The arrows can sometimes mislead. Dr. Stephen Ponder (and T1D) in his Sugar Surfing book advises to follow the dots, not the arrows. Since he’s called my attention to this phenomena, I’ve noticed it more. Just the other day I watched a diagonal up arrow and a 109 reading transition to a diagonal up arrow and a 106 reading. In the next five minutes the system caught up with reality and then showed a sideways arrow and a 105 reading.
Overall, I find the arrows dependable but they sometimes lag reality. The dots usually tell the more accurate story sooner than the arrows. And I always double-check the double up or down arrows with my meter since that situation, if true, can call for a sharp response of a significant insulin correction or some fast acting glucose.
The Dexcom G4 User’s Guide advises:
Do not overreact to the rate of change arrows. Consider recent insulin dosing, activity, food intake, your overall trend graph [the dots!] and your blood glucose value before taking action.
Additionally the User’s Guide reminds us that the flat arrow does not always mean no change. A flat arrow means that the glucose value is not changing by more than 1 mg/dl per minute. It could change by up to 15 mg/dl in 15 minutes and still show a sideways arrow.