Again, full disclosure: I entered your question “what qualifies as a dramatic spike in blood sugar?” into a search engine and Copilot answered:

“A dramatic spike is any rise in blood sugar that:

• jumps more than 50–60 mg/dL from your starting point
• or reaches over ~140 mg/dL at the 1‑hour mark
• or stays above ~120 mg/dL at 2 hours

Those three things together define a spike that stresses your system.

What counts as a dramatic spike (broken down clearly)

1. Height of the rise. This is the most important part.

• Normal: +20 to +40 mg/dL
• Borderline: +40 to +50 mg/dL
• Dramatic spike: +50 to +60 mg/dL or more
• Very dramatic: +80 to +120 mg/dL

Example: Start at 100 → peak at 165 = dramatic spike
Start at 100 → peak at 190 = very dramatic spike

2. Peak number for most adults:

• Under 140 mg/dL at 1 hour = healthy
• 140–160 mg/dL = borderline
• 160–200 mg/dL = dramatic spike
• 200+ mg/dL = very dramatic spike

3. Steepness of the curve

A spike is dramatic if:

• it shoots up fast
• you see a sharp vertical line on your CGM
• you feel symptoms within 20–40 minutes

Fast spikes cause more inflammation than slow ones.

4. How long it stays high

A spike is dramatic if:

• you’re still above 120 mg/dL at 2 hours
• or you don’t return to baseline for 3–4 hours

This means your body struggled to clear the glucose.

@anonymom3
You ask good questions!
Unfortunately equally good answers are sometimes hard to come by, like on this one. Until the CGM nobody really did much study on how normal, that is non-diabetic, that is normal A1C, people digest their carbs. So there was no textbook. The first Dexcom was sold around 2006.
Now there is some data on normals and some on diabetics, but there are still many variables and the textbook is still pretty thin, afaik.

What if a normal person pigs out at a party, what then? I mean, it's probably good not to make a habit of that but people are built to be able to enjoy a big meal now and then, and to be able to go on small meals now and then, with no real damage. But just how fat they are, and how fit they are, is going to affect BG readings.

So to just get a big picture view the relevant metric is probably something like total area under the curve.

If you eat the whole birthday cake, normal or diabetic you're going to have a spike. If you're normal the body restores order quickly enough, if you're diabetic maybe not so much. OTOH if you're a diabetic taking insulin you can adjust your dosage. If you're on metformin or Ozympic or whatever, you're going to see a different curve.

My endo told me, if I get more than one or two readings above 200, call him, but those would be pre-meal readings, let's say at least four hours since finishing the last meal.

They say that your BG should return to normal about two hours after a meal, but I've noticed, just on finger-sticks, that my recovery often takes longer. This is probably a diabetic factor. Given 4-5 hours my finger-stick readings continue to be good, my A1C stays just below the pre-diabetic line of 5.7, so things are working for me. If I pig out on a meal it may be reflected for as long as 24 hours with readings 10-20 points (sometimes more) above my normal.

So it's basically the area beneath the curve, and that's pretty much what your meter must be doing to estimate A1C.