Investigation of Choice to Calculate GFR
For accurate GFR measurement in clinical practice, iohexol plasma clearance is the investigation of choice, having effectively replaced inulin clearance as the gold standard. 1
Why Iohexol Plasma Clearance?
Iohexol fulfills virtually all requirements for an ideal GFR marker 2:
- Minimal extra-renal excretion (< 2%)
- Low protein binding
- Neither secreted nor reabsorbed by the kidney
- Virtually non-toxic
- Low cost
- Stable in plasma, allowing separation of administration and sample analysis in both space and time
The 2024 European Kidney Function Consortium consensus specifically endorses iohexol plasma clearance measurement as the standardized approach for adults 1. This represents a significant shift from historical reliance on inulin clearance, which required cumbersome IV infusions and reliable urine collection.
Practical Protocol Selection
For most patients: Use plasma clearance with multiple samples 2, 3
- Standard protocol: 4-point plasma disappearance curve (10,30,120, and 300 minutes post-injection) 4
- Simplified protocol: 2-point sampling (120 and 300 minutes) works for most cases 4
- Low GFR situations: Include one or more late samples to improve accuracy 2
For patients with large edema or ascites: Use urinary clearance protocols 2
This is critical because volume of distribution affects plasma clearance calculations in these patients.
Alternative Markers (When Iohexol Unavailable)
If iohexol is not accessible, consider these alternatives in order of preference 5, 3:
- 51Cr-EDTA (radioactive) - comparable accuracy but requires radiation safety precautions
- Iothalamate (non-radioactive or 125I-labeled) - ionic contrast agent
- 99mTc-DTPA (radioactive) - widely available but less accurate
Common Pitfalls to Avoid
Do not rely solely on estimated GFR (eGFR) from creatinine or cystatin C for high-stakes decisions 1. While convenient for routine monitoring, eGFR equations:
- Have significant non-GFR variability from muscle mass (creatinine) 1
- Are hampered by non-GFR determinants (cystatin C) 1
- Can be inaccurate at the individual patient level, limiting personalized care 1
Measured GFR is essential for 6:
- Kidney donor approval
- Chemotherapy dosing decisions
- Drug dosing in critical situations
- When eGFR results seem discordant with clinical picture
Key Advantage Over Historical Methods
Unlike inulin clearance (the original gold standard), modern iohexol plasma clearance 7, 4:
- Requires no continuous IV infusion
- Needs no bladder catheterization or timed urine collections
- Avoids errors from incomplete bladder emptying
- Can be performed in outpatient settings
The availability of an external proficiency program (Equalis AB, Sweden) for iohexol facilitates interlaboratory comparison, making it suitable for standardized clinical use 2.