Carbohydrate ratio-calculating and testing made easy

But let’s start again, from the top. If you’re on a course of intensive insulin therapy you’ll normally inject regular bolus insulin or rapid-acting bolus insulin before meals and additional long-acting basal insulin once or twice a day. The bolus insulin includes the meal insulin and the corrective insulin. For people who calculate carbohydrates, the carbohydrate ratio or the carbohydrate-to-insulin ratio is determined by the doctor at the start of the insulin therapy.

What’s the carbohydrate ratio/carbohydrate-to-insulin ratio?

👆 It indicates how much insulin is required to restore blood glucose levels to the target range after consuming carbohydrates.

If carbohydrates are calculated in grams, the carb ratio indicates the amount of carbohydrates (in grams) covered by one unit of bolus insulin. So you might ask:
“How many g of carbohydrates can be eaten for one unit of meal insulin?”

Or, if you calculate carbohydrates in carbohydrate exchanges (e.g.15g), it indicates how many units of insulin are needed to “cover” one exchange. So you might ask:
“How many insulin units do you need for one carbohydrate exchange?”

Many people use the same carbohydrate-to-insulin ratio throughout the day. But since insulin effects fluctuate for each person depending on the time of day it can be helpful to use ratios that vary depending on the time of day as well.

If you adjust the carb ratio to your daily rhythm, the insulin requirement for the carbohydrates will look like this for many adults:¹

These values may differ from one individual to another, of course, depending on how sensitive they are to the insulin effect or what their insulin resistance levels are.
You can use a formula to get a rough initial idea of how much one unit of insulin can lower the blood glucose level in adults. But each individual needs to check this with their diabetes team. If necessary it can also be adjusted to reflect how the insulin requirement varies at different times of day based on tests and historical values.

The ratios may change over time:

It’s important to remember that the insulin requirement doesn’t always stay the same. The bolus insulin requirement as well as the basal insulin requirement can change over time due to fluctuations in body weight or hormonal changes.

👆 So it’s a good idea to check the carb ratio if

• glucose values are regularly too high or too low BEFORE a meal The optimal target range here is 80–130 mg/dL (4.4–7.2 mmol/L).³

or

• glucose values are regularly too high or too low 2–3 hours AFTER a meal. In other words, blood glucose values should ideally be below 180 mg/dL (10 mmol/L) 1–2 hours after starting a meal.³

👆 But before you take a closer look at the carbohydrate ratio, it’s essential to check the following points to see if they might be causing the glucose outliers:

• Before testing the carbohydrate ratios, always start by checking whether the amount of basal insulin is sufficient to keep values stable within the target range without meals as well. The following article contains instructions on how to test for this.
• If your glucose values are sometimes within the target range after a meal and sometimes not, this might have something to do with the estimated carbohydrate amount. Check whether any estimation or calculation errors might have crept in.
• Are you already using an injection-meal interval? If not, ask your diabetes team whether this could help you to scale down peaks in your blood glucose values after meals. 15–20 minute injection-meal intervals are often the quickest way of optimizing blood glucose values, even with rapid-acting insulin analogs.⁴
• Or are there any hardened areas (“lipohypertrophy”) at your preferred injection sites? Do you often inject into these hardened sites, which extends the time needed for the insulin to be absorbed and take effect?

The carbohydrate ratio test:

Before the test:

• baseline glucose value is between 90–140 mg/dL (4–7.8 mmol/L)⁴
• The test should only be conducted if the glucose trend is stable. If you’re wearing a CGM you can check this by looking to see whether the trend arrow is horizontal.
• Do not administer insulin
  • for 3–4 hours before the test if you are using rapid-acting insulin analogs
  • for 5–6 hours before the test if you are using regular insulin⁵
• It should be 3 hours since your last meal, which should be easily digestible and not too high in fat content.⁴
• No values < 70 mg/dL (4 mmol/L) in the last 6 hours
• No unusual physical activity before or during the test
• No alcohol in the last 12 hours.
• Do not conduct the test if you are under severe stress or have a febrile infection.

Conducting the test:

• A meal that consists mainly of carbohydrates but is low in fat and protein is required for the test. A test meal might contain 60 g carbohydrates (5 BE/6 KE). This amount of carbohydrate would be contained, for example, in 85 g uncooked durum wheat pasta, 77 g uncooked rice or 122 g of white bread. Weigh the precise amounts for the test meal.
• The insulin for the test meal is calculated using the current carbohydrate ratio and should be injected 20 minutes (30 minutes if regular insulin is used) before the test meal.⁴
• The glucose values should be checked hourly up to 5 hours after starting the meal. If you are wearing a CGM, you can simply evaluate the glucose curve.
• If hypoglycemia of < 70 mg/dL (3.9 mmol/L) occurs it is essential to discontinue the test and take grape sugar!

Evaluating the test:

The carbohydrate ratio is correct if

• the glucose level remains below 180 mg/dL (10 mmol/L) after eating.
• the glucose value does not differ from the baseline preprandial glucose value by more than 30 mg/dL (1.7 mmol/L) after several hours, or before the next meal.⁴
•   (If rapid-acting insulin analog is used, after approx. 3 h, if regular insulin is used, after approx. 5 h)

If the glucose values are more than 30 mg/dL (1.7 mmol/L) below baseline after several hours, the carbohydrate ratio is too high; if they rise by more than 30 mg/dL (1.7 mmol/L) it is too low.

It is best to correct the ratios in small increments (10–20 %). You can do this yourself as well by agreement with your diabetes team. But if you’re unsure, it’s essential to discuss it with your diabetes team in advance.

It’s worth the effort!

It takes time to check the carb ratio.  But it’s worth the effort as the insulin requirement can change over time and using the wrong carb ratio is one of the main reasons for abnormal glucose values after meals.
Always test the new carb ratio for a few days and check whether the glucose stays below 180 mg/dL (10 mmol/L) after meals and whether the glucose is at the pre-prandial level before the next main meal.

The carb ratio is also an important parameter that you need to program a bolus calculator. A bolus calculator can provide useful support in your everyday life, particularly when using a changing carb ratio!

Sources:

[1] Thurm, Ulrike, Gehr, Bernhard (2020) CGM- und Insulinpumpenfiebel, (4.edition, Mainz
[2] Davidson PC, Hebblewhite HR, Steed RD, Bode BW. Analysis of guidelines for basal-bolus insulin dosing: basal insulin, correction factor, and carbohydrate-to-insulin ratio. Endocr Pract. 2008 Dec;14(9):1095-101. doi: 10.4158/EP.14.9.1095. PMID: 19158048.
[3] American Diabetes Association, 6. Glycemic Targets: Standards of Care in Diabetes—2023. Diabetes Care 1 January 2023; 46 (Supplement_1): S97–S110. https://doi.org/10.2337/dc23-S006
[4] Walsh, John et.al. (2017). Pumping Insulin, everything for success on a pump and CGM. (6.edition), San Diego
[5] SUBITO Schulungsprogramm (Schulungsprogramm für die Insulinpumpentherapie)