The arrow tells you direction. The angle of the arrow tells you speed. A 130 with a gently-up arrow and a 130 with a steeply-up arrow are two completely different situations.
Today's idea
A gentle rise is something you might watch. A fast rise is something to pay closer attention to. A gentle fall is something that often resolves on its own. A fast fall is something else entirely.
Most CGMs use a system roughly like this: one arrow up or down means changing about 1–2 mg/dL per minute. Two arrows in the same direction means 2–3 mg/dL per minute. Three arrows is the rare, urgent case — 3+ mg/dL per minute.
That math means a CGM with two arrows up at 130 is on track to be near 190 in 30 minutes if nothing changes. The same 130 with one arrow up will likely be near 160. The number you see now and the number you'll see in 30 minutes can be very far apart.
The Pro Tip series puts it this way:
Most people see the arrows but were never told what they actually mean. They're a forecast — a rate of change, showing where you'll be in thirty minutes. Reading them is a skill, and one most people are never taught directly.
From Pro Tip 1003
Today, you're being told.
The speed tells me how much time I have.
Today, when you check your CGM, name the direction, then estimate the speed (gentle? medium? fast?), then check the number. Notice if your sense of what's okay or not okay shifts when you read those three pieces of information in that order.
Pro Tip 1006 — Mastering a CGM
Did reading direction, then speed, then number change how I felt about any of today's readings?