29 Immunotherapy: Subcutaneous Immunotherapy
29.1 The Basics of Subcutaneous Immunotherapy
Once the decision is made to treat and the relevant positive allergy tests (in vitro and/or skin tests) are evaluated and correlated with the patient’s symptomatology, one or more treatment vials are created according to a prescription or “recipe” the provider creates (see Chapter 45 on candidate selection for further discussion on selecting candidates for subcutaneous immunotherapy [SCIT]). During escalation and maintenance, usually 5 mL multiallergen, mutidose vials are mixed, each of which contains 10 doses of 0.50 mL volume.
If 10 mL vials are used, then there’s generally 10 doses of 1.0 mL volume. This chapter will focus on 5-mL vial creation.
29.2 To Endpoint or Not to Endpoint
A vial prescription or “recipe” can be created using an “endpoint” to determine a starting dilution for treatment. See Chapter 16 on the definition of an endpoint and how it’s determined. If using an endpoint, the initial treatment vial begins at a treatment dose for each antigen equal to 0.05 mL of the endpoint dilution and escalates to a maximum dose of 0.50 mL.
If you are not using an endpoint to determine a starting point for treatment, then generally the weakest dilution prepared is used. In some practices, all antigens may start at a #6 dilution, in other practices it may be a #4 dilution. If you are just beginning to add allergy to your practice, using the weakest dilution (#6) is recommended until more experience is gained.
Volume reduction is accomplished by using dilutions that are 25 times more concentrated than the endpoint, commonly referred to as “two dilutions to the right.” To prepare 10 doses would require 10 × 0.50 mL, or 5 mL of the endpoint dilution for the maximum dose for each antigen. This is equal to 1 mL of the next more concentrated dilution, or 0.20 mL of the solution that is two dilutions more concentrated. Therefore, a treatment vial for a patient is created by adding 0.20 mL for each antigen, taken for a dilution that is 25 times more concentrated than the endpoint dilution, and the total volume is adjusted to equal 5 mL by adding appropriate amounts of glycerin and phenolated normal saline (PNS) diluent.
29.3 Can You Say That Again?
If the endpoint for cat was #6 and the practitioner adds cat to the treatment vial, he/she should add 0.20 mL of #4 cat to the vial. Add 0.20 mL two dilutions “to the right” (stronger) of each additional allergen that you want in the vial then add enough diluent to bring the volume up to 5 mL (▶Table 29.1).
29.4 A Few Words About Preservatives
There are several factors that can influence the loss of potency over time. Storage temperature is important. Patient vials and practitioner’s “concentrates” should be stored between 2 and 8 °C. Stabilizers and bactericidal agents can also influence potency. There are several additives that can be used in patient vials; these include phenol, human serum albumin (HSA), and glycerin. There are few studies on potency. Therefore, recommendations regarding which preservative to use and expiration dates are not strongly evidence-based.
If PNS is used as a diluent, the potency of the extracts in that vial will last about 6 to 8 weeks even if refrigerated. However, if a 10% glycerin concentration is added, the potency will last about 12 weeks. A 10% glycerin concentration in a patient vial can be achieved by adding 1 mL of 50% glycerin to the vial.
Antigen | Endpoint | Volume | “Two dilutions to the right” |
Cat | #6 | 0.20 mL | #4 |
Dog | #4 | 0.20 mL | #2 |
Oak | #2 | 0.20 mL | Conc |
Ragweed | #6 | 0.20 mL | #4 |
Add diluent | 4.2 mL | ||
To make total | 5.0 mL | ||
1 mL of 50% glycerin added to 4 mL of PNS equals 10% glycerin concentration achieved in the vial.
Human serum albumin also makes a great preservative choice. Some studies have shown evidence that it is a superior allergen extract stabilizer, especially with very dilute extracts. It also inhibits protein aggregation.
29.6 Serious Stuff: Vial Mixing
Any type of allergy vial mixing, whether it is for preparation of a testing/treatment board or for patient vials for sublingual immunotherapy (SLIT) or SCIT therapy or even for oral mucosal immunotherapy (OMIT) toothpaste preparation, falls under United States Pharmacopeia (USP) 797 guidelines that contain the standards for preparing compounded sterile drugs. Please see Chapter 43 for further discussion on these requirements for space, sterility measures, compounding staff training and responsibilities, and environmental monitoring.
29.7 Tools of the Trade (What All Are Needed)
Antigens (“concentrates”) from the manufacturer.
Sterile empty 5 mL vials to be used when preparing the individual patient vials.
23 gauge needles.
Diluent.
The practitioner may choose to use PNS.
The practitioner may choose to use HSA.
50% glycerin (unless HSA is being used as diluent/preservative).
Labels for the patient vials.
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