Ask The Expert
Insulin

1500, 1800, and 500 Rules

NO, You are mistaken on your understanding of the 1500 rule.

1500 Rule, also known as the Correction Factor or Insulin Sensitivity Factor

The 1500 Rule was developed by Dr. Paul C. Davidson, Medical Director of the Diabetes Treatment Center in Atlanta, who based it on his experience managing people with diabetes. The 1500 Rule is defined as the number of mg/dL the blood glucose level will drop after the administration of one unit of regular insulin. In other words: when 1500 is divided by your total daily dose of insulin, the resulting number tells you approximately how far a blood glucose reading will be lowered by 1 unit of regular insulin.

1700, 1800, and 2000 Rules: According to John Walsh, P.A., C.D.E., in his book Pumping Insulin (4th edition), when using rapid-acting insulin, he gives examples of using 1800, 2000, or 2200 in the formula, since it depends on the proportion of basal dose to bolus dose. In addition, other experts have suggested 1700 as a compromise. The point here is that the smaller number (such as in the 1500 rule) gives a larger bolus than would a higher number (such as the 1800 rule) which would give a smaller bolus. Thus, less insulin is needed to bring your blood glucose down when using rapid-acting insulin rather than regular insulin. Most importantly, this formula should be individually determined by your healthcare team.

To illustrate the above definitions:

If you use regular insulin (which is termed a short-acting insulin): If your total daily dose of insulin were 30, then 1500 divided by 30 = 50, meaning that 1 unit of regular insulin would drop your blood glucose approximately 50 mg/dL.

When you use a rapid-acting insulin (such as Humalog®, NovoLog®, or Apidra®), and if your healthcare team gave you the formula of 1800: then if your total daily dose of insulin were 30, then 1800 divided by 30 = 60, meaning that 1 unit of rapid-acting insulin would drop your blood glucose approximately 60 mg/dL.

A word of caution: each person is different, and there are many more details/rules to adjusting insulin that I haven't explained --for example, the unused insulin rule cautions you not to overdo giving yourself extra insulin, when you might still have insulin "on board" from the previous dose. This could result in an overlap of insulin doses.

Therefore, Pumping Insulin advises, and I would agree, that you do not use the above rules to lower your blood glucose:

• Until you first discuss this with your physician;
• If your high blood sugars tend to come down on their own;
• If you have frequent or severe low blood sugars;
• When pending exercise or activity will lower your blood glucose.

500 Rule for estimating the insulin-to-carbohydrate ratio:

This is a rule used by some people only as a starting point-and its purpose is to help you determine how many grams of carbohydrate are covered by one unit of rapid-acting insulin. This rule states: divide the number 500 by your total daily dose of insulin, to find out how many carbs are covered by one unit of rapid-acting insulin. (Correspondingly, one would use the number 450, divided by the total daily dose of insulin, if using regular insulin).

NOTE: In Pumping Insulin, the author also discusses using other numbers in the formula, such as 550 or 600, for rapid-acting insulin, in some cases, since it depends again somewhat on your proportion of basal dose to bolus dose. Keep in mind that a smaller number, such as 450, gives larger carb boluses, while using 600 gives smaller, safer carb boluses.

Example of the 500 rule with Humalog®: Using the previous example of a total daily dose of 30 units, then 500 divided by 30 = 16.6; since carb counting is never that precise, you would round this number off to 17, meaning that 1 unit of Humalog® covers approximately 17 grams of carbohydrate.

Example of the 450 rule using regular: With the same example, 450 divided by 30 = 15, meaning that 1 unit of regular insulin covers approximately 15 grams of carbohydrate.

Caution when using the carb factor rules:

Many factors can affect your insulin to carbohydrate ratio: the presence of insulin resistance, how much long-acting insulin or basal dose you use, your body weight, your level of activity, the fiber and fat content in your diet, as well as many other considerations.

Keep good records: Often, dietitians/diabetes educators prefer to start by having you keep a detailed food record in which you attempt to eat fairly consistently; then you test before and 2- hours after meals, carefully recording the carbohydrate content of the meals, the insulin doses, and the pre-meal and post-meal glucose readings.

This data would enable your diabetes team to retrospectively calculate your current regimen, to determine your current insulin-to-carbohydrate ratio; then, finally, you and your healthcare team could begin to adjust the ratio upward or downward, according to your individual needs.

Conclusion:

As you can see, there are many tools available to assist you in achieving good diabetes control, but there are also many individual factors that must be considered. I would encourage you to meet with your healthcare team to learn how you can incorporate these tools into your daily regimen.