Pretesting and Premedication

Contrast Media & Reactions - Glow Juice Jitters

• Contrast Media Types:
  • Iodinated: Ionic (High Osmolality, ↑ reactions), Non-ionic (Low Osmolality, ↓ reactions).
  • Gadolinium-based (GBCA): For MRI; risk of Nephrogenic Systemic Fibrosis (NSF) in renal impairment.
  • Barium Sulfate: Oral/rectal for GI imaging.
• Reaction Classification & Timing:
  • Acute: < 1 hour post-injection.
  • Delayed: 1 hour to 1 week (e.g., skin rashes).
• Acute Reaction Severity:
  • Mild: Nausea, vomiting, limited urticaria, pruritus, warmth, headache. Self-limiting.
  • Moderate: Diffuse urticaria/erythema, bronchospasm, facial/laryngeal edema (mild), vasovagal syncope. Requires treatment.
  • Severe (Anaphylactoid): Profound hypotension, severe laryngeal edema, severe bronchospasm, arrhythmias, seizure, respiratory arrest, cardiac arrest. Life-threatening. Incidence: 0.01-0.04% with LOCM.

⭐ Non-ionic low-osmolar contrast agents (LOCM) have a significantly lower incidence of severe adverse reactions (approx. 0.04%) compared to high-osmolar contrast agents (HOCA) (approx. 0.2%).

Risk Assessment & Pretesting - Red Flag Radar

Identify high-risk patients to minimize contrast reactions.

• Key Risk Factors:

  • Prior moderate/severe contrast reaction (highest risk)
  • Active/uncontrolled asthma
  • Multiple severe allergies (food, drugs)
  • Renal disease (eGFR < 30 mL/min/1.73m²: CIN risk; NSF with specific GBCAs)
  • Significant cardiac disease (e.g., unstable angina)
  • Drugs: β-blockers, Metformin (hold if eGFR < 30/AKI), IL-2

• Pre-Procedure Checks:

  • Allergy & prior reaction history
  • Renal function (Serum Cr, eGFR)
  • Pregnancy status (if relevant)

• Premedication (prior allergic reaction / high-risk asthma):

  • Steroids: Prednisolone 50mg PO (13,7,1h pre) or Methylprednisolone 32mg PO (12,2h pre)
  • Antihistamine: Diphenhydramine 25-50mg (1h pre)

⭐ Prior allergic-like reaction to same class contrast: strongest predictor of future reaction (risk ↑5-6x).

Premedication & Special Considerations - Shield & Special Ops

• Premedication (Prior Allergic-like Rxn to Iodinated Contrast):
  • Goal: ↓ risk/severity of breakthrough Rxns.
  • Steroids:
    • Oral (Elective): Prednisone 50mg (13h, 7h, 1h pre).
    • IV (Urgent): Hydrocortisone 200mg / Methylprednisolone 40mg IV q4h until scan (last dose 1h pre).
  • Antihistamine: Diphenhydramine 50mg IV/IM/PO 1h pre (adjunct).

• Special Populations:
  • Pregnancy: Shield fetus. Iodinated contrast if essential. GBCAs: avoid unless benefit >> risk.
  • Lactation: Iodinated/GBCAs <1% excreted. Interrupt breastfeeding 12-24h (optional).
  • Metformin:
    • eGFR ≥30 mL/min/1.73m²: Continue metformin.
    • eGFR <30 mL/min/1.73m², AKI, IA contrast (renal exposure): Hold metformin. Resume 48h post if renal function stable.
  • Pheochromocytoma: α & β blockade. Use non-ionic LOCM.
  • Myasthenia Gravis: Risk of exacerbation. Monitor.
  • Hyperthyroidism: Risk of thyroid storm. Prophylaxis if history.

⭐ In patients with prior moderate-severe allergic-like reactions, premedication reduces the incidence of mild reactions, but its efficacy in preventing severe, life-threatening reactions is not definitively established.

High‑Yield Points - ⚡ Biggest Takeaways

• Prior allergic reaction to contrast: strongest predictor of future events.
• Premedication (steroids, antihistamines) for previous moderate/severe reactions.
• Assess renal function (eGFR/SCr) pre-iodinated contrast in at-risk patients.
• Metformin: hold day of contrast & 48h post if eGFR <30 mL/min/1.73m² or Acute Kidney Injury (AKI).
• Hydration is key to prevent Contrast-Induced Nephropathy (CIN).
• GBCAs: screen renal function (eGFR) to prevent Nephrogenic Systemic Fibrosis (NSF).
• Always check pregnancy/lactation status before contrast administration.