Contrast Allergy and Reactions
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- 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.
Etiology and Pathophysiology
- 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.