About This Calculator

The Insulin Pen Days Supply calculator is a clinical support tool designed for pharmacists, pharmacy technicians, and healthcare providers. It helps determine the precise duration an insulin pen or a box of pens will last for a patient based on their prescribed dosing regimen, accounting for factors like insulin concentration and priming waste.

Understanding the Outputs

The calculator provides a detailed breakdown to ensure transparency and aid in prescription verification:

• Total Units per Pen/Box: The total volume of medication available, calculated by multiplying the pen’s volume (in mL) by its concentration (in units/mL).
• Total Daily Insulin Use: This is the sum of the patient’s prescribed therapeutic dose and the units used for priming the needle before each injection. Accurately accounting for this “waste” is crucial for an exact days’ supply calculation.
• Days Supply (Per Pen / Per Box): The final estimated supply duration. This value is calculated by dividing the total available units by the total daily insulin use. Pharmacies often round this number down to the nearest whole day.

How to Use This Tool

To ensure an accurate calculation, enter the following information based on the patient’s prescription and the specific product being dispensed:

• Insulin Pen: Select the patient’s prescribed insulin pen from the dropdown menu. This will auto-populate the pen’s concentration and volume. If the product is not listed, choose “Custom / Unlisted Pen” and enter the values manually.
• Number of Pens per Box: Enter the quantity of pens in the dispensed package, which is typically 5.
• Pen Concentration (units/mL): This indicates the strength of the insulin (e.g., U-100 is 100 units/mL).
• Volume per Pen (mL): The total volume of liquid in a single pen (e.g., 3 mL).
• Total Daily Dose (units): The total number of units the patient is prescribed to inject per day. For variable or sliding-scale doses, enter the average expected daily dose.
• Number of Injections per Day: The total number of times the patient administers an injection each day.
• Priming Dose: The number of units expelled to clear air from the needle before each injection. This is typically 2 units but can vary. Always confirm this value by consulting the manufacturer’s Instructions For Use (IFU) for the specific pen device.

Dosing Overview

Insulin dosing is highly individualized and must be managed by a qualified healthcare professional. Doses are adjusted based on blood glucose levels, diet, activity, and other clinical factors. This tool does not recommend or validate a dose; it only calculates a days’ supply based on the dose entered. Insulin types vary, from long-acting (basal) insulins that provide background coverage to rapid-acting (bolus) insulins taken with meals.

Switching Between Insulin Products

Switching a patient from one insulin product to another, especially between different concentrations (e.g., U-100 to U-200), requires careful medical supervision. Dose adjustments are often necessary, and these changes should only be made by the prescribing physician. This calculator can help model the impact of such a change on the days’ supply, but it does not provide clinical guidance on dose conversion.

Guidance for a Missed Dose

This calculator assumes perfect adherence to the prescribed dosing schedule. It does not account for missed, skipped, or extra doses. Patients who miss a dose of insulin should be advised to follow the specific instructions provided by their doctor or pharmacist, as the correct action can vary depending on the type of insulin and the patient’s clinical situation.

Safety Alerts

• Professional Verification Required: All calculations performed with this tool must be independently verified by a licensed pharmacist or healthcare professional before being used for dispensing or clinical decision-making.
• Not Medical Advice: This tool is for informational purposes only and is not a substitute for professional medical or pharmaceutical judgment.
• In-Use Stability (Beyond-Use Date): Once opened or removed from refrigeration, insulin pens have a limited stability period at room temperature (e.g., 28, 42, or 56 days). The true days’ supply for a single pen cannot exceed this manufacturer-specified in-use stability period, even if there is insulin remaining.
• Importance of Priming: Consistently priming the needle before each injection is critical for ensuring the patient receives the correct dose. Failure to do so can lead to under-dosing and poor glycemic control.

Frequently Asked Questions (FAQ)

How should I calculate the supply for a patient on a sliding scale?

For sliding scale or variable dosing, calculate the patient’s average total daily dose based on their blood sugar logs or typical usage patterns and enter that value into the “Total Daily Dose” field.

Why is the priming dose so important for the calculation?

The priming dose accounts for insulin that is used but does not contribute to the therapeutic effect. Although small, this amount adds up over a month and ignoring it can lead to an overestimation of the days’ supply and potential insurance rejections for early refills.

What does insulin concentration (e.g., U-100, U-200) mean?

Concentration refers to the strength of the insulin. U-100 means there are 100 units of insulin in every 1 milliliter (mL) of fluid. A U-200 insulin is twice as concentrated, containing 200 units per mL.

Can I use this calculator for insulin vials and syringes?

This calculator is specifically designed for insulin pens, as it accounts for priming waste associated with pen needles. Vials and syringes have different waste considerations (e.g., hub loss) that are not factored into this tool.

The calculator shows a 40-day supply, but the pen expires 28 days after opening. What is the correct supply?

The correct days’ supply for a single pen is limited by its in-use stability. In this case, one pen would last for a maximum of 28 days. The total supply for a box of pens would need to be calculated based on this limitation.

What should I do if my patient’s insulin pen is not in the dropdown list?

Select the “Custom / Unlisted Pen” option. You can then manually enter the pen’s concentration and volume, which can be found on the product’s packaging or in its prescribing information.

Does the calculation account for the small amount of overfill in pens?

No, the calculation is based on the labeled volume of the pen (e.g., 3 mL). It does not account for any manufacturing overfill, which should not be relied upon for dosing.

Why might my pharmacy software give a slightly different result?

Pharmacy dispensing software may have different default settings or rounding rules. This tool provides a transparent, step-by-step calculation that can be used to verify software outputs or perform manual checks.

References

• U.S. Food and Drug Administration. Drugs@FDA: FDA-Approved Drugs. Accessed at https://www.accessdata.fda.gov/scripts/cder/daf/
• American Diabetes Association. Insulin Basics. Accessed at https://diabetes.org/healthy-living/medication-treatments/insulin-other-injectables/insulin-basics
• LANTUS® (insulin glargine) injection, for subcutaneous use. Prescribing Information. Sanofi-Aventis U.S. LLC. Accessed via FDA@Drugs.
• Jorgensen C, Seger P, Wallin B. Priming of insulin pens: a new aspect of injection technique. *Curr Med Res Opin*. 2011;27(1):109-113. doi:10.1185/03007995.2010.536838