eGFR and contrast safety
Calculate eGFR with the race-free CKD-EPI 2021 equation, in mg/dL or µmol/L, and read it against the thresholds that govern iodinated contrast, gadolinium and metformin.
Inputs
eGFR
75mL/min/1.73m²
Stage
G2
eGFR 75 mL/min/1.73 m² · G2
75 mL/min/1.73 m² is mildly reduced (G2) and no reason for caution on its own.
An eGFR from a single creatinine assumes stable renal function. In acute kidney injury the creatinine lags behind the true filtration rate and the equation returns a figure that is too flattering — exactly in the situation where you can least afford it.
And the nuance that matters most: the evidence that intravenous contrast genuinely causes kidney injury at moderately reduced function is considerably weaker than was long assumed. Not performing a study, or performing it without contrast, carries its own cost. Follow your local protocol, but weigh the risk of not looking.
How to read the result
The number answers a safety question, not a diagnostic one. Most protocols place their decision point at 30 mL/min/1.73 m², and above that the practical implications for imaging are limited.
The equation
eGFR = 142 × min(Scr/κ, 1)^α × max(Scr/κ, 1)^−1.200 × 0.9938^age × 1.012 (if female)
κ is 0.7 for women and 0.9 for men; α is −0.241 and −0.302 respectively. Creatinine is in mg/dL, so a value in µmol/L is divided by 88.4 first.
The 2021 revision removed the race coefficient present in the 2009 equation.
The result is normalised to 1.73 m² body surface area, which is why an unusual body habitus makes it less reliable.
When to use it
- Checking whether a requested contrast-enhanced CT can proceed on the creatinine available.
- Deciding whether a gadolinium-enhanced MRI needs a different agent or a different conversation.
- Answering the metformin question, which arrives at the reading room more often than it should.
- Converting a laboratory creatinine in µmol/L to an eGFR without a second tool.
⚠️ Pitfalls
- A single creatinine assumes stable function. In acute kidney injury the creatinine lags and the eGFR flatters, precisely when it matters most.
- The equation is normalised to 1.73 m². At the extremes of body size, and in amputees, it drifts.
- It applies to adults only. Laboratories do not report it below the age of 18, and a paediatric estimate such as the bedside Schwartz formula belongs there instead.
- Creatinine depends on muscle mass. A cachectic oncology patient can have a normal creatinine and a genuinely poor filtration rate.
- The gadolinium threshold is not the iodine threshold. Group II macrocyclic agents carry a very low nephrogenic systemic fibrosis risk even below 30.
- Thresholds differ between national guidelines. Use these as orientation and let your local protocol decide.
Frequently asked questions
Which equation does this calculator use?
CKD-EPI 2021, the race-free revision. It uses serum creatinine, age and sex, and reports a rate normalised to 1.73 m².
Can I enter creatinine in µmol/L?
Yes. The calculator converts µmol/L to mg/dL by dividing by 88.4, so you can enter the value exactly as the laboratory reported it.
At what eGFR is iodinated contrast a concern?
Most protocols place their decision point at 30 mL/min/1.73 m². Above that the risk of post-contrast acute kidney injury from intravenous contrast is considered low, and recent evidence suggests it was historically overestimated.
Does gadolinium use the same threshold?
No. Group II macrocyclic agents carry a very low risk of nephrogenic systemic fibrosis even below an eGFR of 30, and most protocols permit them with deliberate dosing rather than forbidding them.
When should metformin be withheld?
The common line is to withhold below an eGFR of 30, or in acute kidney injury, rather than routinely in everyone receiving contrast.
References
📄Inker LA, Eneanya ND, Coresh J, et al. New Creatinine- and Cystatin C-Based Equations to Estimate GFR without Race. N Engl J Med. 2021;385(19):1737–1749.
📄Davenport MS, et al. Use of Intravenous Iodinated Contrast Media in Patients with Kidney Disease: Consensus Statements from the ACR and the National Kidney Foundation. Radiology. 2020;294(3):660–668.
📄ACR Manual on Contrast Media, current edition.↗📄ESUR Guidelines on Contrast Agents.↗This calculator supports clinical judgement and does not replace it. Always check the result against your own measurements and the guideline in force.
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