How In-Vitro GFR is Measured
I need to clarify an important terminology issue: there is no such thing as "in-vitro GFR" in clinical practice—GFR measurement is an in-vivo procedure performed in living patients. The term you're likely referring to is "measured GFR" (mGFR) using exogenous filtration markers, which involves laboratory analysis of blood samples but requires the patient to receive an injection and have blood drawn over time 1.
The Actual Procedure: Measured GFR (mGFR)
The gold standard approach uses iohexol plasma clearance with multiple blood samples collected at specific time intervals after a single intravenous injection 1.
Step-by-Step Protocol:
Initial injection: A single intravenous dose of iohexol (an exogenous filtration marker) is administered 1
Blood sampling timing depends on the patient's estimated GFR 1:
- eGFR >60 ml/min/1.73 m²: Collect samples at 2 and 4 hours with 1-2 additional samples in between
- eGFR 30-59 ml/min/1.73 m²: Collect samples at 3 and 7 hours with 1-2 additional samples in between
- eGFR <30 ml/min/1.73 m²: Collect samples at 4 and 10 hours with 1-2 additional samples in between
- Minimum of 4 samples recommended to ensure quality control 1
Laboratory analysis: Blood samples are analyzed using high-performance liquid chromatography (HPLC) to measure iohexol concentrations 2
GFR calculation: Uses a one-compartment kinetic model with Bröchner-Mortensen correction before body surface area indexation 1
Alternative Methods:
- Single-sample protocols: Simpler but less accurate, using only 1-2 blood samples 1
- Urinary clearance: Can be used in patients with severe volume overload (grade 3-4 edema or ascites) 1
- Other markers: ⁵¹Cr-EDTA, ⁹⁹ᵐTc-DTPA, or iothalamate can be used but may involve radiation exposure 3
Possible Side Effects
Iohexol is remarkably safe with minimal side effects when used for GFR measurement 1. The evidence shows:
Common/Expected Effects:
- Essentially none at the low doses used for GFR measurement (much lower than diagnostic imaging doses)
- The procedure is non-invasive except for the initial IV injection and blood draws
Theoretical Risks (extremely rare at GFR measurement doses):
- Allergic reactions: Possible but exceedingly uncommon with non-ionic contrast agents like iohexol
- Local injection site reactions: Minor irritation or inflammation at IV site
- Venipuncture complications: Bruising, discomfort from multiple blood draws
Important Context:
The safety profile is so favorable that iohexol has replaced radioactive markers (which carried radiation exposure risks) as the preferred exogenous marker 1, 3. Studies specifically evaluating similar fluorescent markers like MB-102 showed "negligible toxicity" with no adverse inflammation, irritation, or preferential tissue accumulation 4.
Practical Considerations:
- Patient burden: The main "side effect" is the time commitment (2-10 hours depending on kidney function) and multiple blood draws 1
- Contraindications: Severe iodine allergy (theoretical concern, though doses are much lower than imaging studies)
- Volume overload patients: Should use urinary clearance method instead 1
The procedure is so safe that it's recommended as the confirmatory test when accurate GFR assessment is needed beyond routine estimated GFR equations 5, 6.