About This Calculator

The IVIG → SCIG Conversion calculator estimates an equivalent subcutaneous immunoglobulin (SCIG) dose for a patient who is stable on an intravenous immunoglobulin (IVIG) regimen. The primary goal of conversion is to maintain similar systemic IgG exposure and clinical efficacy while offering the convenience of subcutaneous administration. This tool uses a standard dose adjustment factor to account for differences in bioavailability between the two routes of administration.

Outputs Explained

After entering the required patient and product data, the calculator provides the following key outputs:

• Total Weekly SCIG Dose (g): This is the foundational dose, calculated by converting the patient’s IVIG dose to a weekly equivalent and then multiplying by the Dose Adjustment Factor (DAF).
• Dosing Options Table: The tool presents a table with various administration schedules (e.g., weekly, bi-weekly, frequent) to help tailor the therapy to the patient’s lifestyle and tolerability. For each schedule, it calculates:
• Dose (g): The amount of immunoglobulin in grams for each infusion.
• Volume (mL): The total liquid volume to be infused, based on the dose and SCIG product concentration.
• Min. Sites: The minimum number of injection sites required, calculated by dividing the total volume by the maximum volume allowed per site.

How to Use the Calculator

To ensure an accurate calculation, follow these steps and input the correct information into each field:

1. Total IVIG Dose (g): Enter the total grams of IVIG the patient receives per infusion.
2. IVIG Dosing Interval (weeks): Select the number of weeks between the patient’s IVIG infusions (e.g., 3 or 4 weeks).
3. Dose Adjustment Factor (DAF): The default is 1.37, which is standard in clinical practice. This can be adjusted based on specific product guidelines or clinical judgment.
4. SCIG Product Concentration (%): Select the concentration of the target SCIG product (e.g., 10% or 20%). A higher concentration results in a lower infusion volume for the same dose.
5. Max Volume per Site (mL): Enter the maximum volume that can be administered at a single injection site. This is product-specific and should be confirmed in the prescribing information.

Dosing Overview

Switching from IVIG to SCIG requires a dose adjustment to maintain equivalent serum IgG trough levels. Due to lower bioavailability, SCIG doses are typically higher than the equivalent IVIG dose. A Dose Adjustment Factor (DAF) of 1.37 is commonly used, meaning the total monthly SCIG dose will be approximately 137% of the total monthly IVIG dose. This factor is derived from pharmacokinetic studies comparing IVIG and SCIG formulations. The final dose may require titration based on clinical response and monitoring of IgG levels.

Switching and Administration

The transition from IVIG to SCIG should be managed carefully. The first SCIG dose is typically administered one week after the patient’s last IVIG infusion. For patients new to subcutaneous administration, a gradual dose ramp-up over several weeks is often recommended to enhance tolerability and minimize local site reactions. The total weekly dose can be divided into more frequent infusions (e.g., twice weekly or daily) to reduce the volume per injection site. Patients and caregivers must be trained on proper aseptic technique for preparing and administering the infusion.

Missed Dose Protocol

If a patient misses a scheduled SCIG dose, they should be advised to administer the dose as soon as they remember and then resume their normal dosing schedule. They should not infuse a double dose to make up for a missed one. If several days have passed or if they are unsure what to do, they should contact their healthcare provider for guidance.

Safety Alerts

Immunoglobulin products, including SCIG, carry potential risks. Healthcare professionals should be aware of the following:

• Local Site Reactions: Swelling, redness, itching, and discomfort at the injection site are common but usually mild and transient.
• Systemic Reactions: Though less common with SCIG than IVIG, systemic reactions like headache, fever, chills, and fatigue can occur.
• Thrombosis: Thromboembolic events have been reported with IG products. Patients with known risk factors for thrombosis should be monitored closely.
• Aseptic Meningitis Syndrome (AMS): AMS can occur, typically within several hours to 2 days following IG treatment.
• Renal Dysfunction: Acute renal dysfunction and failure have been reported, particularly with IG products containing sucrose.

Frequently Asked Questions (FAQ)

Why is the calculated SCIG dose higher than the IVIG dose?

The SCIG dose is higher to compensate for its lower bioavailability compared to IVIG. Not all of the subcutaneously administered immunoglobulin reaches the bloodstream. The Dose Adjustment Factor (typically 1.37) ensures that the patient achieves similar systemic IgG exposure and trough levels as they had on their stable IVIG regimen.

What is the Dose Adjustment Factor (DAF) and can I change it?

The DAF is a multiplier used to convert the IVIG dose to an equivalent SCIG dose. The default value of 1.37 is based on clinical trials. While the calculator allows you to change this value, any modification should be based on official prescribing information for a specific SCIG product or established institutional guidelines.

How is the minimum number of injection sites determined?

The calculator determines the minimum number of sites by dividing the total infusion volume (mL) by the “Max Volume per Site (mL)” you entered. The result is always rounded up to the next whole number (ceiling function) to ensure no single site receives more than the maximum allowed volume.

How do I choose the best SCIG infusion frequency?

The choice of frequency (e.g., weekly, bi-weekly, daily) depends on several factors, including patient preference, the total volume to be infused, and tolerability. More frequent dosing involves smaller volumes per infusion, which may improve comfort and reduce local site reactions.

What happens if I use a custom SCIG concentration?

If you select “Custom” and enter a different SCIG concentration, the calculator will adjust the total infusion volume accordingly. A lower concentration will result in a larger volume for the same gram dose, which may increase the number of required injection sites.

Is a dose ramp-up required when starting SCIG?

A dose ramp-up is not always required but is strongly recommended for many patients, especially those new to SCIG. A gradual increase to the target dose over the first few infusions can significantly improve tolerability and reduce the incidence and severity of local site reactions. Always consult the product’s prescribing information for specific ramp-up schedules.

Does this calculator work for converting from SCIG to IVIG?

No, this tool is designed exclusively for one-way conversion from a stable IVIG dose to an initial SCIG dose. Converting from SCIG to IVIG requires different clinical considerations and calculations.

What should I do if the calculated infusion volume is very large?

If the volume per infusion is large, consider either increasing the number of simultaneous injection sites (if the patient is able to manage them), or selecting a more frequent dosing schedule (e.g., changing from weekly to twice-weekly) to divide the total weekly volume into smaller, more manageable infusions.

References

• FDA. Immune Globulin Products. U.S. Food & Drug Administration.
• EMA. Human normal immunoglobulins for subcutaneous use. European Medicines Agency.
• Wasserman RL, Melamed I, Stein MR, et al. Recombinant human hyaluronidase-facilitated subcutaneous infusion of human immunoglobulins for primary immunodeficiency. J Allergy Clin Immunol. 2012;130(4):951-957.e11.
• Hizentra® (Immune Globulin Subcutaneous [Human]) 20% Liquid. Full Prescribing Information. CSL Behring.
• Berger M. Principles of and advances in immunoglobulin replacement therapy for primary immunodeficiency. Immunol Allergy Clin North Am. 2011;31(2):265-281.