How to Calculate GFR in ICU Patients
The most accurate method to calculate glomerular filtration rate (GFR) in ICU patients is using the formula U × V/P (urinary creatinine × urine volume / plasma creatinine) from a timed urine collection of at least 1 hour. 1
Recommended Method for ICU Patients
Calculate creatinine clearance using the formula: Ucreat × V/Pcreat, where:
- Ucreat = urinary creatinine concentration (mmol/L)
- V = urinary volume (mL per time unit)
- Pcreat = serum creatinine concentration (mmol/L) 1
Collect urine over a period of at least 1 hour, though longer collection periods (8-24 hours) may provide more stable results 1, 2
Measure serum creatinine at the same time as the urine collection 1
Express the result in mL/min or mL/min/1.73m² if normalized to body surface area 1
Why Standard eGFR Equations Are Inappropriate for ICU Patients
Estimated GFR formulas (sMDRD, CKD-EPI, Cockroft-Gault) were developed for stable patients with chronic kidney disease and should NOT be used in critically ill patients 1
ICU patients frequently have normal creatinine levels despite altered GFR, making standard equations unreliable 1, 3
Studies show poor correlation between measured creatinine clearance and eGFR equations in ICU patients (correlation coefficients of only 0.2-0.34) 3
Standard equations significantly underestimate GFR in patients with augmented renal clearance (>130 mL/min/1.73m²), which affects up to 40% of septic ICU patients 1, 3
Factors Affecting GFR Calculation in ICU Patients
Systemic inflammatory response syndrome (SIRS) often increases cardiac and renal blood flows, raising GFR 1
IV fluids and vasoactive drugs contribute to GFR increases 1
Hypoalbuminemia affects drug binding and clearance, particularly for medications highly bound to plasma proteins 1
Patient clinical condition may change rapidly during ICU stay, altering pharmacokinetics 1
Unstable creatinine kinetics from variable urine output and creatinine production make accurate GFR estimation challenging 4
When to Calculate GFR in ICU Patients
At the onset of treatment with medications requiring renal dosing (particularly beta-lactam antibiotics) 1
Every time the clinical condition and/or renal function of the patient significantly changes 1
When performing therapeutic drug monitoring to help interpret results 1
When measuring albumin levels (or plasma proteins) to better interpret GFR in the context of drug dosing 1
Common Pitfalls to Avoid
Do not rely on serum creatinine alone as it may appear normal despite significantly altered GFR 1
Do not use standard eGFR equations (CKD-EPI, MDRD, Cockroft-Gault) as they have unacceptable error rates in ICU patients 2, 3
Be aware that measured creatinine clearance has inherent variability (±52%) due to unstable creatinine kinetics in critically ill patients 4
Remember that augmented renal clearance is common in ICU patients and can lead to underdosing of renally cleared medications if not recognized 1
Consider that the coefficient of variation for creatinine production (28%) and urine output (34%) in ICU patients introduces significant error in GFR calculations 4