Immunoglobulin therapy can be delivered into a vein or under the skin. Intravenous immunoglobulin (IVIG) and subcutaneous immunoglobulin (SCIG) draw on the same class of plasma-derived product, but they differ in infusion frequency, infusion time, the shape of the immunoglobulin G (IgG) curve in the blood, the adverse-effect pattern, and the scope of the FDA boxed warning. This article sets out those differences as the FDA labels, the Immune Deficiency Foundation and the published pharmacokinetic literature describe them.
What is the difference between IVIG and SCIG?
IVIG is infused into a vein, usually every three to four weeks. Conventional SCIG is infused into the fatty tissue under the skin, usually weekly or every other week, in smaller volumes and often across more than one infusion site. Facilitated subcutaneous immunoglobulin (fSCIG) pairs immunoglobulin with recombinant human hyaluronidase, which allows a larger volume under the skin in a single session and a dosing interval of every three to four weeks.
IVIG and SCIG are routes of administration, not brands. Gammagard Liquid, Gamunex-C and Gammaked are labeled for both intravenous and subcutaneous use; Hizentra is a 20% subcutaneous immunoglobulin; HyQvia is the facilitated subcutaneous product. The Immune Deficiency Foundation notes that subcutaneous immunoglobulin "does not require access to veins," which is the most concrete physical difference between the two routes. Our IVIG treatment page describes the intravenous route in more detail.
How do IVIG, conventional SCIG and facilitated SCIG compare?
The Immune Deficiency Foundation publishes the clearest patient-facing comparison of the three delivery methods. According to the Immune Deficiency Foundation, an IVIG infusion typically takes between 2 and 6 hours, a conventional subcutaneous infusion typically takes 1 to 2 hours, and a facilitated subcutaneous infusion generally takes 1 to 3 hours.
| Feature | IVIG | Conventional SCIG | Facilitated SCIG (fSCIG) |
|---|---|---|---|
| Vein access required | Yes, a single site directly into a vein | No | No |
| Frequency | Every 3-4 weeks | Weekly to biweekly | Every 3-4 weeks |
| Typical duration (Immune Deficiency Foundation) | 2-6 hours | 1-2 hours | 1-3 hours |
| Setting | Infusion center, hospital, physician office, or home with nursing support | Usually self-administered at home | Home or clinic |
| IgG level pattern | Peaks and troughs | Steady level, without marked peaks or troughs | Peaks and troughs, less pronounced than IVIG |
| Adverse-effect pattern | Predominantly systemic | Predominantly local infusion-site reactions | Not separately characterized by the Immune Deficiency Foundation |
| Premedication | Often used | Usually not needed | Not separately characterized |
| Boxed warning scope | Thrombosis plus renal dysfunction and acute renal failure | Thrombosis only | Thrombosis only |
Setting follows the route rather than a preference: subcutaneous infusions are usually given by the patient or a caregiver, while IVIG is given in an infusion center, a hospital, a physician office, or at home with nursing support. Prime Infusions covers the practicalities of the latter separately under home infusion services.
Why does IVIG carry a boxed warning that subcutaneous immunoglobulin does not?
Every IVIG product carries an FDA boxed warning headed "WARNING: THROMBOSIS, RENAL DYSFUNCTION, and ACUTE RENAL FAILURE." Subcutaneous and facilitated subcutaneous immunoglobulin products carry the thrombosis portion only. The renal dysfunction and acute renal failure portion applies to intravenous products, and the Hizentra prescribing information shows this directly: its boxed warning addresses thrombosis and does not include renal dysfunction. This is a real difference in labeling, not a technicality.
Thrombosis risk follows the product to either route. The Gamunex-C boxed warning states that "thrombosis may occur in the absence of known risk factors" and lists advanced age, prolonged immobilization, hypercoagulable conditions, prior venous or arterial thrombosis, estrogen use, indwelling central vascular catheters, hyperviscosity and cardiovascular risk factors. The renal portion names pre-existing renal insufficiency, diabetes mellitus, age over 65, volume depletion, sepsis, paraproteinemia and nephrotoxic drugs as predisposing factors, and directs that at-risk patients receive the minimum concentration and the slowest infusion rate practicable. Hemolysis, aseptic meningitis and transfusion-related acute lung injury sit in Warnings and Precautions on both routes, not in the boxed warning.
How do IgG blood levels differ between the two routes?
Beydoun SR and colleagues, writing in Frontiers in Neurology (2021;12:638816), put numbers on the difference. A 2 g/kg IVIG induction dose can raise immunoglobulin concentration within minutes to about 4 times pre-infusion levels, with peaks documented in the range of 16.7 to 41.0 g/L within 2 hours. Subcutaneous immunoglobulin peaks at around 60% of the typical post-IVIG peak, and reaches that peak after 36 to 72 hours rather than during the infusion. Subcutaneous trough levels run 12% to 15% higher than the troughs seen with monthly IVIG. IgG has a half-life of 21 to 30 days, which is why IVIG intervals are set at three to four weeks.
That pattern is what "end of dose wear-off" describes. With IVIG, IgG is highest in the days after an infusion and lowest in the days before the next one, and some patients notice fatigue or returning symptoms in that last week of the cycle. Beydoun and colleagues attribute the reduction of wear-off with subcutaneous dosing to its narrower peak-to-trough range. Steadier levels matter a great deal to some patients and very little to others, which is why the pattern is discussed rather than assumed.
Are SCIG side effects different from IVIG side effects?
The honest summary is systemic versus local. A meta-analysis cited by Guo Y and colleagues in Frontiers in Immunology (2018;9:1299) found the risk of moderate or systemic adverse effects about 28% lower with subcutaneous administration. What takes their place is reaction at the infusion site. The Gamunex-C prescribing information reports local infusion-site reactions in 75.0% of the subcutaneous primary immunodeficiency population, while systemic reactions dominate the intravenous primary immunodeficiency population: increased cough 31.0%, rhinitis 24.1%, pharyngitis 16.1%, headache 14.9% and asthma 14.9%. The Hizentra label reports local reactions in 19.3% of subjects in its CIDP study against 7% on placebo.
Local reactions are usually redness, swelling, itching or soreness where the needle sat. The Immune Deficiency Foundation reports that systemic side effects occur with up to 34% of IVIG infusions and that only about 1% of IVIG infusions result in serious systemic side effects, and states that premedication is usually not needed for subcutaneous infusions.
Which immunoglobulin products are labeled for which route and condition?
Route and indication are labeled together, and they do not always travel as a pair. Gamunex-C and Gammaked are labeled for primary humoral immunodeficiency by either the intravenous or the subcutaneous route, but their chronic immune thrombocytopenia and chronic inflammatory demyelinating polyneuropathy (CIDP) indications are intravenous only. Hizentra is labeled for primary immunodeficiency and for CIDP as maintenance therapy to prevent relapse in adults. HyQvia was FDA-approved on January 16, 2024 for CIDP maintenance therapy in adults.
The distinction to hold onto is maintenance versus induction: subcutaneous immunoglobulin in CIDP is labeled as maintenance therapy, not as the initial loading course. The EAN/PNS 2021 CIDP guideline strongly recommended using subcutaneous immunoglobulin for maintenance treatment in CIDP and recommended no preference between the two routes for maintenance. Our articles on IVIG for CIDP and IVIG for primary immunodeficiency and CVID cover the intravenous side of each indication.
How is switching from IVIG to SCIG calculated, and what does a prescriber weigh?
The labeled conversion is arithmetic the prescriber performs, not something a patient works out. On Gamunex-C, Gammaked, Gammagard Liquid and Hizentra, the initial subcutaneous dose in grams equals 1.37 multiplied by the previous IVIG dose in grams, divided by the number of weeks between IVIG doses. The 1.37 coefficient exists because subcutaneous bioavailability is lower than intravenous bioavailability, so more grams per month are required to reach a comparable IgG level.
Beyond the arithmetic, the factors a prescriber weighs include venous access, whether systemic infusion reactions have recurred on IVIG, whether end-of-cycle wear-off is a problem, travel and scheduling, willingness and ability to self-administer or to have a caregiver administer, and whether the specific indication is labeled for the route at all. Volume is a practical constraint as well: immunomodulatory dosing at 1 to 2 g/kg makes the intravenous route the workable one. The Immune Deficiency Foundation's stated position is that quality of life is similar across regimens as long as each regimen is individualized, and it recommends shared decision-making. Neither route is presented here as the better one.
Where can New York patients discuss IVIG and SCIG options?
Prime Infusions operates 13 infusion centers across New York, including our Manhattan infusion center on the Upper East Side, our Brooklyn infusion center on Foster Avenue and our Great Neck infusion center in Nassau County. Patients and prescribers can reach the Prime Infusions clinical team at 718-443-4000 or office@primeinfusions.com, and IVIG at Prime Infusions is described on our medication page. Referring physicians can start a case through physician referrals, and a patient or prescriber can submit the IVIG request form. Coverage and site of care are determined by your health plan; the Prime Infusions team verifies benefits and submits the authorization request.
This article is for general education and is not medical advice, a diagnosis, or a treatment recommendation. IVIG products carry an FDA boxed warning for thrombosis and for renal dysfunction and acute renal failure; review the full prescribing information for the product prescribed to you and discuss your individual risk factors with your treating clinician. Coverage and out-of-pocket cost are determined by your health plan.