Distinguishing ICR and ISF problems in type 1 diabetes graphs

In type 1 diabetes, high glucose after meals does not always have the same cause. A meal bolus may be too small or too large because of the insulin-to-carb ratio (ICR). Alternatively, the meal may be covered adequately, but later corrections may act too weakly or too strongly because of the insulin sensitivity factor (ISF). Look for recurring patterns in what happens after meals and after corrections, rather than relying on a single graph.

When ICR is more likely to be involved

Glucose repeatedly rises high after similar meals — if glucose repeatedly rises too high after meals despite a timely bolus, ICR may need attention.
Later corrections still work — if post-meal high glucose then responds normally to a correction, this suggests an insufficient meal bolus more than an ISF setting that produces too little correction.
The problem is linked to meals — a pattern that follows breakfast, lunch or dinner rather than occurring at any time also points more toward ICR.
The same meal produces a similar pattern — comparing several similar meals is more useful than comparing days with different foods.

When ISF is more likely to be involved

A correction barely lowers glucose — a correction bolus that repeatedly has less effect than expected under similar conditions may point to an ISF setting that produces too little correction.
A correction acts too strongly — glucose falling too low after a usual correction may indicate that the ISF setting produces an overly aggressive correction.
The problem occurs outside meals too — an unusual response to corrections that also occurs away from meals points more toward ISF than ICR.
The same correction behaves similarly under similar conditions — the repeated pattern matters more than an isolated episode.

How to avoid confusing ICR and ISF

Do not draw a conclusion from one graph: review several similar episodes.
First separate meals from corrections: compare the first hours after food with what happens after an additional bolus.
Account for bolus timing: a late bolus can look like an ICR problem when the cause is timing.
Consider meal composition: fat and protein can complicate the pattern and make early conclusions unreliable.
If you use a pump or AAPS, account for automatic corrections and additional system actions.

What to do next

This review can help identify whether the meal bolus, insulin sensitivity, bolus timing or meal composition needs attention. Changes to ICR or ISF are better based on several similar observations than on one difficult day.