Creatinine-based estimation of rate of long term renal function loss in lung transplant recipients. Which method is preferable?

Background: Progressive renal function loss during long-term follow up is common after lung transplantation and close monitoring is warranted. Since changes in creatinine generation and excretion may occur after lung transplantation, the reliability of creatinine-based methods of renal function assessment to serial measurements of glomerular filtration rate (GFR) were compared in this population.
Methods: Renal function with serial measurements of GFR by iothalamate clearance every 6 months after transplantation was studied in a cohort of 40 lung transplant recipients with at least 24 months of follow up, transplanted between November 1990 and October 1995 in this center. The correlation between the rate of renal function loss calculated from the slope of GFR and the following creatinine-based indices: 1/S-creatinine, Cockcroft clearance and Levey estimation were analyzed. The absolute difference between GFR and Cockcroft clearance and Levey estimation pre- post-transplantation at several points was also studied.
Results: The slopes of 1/S-creatinine (r = 0.85), Cockcroft clearance (r = 0.86), and the Levey estimation (r = 0.84) correlated significantly with the slope of GFR as measured by iothalamate clearance. However, all creatinine-based slopes underestimate the rate of GFR loss. Cockcroft clearance and the reciprocal value of serum creatinine do not detect small GFR losses. During long-term follow up a time-dependent discrepancy between Cockcroft clearance, Levey estimation and GFR was observed which may partially explain the observations for this population.
Conclusion: Creatinine-based slopes correlate with GFR slopes after lung transplantation, but consistently underestimate the rate of GFR decline. The Levey estimation is the most sensitive method used to detect small GFR losses and may be preferable when no GFR measurement is available. In special conditions when an accurate renal function assessment is needed true GFR may be necessary.