15 to 20 minutes. Let your glucose stabilise before adding insulin. The temporary post-meal rise is easier to manage than a low that starts from a falling baseline.
The pre-exercise CGM check is one of the most important applications of trend data. Before any session, your trend arrow is as important as your glucose number. A flat reading at 6 mmol/L is a safe starting point for aerobic exercise. A falling reading at 6 mmol/L with a single down arrow is a different situation entirely — you need carbs before you start, regardless of the number.
The Diabetic Wolf CGM Decision Tool uses this logic explicitly, factoring both your current glucose and your trend arrow into the GO, PROCEED WITH CAUTION, or HARD STOP decision.
The Diabetic Wolf bolus calculator applies everything in this guide automatically. ICR by time of day, CGM trend adjustment, IOB correction, fat and protein split dosing, and exercise modifier.
Open the Bolus Calculator CGM Decision Tool