From the Guidelines
For patients with a known contrast allergy who need a CT scan with contrast, premedication with a regimen of 50 mg of prednisone administered 13 hours, 7 hours, and 1 hour before the procedure, as well as 50 mg of diphenhydramine 1 hour before the procedure, is recommended to reduce the risk of allergic reactions. This recommendation is based on the evidence from the 2011 ACCF/AHA/SCAI guideline for percutaneous coronary intervention 1, which suggests that adequate pretreatment of patients with prior anaphylactoid reactions reduces the recurrence rate to close to zero. The use of corticosteroids and antihistamines has been shown to be effective in preventing recurrent anaphylactoid reactions to radiographic contrast material, as stated in the 2005 practice parameter on the diagnosis and management of anaphylaxis 1.
Key Considerations
- Patients with a history of prior anaphylactoid reaction to contrast media are at increased risk of recurrence, with rates ranging from 16% to 44% without prophylaxis 1.
- The premedication regimen should be individualized based on the patient's history of contrast reactions and other factors, such as age and medical conditions.
- In emergency situations, an accelerated protocol using methylprednisolone and diphenhydramine can be used, as described in the 2005 practice parameter 1.
- Patients should be closely monitored during and after the procedure, as breakthrough reactions can still occur despite premedication.
Pretreatment Regimens
- The standard premedication regimen consists of prednisone 50 mg orally at 13 hours, 7 hours, and 1 hour before the procedure, plus diphenhydramine 50 mg orally or intravenously 1 hour before the procedure.
- For emergency situations, an accelerated protocol can be used: methylprednisolone 40 mg IV immediately and every 4 hours until the procedure, plus diphenhydramine 50 mg IV 1 hour before the scan.
- The choice of premedication regimen should be based on the patient's individual needs and medical history, as well as the severity of the prior contrast reaction.
From the Research
Preparation for CT with Contrast Allergy
To prepare for a CT scan with contrast in patients who have a known allergy to contrast media, several steps can be taken:
- Pretreatment regimens: Studies have shown that pretreatment with certain medications can reduce the risk of anaphylactoid reactions to radiographic contrast media 2.
- Medication administration: The use of a visual aid can increase the subjective confidence of radiologists in the dose and route of medication administration and lead to faster administration of epinephrine in the event of a contrast medium reaction 3.
Pretreatment Regimens
The following pretreatment regimens have been studied:
- Prednisone and diphenhydramine: This combination has been shown to decrease the reaction rate to 9.0% 2.
- Prednisone, diphenhydramine, and ephedrine: The addition of ephedrine to the pretreatment regimen has been shown to provide a statistically significant reduction in reaction rate, to 3.1% 2.
Contrast Medium Administration
The administration of contrast medium is an important aspect of CT scans:
- Indications for contrast medium enhancement: Contrast medium enhancement is not always necessary and should be used with discrimination, taking into account the clinical demand and scanning time available 4.
- Contrast medium administration strategies: Different strategies for administering contrast medium, such as dilution, low flow, or low concentration, have been compared in terms of their effect on vascular attenuation 5.