High Yield Overview
Contrast Agents: Indications & Safety
—Iodinated Contrast Reaction
1-3%mild, 0.04% severe
—NSF Risk (Gadolinium)
30eGFR less than
—Metformin Withhold
45If eGFR less than
—Contrast-Induced Nephropathy
45eGFR less than risk
Contrast Reaction Severity
Mild: Urticaria, pruritus, nausea, flushing
Moderate: Bronchospasm, facial oedema, tachycardia
Severe: Anaphylaxis, laryngeal oedema, shock, arrest
Key: Severe reactions require immediate ABCDE and adrenaline
Critical Must-Knows
- Iodinated contrast: Risk of allergy, nephrotoxicity
- Gadolinium: Risk of NSF in severe renal impairment
- Pre-hydration reduces contrast nephropathy risk
- Severe prior reaction: Premedication and alternative agent
- Metformin held 48 hours post-contrast if eGFR less than 45
Examiner's Pearls
- "Mild reaction: Urticaria, nausea - treat and observe
- "Severe reaction: Anaphylaxis - ABCDE approach, adrenaline
- "NSF: Fibrosing skin condition, now rare with screening
- "Group 1 gadolinium agents lowest NSF risk
- "Creatinine/eGFR required before iodinated contrast
Exam Warning
Contrast safety is commonly examined. Know the risk factors for contrast reactions, how to manage anaphylaxis, and the renal thresholds for contrast use. Gadolinium-related NSF is now rare due to screening and avoiding high-risk agents in renal impairment.
Iodinated Contrast (CT)
CT Contrast in Orthopaedics
| Indication | Reason for Contrast | Alternative |
|---|---|---|
| Tumour assessment | Vascularity, extent, necrosis | MRI often preferred |
| Infection | Abscess rim enhancement | MRI with Gd preferred |
| Vascular injury | CT angiography | None (contrast essential) |
| CT arthrography | Joint visualisation | MR arthrography |
Risk Factors for Reaction
Previous contrast reaction (strongest predictor), asthma (especially unstable), multiple allergies, cardiac disease. Shellfish/seafood allergy is NOT a specific risk factor (myth). Previous reaction: 17-35% will react again without premedication.
Contrast-Induced Nephropathy
CIN Risk Factors and Prevention
| Factor | Risk Level | Mitigation |
|---|---|---|
| eGFR greater than 45 | Low | Routine contrast use acceptable |
| eGFR 30-44 | Moderate | Hydration, limit volume, consider alternatives |
| eGFR less than 30 | High | Avoid if possible, nephrology input, dialysis planning |
| Diabetes + CKD | Increased | Extra caution, good hydration |
| Multiple myeloma | Increased | Hydration critical, dehydration increases risk |
CIN Prevention Protocol
Pre-hydration: IV normal saline 1mL/kg/hr for 6-12 hours before and after. Limit contrast volume. Use iso-osmolar agents if high risk. Avoid nephrotoxics (NSAIDs, aminoglycosides) peri-procedure. Monitor creatinine 48-72 hours post if high risk.
Metformin and Contrast
If eGFR greater than 45 and normal renal function: No action needed. If eGFR 30-45: Withhold metformin from time of contrast and restart 48 hours after if renal function stable. If eGFR less than 30: Withhold metformin, contrast generally avoided. Risk is metformin accumulation causing lactic acidosis if CIN develops.
Gadolinium Contrast (MRI)
Nephrogenic Systemic Fibrosis (NSF)
Rare but serious fibrosing condition affecting skin and internal organs. Only occurs with gadolinium exposure in severe renal impairment (eGFR less than 30). Group 1 agents (highest risk) now avoided in renal impairment. With current screening protocols, NSF is very rare.
Gadolinium Agent Risk Classification
| Risk Group | Agents | Recommendation (eGFR less than 30) |
|---|---|---|
| High risk (Group 1) | Gadodiamide, gadopentetate, gadoversetamide | Contraindicated |
| Intermediate (Group 2) | Gadobenate, gadofosveset, gadoxetate | Avoid if possible |
| Low risk (Group 3) | Gadobutrol, gadoterate, gadoteridol | Use if essential with caution |
Premedication Protocols
Premedication for Previous Contrast Reaction
| Timing | Medication | Dose |
|---|---|---|
| 13 hours before | Prednisolone | 50 mg PO |
| 7 hours before | Prednisolone | 50 mg PO |
| 1 hour before | Prednisolone + Antihistamine | 50 mg PO + Promethazine 25mg |
| Alternative (Emergency) | Hydrocortisone IV | 200mg IV (plus antihistamine) |
When to Premedicate
Previous moderate/severe contrast reaction. May reduce but does NOT eliminate reaction risk. Use alternative contrast agent (different manufacturer/type). Consider whether contrast is truly necessary. Ensure emergency equipment available.
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
VIVA SCENARIOStandard
EXAMINER
"A patient scheduled for CT with contrast has eGFR of 35 mL/min and takes metformin for diabetes."
EXCEPTIONAL ANSWER
This patient has moderate CKD (eGFR 30-44) putting them at moderate risk for contrast-induced nephropathy. Management: (1) Confirm the clinical indication justifies contrast - consider whether non-contrast CT or MRI could answer the question. (2) If contrast essential: Pre-hydration with IV normal saline 1mL/kg/hr for 6-12 hours before and after. (3) Use iso-osmolar or low-osmolar contrast agent, minimise volume. (4) Metformin: Withhold from the time of contrast administration. Do not restart for 48 hours after contrast and only if renal function remains stable (repeat creatinine). (5) Monitor creatinine 48-72 hours post-procedure. (6) Avoid other nephrotoxics (NSAIDs, aminoglycosides) peri-procedure.
KEY POINTS TO SCORE
eGFR 30-44 = moderate CIN risk
Pre-hydration with IV saline recommended
Metformin withheld until 48h post if eGFR less than 45
Minimise contrast volume, use low-osmolar
Monitor creatinine 48-72h post
COMMON TRAPS
✗Not withholding metformin
✗Not pre-hydrating high-risk patients
✗Not rechecking renal function
VIVA SCENARIOStandard
EXAMINER
"During a CT scan, a patient develops facial swelling, stridor, and hypotension after contrast injection."
EXCEPTIONAL ANSWER
This is anaphylaxis requiring immediate ABCDE management: (A) Airway - Assess for obstruction, consider early intubation if stridor worsening. (B) Breathing - High-flow oxygen, assess for bronchospasm, nebulised salbutamol if needed. (C) Circulation - Large bore IV access, fluid bolus (1-2L crystalloid), monitor BP. (D) Disability - Check GCS. (E) Exposure - Look for urticaria. ADRENALINE: 0.5mg (0.5mL of 1:1000) IM into lateral thigh immediately. Repeat every 5 minutes if no improvement. Call resuscitation team. Secondary medications: Antihistamine (chlorpheniramine 10mg IV), Hydrocortisone 200mg IV. Transfer to resuscitation area. Monitor for biphasic reaction (observe 6+ hours).
KEY POINTS TO SCORE
ABCDE approach immediately
Adrenaline 0.5mg IM (1:1000) into lateral thigh
Repeat adrenaline every 5 min if no response
High-flow O2, IV fluids, antihistamine, steroids
Observe 6+ hours for biphasic reaction
COMMON TRAPS
✗Delaying adrenaline
✗Wrong adrenaline route or dose
✗Not calling for help
VIVA SCENARIOStandard
EXAMINER
"A patient with chronic kidney disease (eGFR 25) requires an MRI with gadolinium contrast for tumour assessment."
EXCEPTIONAL ANSWER
The main concern is nephrogenic systemic fibrosis (NSF), a rare but serious fibrosing condition that only occurs in patients with severe renal impairment (eGFR less than 30) exposed to gadolinium. Management: (1) Question necessity - is contrast essential? Can the clinical question be answered without? (2) If essential, use only Group 3 (low-risk) gadolinium agents such as gadobutrol, gadoterate, or gadoteridol. (3) Avoid Group 1 agents (gadodiamide, gadopentetate) which are contraindicated in eGFR less than 30. (4) Use the minimum dose needed. (5) Document the discussion and consent specifically regarding NSF risk. (6) No specific post-procedure treatment for gadolinium (unlike iodinated contrast, dialysis doesn't reliably remove gadolinium).
KEY POINTS TO SCORE
NSF risk in eGFR less than 30
Use only Group 3 (low-risk) gadolinium agents
Group 1 agents contraindicated
Minimum dose necessary
Dialysis doesn't reliably remove gadolinium
COMMON TRAPS
✗Using high-risk gadolinium agent
✗Not documenting consent for NSF risk
✗Assuming dialysis removes gadolinium
Contrast Safety Quick Reference
High-Yield Exam Summary
CIN Prevention
- •eGFR greater than 45: Routine use OK
- •eGFR 30-44: Hydrate, limit volume
- •eGFR less than 30: Avoid if possible
- •Pre-hydration: Saline 1mL/kg/hr
Metformin Protocol
- •eGFR greater than 45: No action needed
- •eGFR 30-45: Hold, restart 48h if stable
- •eGFR less than 30: Hold, contrast avoided
Anaphylaxis Management
- •ABCDE approach
- •Adrenaline 0.5mg IM (1:1000)
- •Repeat every 5 minutes
- •O2, fluids, antihistamine, steroids
Gadolinium NSF Risk
- •Only in eGFR less than 30
- •Group 1 agents: Contraindicated
- •Group 3 agents: Lowest risk
- •Dialysis doesn't remove gadolinium