Bloodwork for CT Scan with Contrast
Order serum creatinine and calculate eGFR before administering IV contrast in all patients at risk for contrast-induced nephropathy, using an eGFR threshold of 30 mL/min/1.73 m² as the level with greatest evidence for CIN risk. 1
Essential Pre-Contrast Laboratory Testing
Renal Function Assessment
The primary bloodwork needed is serum creatinine with calculated eGFR, as eGFR is recognized as a better indicator of baseline renal function than serum creatinine alone. 1
Key thresholds to guide contrast administration:
- eGFR ≥45 mL/min/1.73 m²: Intravenous iodinated contrast is not an independent nephrotoxic risk factor in patients with stable baseline function at this level 1
- eGFR 30-45 mL/min/1.73 m²: Conflicting evidence exists, but reduced iodine dose should be considered 1
- eGFR <30 mL/min/1.73 m²: The ACR Manual on Contrast Media notes this threshold has the greatest level of evidence for CIN risk 1
Risk-Stratified Approach to Testing
You can selectively omit routine creatinine testing in low-risk patients who answer negatively to all of the following screening questions: 2
- History of renal disease (strongest predictor, OR 13.6)
- Proteinuria (OR 8.7)
- Prior kidney surgery (OR 8.1)
- Hypertension (OR 5.4)
- Gout (OR 4.6)
- Diabetes mellitus (OR 3.2)
If all six questions are negative, 99% of patients will have creatinine values ≤1.7 mg/dL, which could reduce routine creatinine determinations by 67% and decrease costs and delays. 2
Additional Risk Factors Requiring Creatinine Testing
Always obtain baseline creatinine in patients with these high-risk features: 3, 4
- Pre-existing renal impairment (most critical risk factor)
- Diabetes mellitus, especially with coexisting renal disease (20-50% CIN risk)
- Age >70 years
- Dehydration or volume depletion
- Concomitant nephrotoxic medications
- Cardiovascular disease
- High planned contrast dose
Metformin Management
For patients taking metformin, verify eGFR before contrast administration—never assume renal function is adequate. 3
Metformin management protocol based on renal function:
- Normal renal function (eGFR ≥60): Stop metformin at time of contrast administration, hold for 48 hours post-procedure, restart if renal function remains stable 3
- Impaired function (eGFR 30-60): Withhold metformin for 48 hours after procedure, re-evaluate eGFR at 48 hours before restarting, consider alternative glucose medications during hold period 3
- Severe impairment (eGFR <30): Only restart metformin after kidney function re-evaluated and confirmed normal 3
Post-Contrast Follow-Up Testing
Re-evaluate kidney function (serum creatinine) within 48-96 hours after contrast administration in patients with baseline eGFR <60 mL/min/1.73 m² or other high-risk features. 3
Contrast-induced AKI typically manifests within 24-48 hours (up to 2-5 days) after contrast administration, so this timing is critical for detection. 4
Common Pitfalls to Avoid
Do not rely on reported eGFR in acute or unstable conditions—in the absence of steady state (such as postoperative patients or those with recent creatinine increases), reported eGFRs cannot be used accurately. 1
Do not use estimated baseline creatinine indiscriminately—when premorbid baseline is unavailable, KDIGO recommends estimating creatinine based on MDRD GFR of 75 mL/min per 1.73 m², but this should be applied cautiously. 1
Do not withhold nephrotoxic medication screening—suspend NSAIDs and aminoglycosides at least 24 hours before the procedure in at-risk patients. 3