Question 1

Should I use the 500 rule or the 400 rule?

Accepted Answer

The 500 Rule is the most widely used for adults. The 400 (or 300) Rule may be used in children, adolescents, and very insulin-sensitive adults. The ratio should always be individualised through glucose monitoring.

Question 2

What if the calculated ratio doesn't work well in practice?

Accepted Answer

Adjust empirically: if post-meal glucose rises >2–3 mmol/L above target, reduce the ratio (more insulin per gram). If hypoglycaemia occurs within 3–4 hours of eating, increase the ratio (less insulin per gram).

Question 3

Does the ratio change throughout the day?

Accepted Answer

Yes. Many patients need a lower ratio (more insulin) at breakfast due to dawn phenomenon and higher ratios (less insulin) at lunch and dinner. Separate ratios for each meal can be programmed into insulin pumps or defined in multiple daily injection regimens.

Question 4

How do I estimate a starting ratio for someone not yet on insulin?

Accepted Answer

The ratio is only meaningful once the total daily dose is established. For new starter patients, prescribe initial fixed prandial doses and refine the ratio over 2–4 weeks based on glucose records.