I have found, in my years of practicing correctional medicine, that few practitioners who come to corrections are comfortable with insulin dosing. In my experience, this is especially true for physician assistants and nurse practitioners, but many physicians have problems, too. Insulin dosing can be complicated and tricky at times, but for most patients, 10 simple rules will get you to where you need to be.
We first need to cover some groundwork and some terms. Insulin terminology can be confusing. First, it is very important to remember that this discussion applies to type 1 diabetics only. Type 2 diabetics sometimes use insulin, but that’s a “whole ‘nother ballgame.”
There are two types of insulin used for two very different purposes when treating type 1 diabetics. The first is basal insulin, which is used to replace the insulin that the normal pancreas releases constantly—whether we eat or not. Long-acting insulin is used to provide coverage for the basal metabolic needs of type 1 diabetics. Examples are insulin glargine (Lantus) and insulin detemir (Levemir). The most commonly used long-acting insulin is Lantus, so I am going to use that name in this article. (I have no financial ties to the maker of Lantus—I use that name because it is the name most commonly used by patients).
The second type of insulin that type 1 diabetics need is short-acting insulin, which is given to cover the carbohydrates in the food they eat. Short-acting insulins are given just before a meal or snack and, ideally, the dose should vary depending on how many carbohydrates are in the food. Examples of short acting insulins are insulin regular, insulin aspart (Novolog) and insulin lispro (Humalog). Again, I will use the term Humalog in this article because it is the term most often used by patients themselves. And again, I have no ties to the maker of Humalog.
Finally, I must point out that there are other insulin dosing systems besides the Lantus-Humalog system I am presenting here. For example, many patients now use insulin pumps, and some still use the older NPH-Humulin system.
10 Rules of Insulin
We are going to apply our rules to two imaginary patients. “Jeffrey” has been newly diagnosed as being a type 1 diabetic. “Ernest” has been taking insulin since he was a child. Both have come to our jail and need insulin orders written. By going through the rules of insulin, we can quickly and easily calculate appropriate insulin orders for these two patients.
Rule No. 1
The basic unit of insulin dosing is the total daily dose, abbreviated TDD. If you count up every unit of insulin that a patient takes in one day, that is that patient’s TDD. Example: Yesterday, Ernest took 33 units of Lantus insulin and 57 units of Humalog. His TDD was 90.
Rule No. 2
The TDD for most patients should be approximately 0.5-1.0 unit per kilogram of weight. Ernest weighs 100 kg. Multiply this by 0.5 to1.0 units and you get a range of 50 units to 100 units of insulin a day. Ernest’s TDD is 90, which is about right, though at the high end of the range.
Jeffrey is a newly diagnosed type 1 diabetic. What should his beginning insulin dosage be? Generally, you should begin dosing at the low end of the range and gradually move up. Jeffrey weighs 80 kg and we want to start at around 0.5 units per kilogram, which equals a TDD of 40 units per day.
Rule No. 3
Approximately half of each patient’s TDD should be Lantus and half Humalog. We calculated Jeffrey’s initial TDD at 40 units, so half of this, 20 units, should be given as Lantus and the other 20 units will be given as Humalog. Lantus normally is dosed once a day.
Notice that Ernest is taking too little Lantus. He is taking 33 units of Lantus per day, but since his TDD is 90, his dose should be around 45. That change should reduce his need for Humalog, which should fall to around another 45 units.
Rule No. 4
Humalog covers what the patient eats. It should be split between the meals the patient eats during the day. If a patient eats six equal meals, the Humalog units should be split by six. However, since inmates generally eat only three meals a day (and assuming that the three meals contain approximately the same number of carbohydrate grams), the Humalog should be split into thirds.
We have calculated that Ernest’s daily Humalog dose is 45 units. Divide that by 3 and Ernest should get approximately 15 units before each jail meal.
We estimated Jeffrey’s initial daily Humalog dose to be 20 units. Let’s
add one to that (these are estimates, after all, so let’s make things easy) to get 21 units. Divide that by three and Jeffrey will get 7 units before each meal. By the way, it is OK to unbalance the 50-50 ratio a little to make dosing work. When we do, the patient should usually take a little more Humalog.
So, to summarize, the initial orders for Jeffrey will be Lantus 20 units daily and Humalog 7 units before each meal. The initial orders for Ernest will be Lantus 45 units daily and Humalog 15 units before each meal.
Rule No. 5
We can use the TDD and the “Rule of 500” to calculate how many grams of carbohydrate each unit of Humalog will cover. 500 divided by the TDD equals the number of carbs covered by one unit of Humalog. Jeffrey has a TDD of 41. 500 divided by 41 equals about 12. Each unit of Humalog will cover approximately 12 grams of carbohydrates. For Ernest, 500 divided by 90 (his TDD) equals about 6. Each unit of Humalog will cover approximately 6 grams of carbs. This becomes important if, as sometimes is done, these patients are given a diabetic snack at bedtime. Snacks should be covered by Humalog! So if the diabetic snack is, say, a peanut butter sandwich containing 24 grams of carbohydrates, Jeffrey should get 2 units of Humalog to cover the snack and Ernest should get 4 units.
Rule No. 6
Let’s say that we check Jeffrey’s blood sugar before lunch and discover that it is a whopping 500. Jeffrey is already getting 7 units of Humalog to cover his meal. How much extra should he get to bring his blood sugar down to normal? The “Rule of 1800” will solve this problem. Here, 1800 divided by the TDD tells you how far the patient’s blood sugar will drop with each unit of Humalog. In Jeffrey’s case, 1800 divided by 41 equals about 44. Since each unit of extra Humalog will be expected to drop his blood sugar by around 44, if we want to drop him from 500 to 100, we will need to give him an extra 9 units of Humalog along with his regular dose of 7 units. So we can give him 16 units of Humalog before lunch and expect his blood sugar to fall to around 100 after lunch.
In Ernest’s case, 1800 divided by his TDD of 90 equals 20. So his blood sugar will only drop by around 20 points with each extra unit of Humalog he is given. To drop his blood sugar from 500 to 100, he will need 20 extra units of Humalog. Add these to the 15 units we are giving him to cover lunch, and the total is 35 units of Humalog.
The rule of 1800 can be used to create a patient-specific sliding scale so that high blood sugar levels are addressed each time they arise.
Rule No. 7
This rule tells us how fast to adjust a patient’s insulin dose. Let’s say Jeffrey’s blood sugars are consistently in the 300s. His insulin dosage needs to be increased. This rule says to increase the dosage by no more than 5% to 10% every two to three days until you reach your goal. You must remember to keep the (approximate) 50-50 balance between Lantus and Humalog. Since Jeffrey was taking a TDD of 41 units, 5% to 10% of that is 2 to 4 units. One way to add another 4 units to Jeffrey’s TDD would be to increase his Lantus dose to 21 units a day and increase his Humalog to 8 units each meal. If he is still running too high in 2 to 3 days, we can increase his Lantus by 3 units to bring him back to perfect 50-50 balance (24 units of Lantus, 24 units of Humalog divided between three meals).
If you have been using a sliding scale, another way to make insulin dose adjustments is to add all of the sliding scale doses the patient has been given to the previous TDD and then recalculate the 50-50 Lantus-Humalog split based on the new TDD. For example, Ernest was taking a TDD of 90 units. However, he also received an average of 10 units of sliding scale insulin every day. His new TDD is 90 plus 10 for a total of 100 units. Divide this in half and his new Lantus order is 50 units and his new Humalog dose is 50 units divided between three meals. Notice that this is a much more aggressive increase of insulin dose. Be sure to consider the next rule, No. Eight, each and every time before you make a large insulin dose adjustment.