Contrast Allergy and Reactions

Basics

A contrast allergy or reaction is the immunologic or physiologic response to intravenous (IV) iodinated contrast media (ICM) administered during a CT scan.
Reaction timing can be acute or delayed (1).
Acute reactions can be allergic, allergic-like (a.k.a. anaphylactoid), idiosyncratic or physiologic (a.k.a. chemotoxic or osmotoxic) (1).
Acute reactions are categorized as mild, moderate, or severe.
Delayed reactions occur from 1 hour to 1 week after ICM administration (2).
Contrast-induced nephropathy (CIN) is a sudden decrease in renal function following IV ICM administration without another known cause for renal insult (3).
A breakthrough reaction refers to a reaction that occurs in a patient who has been premedicated for such reactions (2).
MRI contrast media is not included in this chapter.

High-osmolality contrast media (HOCM) has associated acute adverse events in 5–15% of cases. It is rarely or ever used now because of its side effects (1).
Low-osmolality contrast media (LOCM) has associated acute adverse events incidence reported between 0.2% and 0.7% in three different studies reviewed by the ACR Committee Manual. Serious acute adverse events are rare (0.04%) (1).
CIN incidence is difficult to assess because there is no standard definition for diagnosing CIN, and the criteria used in different studies vary.

The pathogenesis of allergic-like reactions is not known, with several possible mechanisms resulting in immunologic mediator activation. Most are not associated with increased IgE (and therefore are not true allergic reactions). Allergic-like reactions are not dose or concentration dependent (1,4).
The pathogenesis of physiologic reactions is probably the result of the ICM physiologic properties (i.e., osmolality, molecular binding, or chemotoxicity). Physiologic reactions are dose and concentration dependent (1,4).
The pathogenesis of CIN is not well understood at this time, but several factors have been suggested.

Prior ICM reaction
History of asthma, atopy or allergy (attention to significant allergic reactions), cardiovascular disease (CVD), anxiety
For CIN: renal insufficiency/disease, diabetes mellitus, dehydration, CVD, diuretic use, advanced age, hypertension, hyperuricemia, and multiple doses of ICM in <24 hours
Metformin (risk of lactic acidosis)
Contraindicated with myasthenia gravis
Package inserts suggest necessary precautions for known or suspected pheochromocytoma, thyrotoxicosis, dysproteinemias (like multiple myeloma), or sickle cell disease (1).
An allergy to shellfish itself is not associated with an increased risk of ICM allergic-like reaction (3,4).

For patients with risk factors for CIN (including age >60 years, history of renal disease, treated hypertension, diabetes mellitus, and patients taking metformin or metformin-containing drugs), precontrast administration serum creatinine (Cr) and GFR measurements should be obtained for risk stratification (1)[C].
Consult radiology for patients with GFR <45 mL/min to evaluate need for CIN prevention steps/strategies (which may include PO or IV hydration, decreasing contrast dose or alternative imaging depending on the level of risk). A stable baseline GFR >45 mL/min is not an independent nephrotoxic risk factor (1)[C].
Consult radiology for patients taking metformin regarding the need for temporary discontinuation of medication and renal function follow-up.
For prevention of allergic-like reactions, a frequently used elective premedication regimen is prednisone 50 mg PO at 13 hours, 7 hours, and 1 hour prior to IV ICM injection PLUS diphenhydramine 50 mg IV/IM/PO 1 hour prior to ICM injection. If the patient cannot take PO, hydrocortisone 200 mg IV may be substituted for oral prednisone (1)[C].
For patients with allergic-like reaction history, consideration changing the type of ICM used
Assess alternative imaging modalities.
Avoid HOCM.

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