Bloodwork for CT Scan with Contrast

Essential Pre-Contrast Laboratory Testing

The American College of Radiology recommends ordering serum creatinine and calculating 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, 2
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, 2
Key thresholds to guide contrast administration include: eGFR ≥45 mL/min/1.73 m², where intravenous iodinated contrast is not an independent nephrotoxic risk factor in patients with stable baseline function at this level 1, 2
For patients with eGFR 30-45 mL/min/1.73 m², conflicting evidence exists, but reduced iodine dose should be considered 1, 2
The ACR Manual on Contrast Media notes that an eGFR <30 mL/min/1.73 m² has the greatest level of evidence for CIN risk 1, 2

The American College of Radiology recommends always obtaining baseline creatinine in patients with high-risk features, including pre-existing renal impairment, diabetes mellitus, age >70 years, dehydration or volume depletion, concomitant nephrotoxic medications, cardiovascular disease, and high planned contrast dose 3, 4

For patients taking metformin, the American College of Radiology recommends verifying eGFR before contrast administration—never assume renal function is adequate 3
The metformin management protocol based on renal function includes: stopping metformin at time of contrast administration and holding for 48 hours post-procedure for patients with normal renal function (eGFR ≥60) 3
For patients with impaired renal function (eGFR 30-60), metformin should be withheld for 48 hours after procedure, and re-evaluated at 48 hours before restarting 3
For patients with severe renal impairment (eGFR <30), metformin should only be restarted after kidney function has been re-evaluated and confirmed normal 3

The American College of Radiology recommends re-evaluating 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

The Kidney Disease: Improving Global Outcomes (KDIGO) guidelines recommend not relying on reported eGFR in acute or unstable conditions, as reported eGFRs cannot be used accurately in the absence of steady state 5
The KDIGO guidelines also recommend not using estimated baseline creatinine indiscriminately, and instead estimating creatinine based on MDRD GFR of 75 mL/min per 1.73 m², but this should be applied cautiously 5